BOILER AND FACTORY CHIMNEYS,

[Stanley]

BOILER AND FACTORY CHIMNEYS, THEIR DRAUGHT, POWER AND STABILITY:
[WITH A CHAPTER ON LIGHTNING CONDUCTORS.]

BY, ROBERT WILSON, A.I.C.E.,
LONDON:
CROSBY LOCKWOOD & SON,

7, STATIONERS' HALL COURT, LUDGATE HILL

1892.

PREFACE.

The following Chapters were commenced with the intention of adding them to the 5th Edition of my “Treatise on Steam Boilers” in consequence of having had numerous enquiries respecting the proper size of chimney for boiler-work. The information is, however, likely to be more useful in its present form, hence the appearance of this little book.

I had some diffidence in calling in question the correctness of the theory of draught adopted by Rankine, by Morin, and by Peclet in the 2nd Edition of his “Traite de la Chaleur," but after going to press I find that Peclet in the 3rd Edition of his work has altered his theory and adopted the same as I have arrived at.

ROBERT WILSON.

CONTENTS.


CHAPTER I. CHIMNEY DRAUGHT .

CHAPTER II. STABILITY OF CHIMMYS

CHAPTER III. DESCRIPTIONS OF CHIMNEYS.

CHAPTER IV. LIGHTNING CONDUCTORS

[Like all books of this age there are problems with scanning in text which is larded with formulae and diagrams. The first chapter on draught of chimneys is particularly bad and I have omitted it on the grounds that whilst it is interesting to see what the state of thinking was in 1892, the formulae are useless in that modern principles are different and in my personal opinion all the formulae used gave insufficient draught, probably on cost grounds. This saved money on the initial investment but cost more in coal in the long run. Wilson himself admits in the text that he had difficulty reconciling his calculations with other experts. So this is not a complete scan but I think we have got the juice even though I have cut out the first chapter and all the small diagrams. Thanks to Richard Adamek for lending me the original copy.]

CHAPTER II. STABILITY OF CHIMNEYS.

WHEN the proper height and size of chimney have been decided upon to ensure a sufficient draught for the furnace, and also to satisfy the sanitary requirements of the case, the designing with respect to ornamentation, beauty of outline, and harmonising with surrounding buildings, belongs to the architect rather than to the engineer ; but the design, so far as the stability of the structure is concerned, still lies within the engineer's province. The principles of stability have been laid down by Professor Rankine, who, up to the time of his death, was regarded as the first authority on this subject.

In estimating the safety and stability of a tall chimney shaft, the strains to be considered are- 1st, the pressure exerted by the weight of the masonry or brickwork; and 2nd, the lateral pressure of the wind.

In order to resist the former strain, the best form of structure is that which gives an equal pressure per square unit of area in every section or "bed-joint." Taking, for simplicity, a solid cylinder, the weight evidently increases from the top downwards. This increase of weight must therefore be provided for by an increase in the sectional area as we descend. But this very increase of area augments the rapidity of growth of the mass as we descend, and the sectional
area below must be further increased in consequence.

For a structure whose centre of figure is the same as its centre of pressure, this law of increase may be deduced with the aid of the differential calculus as follows -

Let W = weight of top layer of chimney. A = area of top section, K = coefficient of safety (for brick say 10 tons per square foot), c = weight of a cubic foot of brickwork, a = any given section at distance h from the top, e = basis of natural logs = 2.71828. then W=AK or A= W/K which gives us the first sectional area from the top to resist crushing. Any other section can be found by the formula; a = Ae X c/kh. Or log a = log A + 0.4343 h. e/k h.

This formula applies also to hollow cylinders or cones having a straight batter inside.

It is evident from this formula that the outline of the structure will be a logarithmic line, practically straight at the top, and increasing in concavity as it approaches the bottom, giving what is called a "hollow batter." For ordinary chimneys 100 ft. high the amount of concavity required is not worth considering. For tall shafts, 300 ft. high and over, it is sometimes used. There is, however, this great advantage in using a straight batter instead of the theoretically correct hollow batter; viz., the accuracy of the construction can be detected at any stage at a glance of the eye without the aid of instruments. Yet it must be conceded that the hollow batter is much more shapely, and may be worth the extra expense of building.

In a chimney made of blocks of stone or brick separated by plain joints, where there is no lateral pressure, the conditions of stability are - 1st, that no joint shall be inclined to the horizon at a greater angle than that of repose, which in this case may be taken as 36 ½ and 2nd, that in any given bed-joint the centre of downward pressure, or point which is vertically below the centre of gravity of the super incumbent mass, shall not depart from the centre of figure of the joint more than a certain distance, which for round chimneys may be taken at ¼ the diameter of the joint, and for square chimneys 1/3 the length of the joint.

In order to be able to calculate the strains caused by the lateral pressure of the wind, we must first consider the manner in which the chimney will fail by this pressure. If the joints between the blocks of the material composing the structure had any tenacity such as the riveted or bolted joints of wrought or cast-iron, or of brick or stone held together by wrought-iron cramps, or by cement of a strength equal to that of the material it joins, the structure should be considered as one piece, and its strength determined by an investigation based on the theory of the strength of materials. But chimneys are usually made of brick or stone, the blocks of which, laid in mortar, touch each other at their joints, which are flat surfaces, held together by pressure and friction, but not by tension, so long as the mortar is fresh, and on this basis the stability ought to be considered. Even a year after mixture, the strength of good mortar is only about 50 lbs. per square inch, and a large proportion of failures of chimneys have occurred before the mortar has had time to set, which shows that the strength of the mortar should never be taken into account in designing a new chimney; but for old chimneys the strength of the mortar may also be considered, and taken at 8000 lbs. per square foot when not less than eighteen months old. In cases where chimneys have been sawn to restore them to the perpendicular, and the joints have not been properly remade with mortar or cement, the weight of the chimney can alone be depended upon for its stability.

In designing a new chimney, we may then disregard the tenacity of the mortar, and consider the chimney as being simply set upon its foundation and held down only by its own weight, upon which alone it is dependent for its stability. The moment of stability for a new chimney at any point is evidently half the diameter of the bed-joint, at this point, X the weight of the chimney above this joint, or W x B/2

For an old chimney, if we make the solid area in square feet at the joint = B, we have the moment of stability = (W + B, x 8000) B/2 .

The lateral pressure of the wind may be assumed to act horizontally, and to be of uniform intensity at all heights above the ground. The greatest intensity in this country, against a flat surface directly opposed to it, used to be taken by Rankine at 55 lbs. per square foot, but in 1868 the pressure of the wind at Liverpool was registered at nearly 80 lbs. per square foot, the highest ever known in this country. For new chimneys the pressure of the wind may still be taken at 55 lbs.

The inclination of the surfaces due to the batter or slope of the chimney is usually not sufficient to be taken into account in estimating the pressure of the wind against it.

A circular chimney may be considered as cylindrical in plan, and the total effect of the pressure against the side of a cylinder may be taken as being equal to one half the total pressure against its diametric plan, or against the side of a square chimney of equal diameter which for the strongest winds gives us 40 lbs. per square foot. This result is arrived at thus: Let A B = p, the force of the wind in a direction parallel to the diameter of the chimney. Resolving A B into its component parts at right angles, and with one of them BD , as a normal to the



them, B D, as a normal to the curve at the point B, we have B D as the measure of force exerting pressure towards the centre of the chimney, and BD= p sin L angle A B D. We have now to resolve this force again to get the component as measuring the effective pressure in a direction parallel with that of the wind, whence we have p sin2 angle ABD. Taking a number of points affected by the wind, the mean sine of the arcs will be about .75. The square of' .75 = .56, whence the mean effective pressure on the semi-circumference =p X .56.


The manner in which a chimney yields to the pressure of the wind is, however, by the opening or cracking of one of the bed joints at the windward side, without completely overturning. This opening gradually extends, in a more or less regular zigzag course diagonally downwards towards the lee side. The
complete destruction eventually takes place, either by the shifting of the upper portion past its support below, or by the crushing of the brick-work at the lee side by the too great pressure concentrated there, or in many cases, from both causes acting together, and in all cases the upper portion of the chimney falls to pieces inside and out, filling the interior of the portion left standing.

The resistance to the horizontal shifting of a bed joint is due to the friction of the horizontal faces of the blocks of stone or brick, and is called "frictional tenacity" whose amount at any given joint is the product of the vertical load on the joint into the coefficient of friction, which for masonry and brick-work, with damp mortar, is about 0.74.

The tendency of the wind pressure being to open the bed joint at the windward side, and to crush the material at the leeward side, or to overturn the structure above the joint in a plane parallel with the direction of the wind, it is evident that the centre of resistance of the structure will be moved towards the lee side.

It has been found by experience necessary to limit this deviation of the centre of resistance from the centre of figure, so that the maximum intensity of pressure at the leeward edge shall not exceed twice the mean intensity. Denoting by q the ratio which the distance of this deviation bears to the diameter of the joint j, we have for round chimneys q = ¼ the diameter of the joint. For square, use 1/3 the diameter.

The moment of stability of a chimney at any given bed joint is the product of the weight of the structure, or of the weight of the structure plus the tenacity of the mortar as the case may be, above that joint into the horizontal distance qj. If the axis of the chimney be vertical the limiting distance q j, for the centre of pressure will be the same in all directions.

But most chimneys are found to have their axes not quite vertical, and the least moment of stability is evidently that which resists the pressure in that direction towards which the axis of the chimney leans. In estimating the stability of existing chimneys this must be taken into account.

When we require to know the mean thickness of brickwork for a new chimney, the form and dimensions being given, we have the following tentative formulae, the results being multiplied by d/d-t for the actual thickness, when t is so found.

The outside diameter of the chimney at the ground line should not, as a rule, be less than one tenth the height. The batter varies from 1 in 60 to 1 in 10 : 1 in 24 is very common.

A chimney shaft is made up of a series of steps or courses, one above the other. Each step is of uniform thickness, but as we ascend, each succeeding step is thinner than that which it rests upon, so that the bed joints between the steps, where the thickness changes, have less stability than the intermediate bed joints, and it is only these former to which it is necessary to apply the formula in determining the strength and stability of the structure.

The height of the different steps of uniform thickness varies greatly according to the judgment and practice of the architect or builder, and the custom in different districts.

If we take the safe load that can be borne by good brickwork, at 20,000 lbs. per square foot, and the weight of brickwork with little mortar at the joint at 115 lbs. per cubic foot, we have 20,000/115 = 170, approximately, as the extreme height in feet, we should make any single division, or length, of uniform thickness. This length is very seldom approached even in the tallest shafts, as the brickwork has also to bear the weight due to force of wind acting against the opposite side of chimney, in addition to the chimney itself. The steps, or courses, should not exceed 90 feet in height, except in cases where the chimney shaft is inside a tower which protects it from the wind. In chimneys from 90 to 120 ft. high, the lengths vary from 17 to 25 ft., the top height being 1 brick thick; in chimneys from 130 to 150 ft., the lengths are from 25 to 35 ft.; in chimneys from 150 to 200 ft. the lengths are from 35 to 50 ft. the top length being 1 ½ brick thick, and in very tall chimneys, that is, from 200 to 300 ft., and over, the lengths vary from 50 to 90 ft., the thickness of top length being 1 ½ bricks. As the crushing strength of ordinary brickwork is much less than 90 tons to the square foot, the strength of the bricks used in very high structures should be ascertained by experiment.

The binding of the masonry [The bond usually adopted is 1 course of headers to 4 of stretchers. In circular chimneys a uniform bond for the outside course of brickwork is sometimes recklessly adopted] is often increased by laying at intervals hoop-irons, tarred and sanded, in the bed joints, the ends being turned down into the side joints. The length of the hoop-iron in each joint, should be twice the circumference of the chimney at that part.

Forge chimneys are often strengthened by strong bands of wrought iron, placed at intervals outside but these are not necessary for boiler chimneys. Sometimes strong hoops about 3 " x ½” are built in at intervals of 12 to 18 feet, to prevent cracks, but unless the chimney is provided with a very efficient lightning conductor, these masses of iron are apt to prove dangerous during a thunder-storm.

It is usual, and expedient, to protect the inside of the chimney with a lining of fire brick. For forge and ironworks' chimneys, where the gases escape at a very high temperature, the lining should be carried all the height. If care be taken to heat the chimney gradually for the first time of working, and subsequently, when it has been allowed to become cold, it is not necessary to have an air space between the firebrick lining and the shaft proper. In this case the lining may be included in the thickness of brickwork necessary for the strength and stability of the shaft. The firebrick lining may then be bonded into the other brickwork in the ordinary way, the thickness of the lining being ½ brick in the upper portion, and 1 brick in the lower portion, and should be laid in fire clay, and not in mortar like the other brickwork.

In boiler chimneys it is, however, unnecessary to carry the lining all the way to the top. In small chimneys, under 100 feet high, it may be carried one third, or one-half the way up, and in chimneys of great height, the lining need not be carried higher than from 50 to 80 feet, according to the height of the chimney shaft.

An arrangement often used, is to carry the lining up parallel, that is, without taper, and to let the outside shaft meet it at a very acute angle. This leaves an air space between the lining and the main body of the shaft, which should be provided with air holes, communicating with the external atmosphere, but carefully sealed from communication with the inside. This caution is necessary, because when the gases that pass through the flues have access to the air space round the lining there is always a risk of damage being done by the explosion of inflammable mixtures of gas and air that may collect. It is, therefore advisable, when an air space is provided, not to allow any communication between it and the inside flue, consequently the plan of leaving the air space open to flue at top, so often done, cannot be recommended.

It is, however, perhaps, best to dispense with the air space altogether, as the, difference between the temperature of the portions of the chimney above and below the top of the air space, renders the masonry liable to fracture at this part, and many chimneys may be found so fractured without the actual cause being suspected.

In erecting a chimney, care should be taken that the building is not proceeded with too rapidly. It is sometimes restricted to a rate not exceeding 6 feet a day in height. It is advisable to build chimneys when the work can be most steadily proceeded with. When the structure is built up too rapidly, and the mortar has not time to set, a gale of wind is liable to press the chimney over to one side, where it stays - the compressible nature of the mortar offering little or no resistance. Consequently, the less mortar used the better. Cement, owing to its crumbling when exposed to a high temperature, cannot be recommended except for the top of the chimney, where it may, however, be usefully employed. Grouting should, as a rule, not be attempted.

In order to gain admission into flues and chimneys many engineers and architects make doors in the sides or crown of the horizontal fines leading to the chimney. A much better plan, however, is to make an arched opening at the bottom of the chimney, or in the pedestal, at one side of, or opposite the flue entrance. This opening can be readily built up air tight. Such a provision in the chimney may be required for introducing a fire of wood and shavings, to
cause the boiler fire to draw on first lighting up, after the flues have been allowed to become cold and damp, especially when they descend to the chimney. This opening would be of service when it is advisable to make experiments on the draught, which are too often overlooked, but which may be of great service in detecting a falling off in the draught, and lead to the
detection of air leakages, and other defects that may operate strongly against the economical or efficient working of the boilers.

With respect to the best position for the dampers for regulating the draught, apart from all consideration of economy and efficiency, it is most convenient to have them. at the end of the external flues of the boilers, or between the boiler and the chimney. The dampers are therefore generally so placed, but this is the least advantageous position for preserving the temperature in the flues and chimney, and the energy of the draught, which would be best preserved at meal-times and over night, by placing the damper on the top of the chimney. There, however, it would not keep the flues under the boiler so warm as when placed in its usual position behind the boiler, and the effect would be felt in lighting up on Monday mornings. [SG note: Now there’s an eccentric suggestion!]

Dampers at the ashpit might be used with some advantage in maintaining the temperature of the boiler flues.

The depth and area of the foundation will depend very much upon the nature of the ground upon which the chimney is built. In many cases where chimneys are built on the banks of a river, and other places where the upper strata are of alluvial clays, soft silts and made ground, it is necessary to go to a depth of 25 or 30 feet, or even more, to reach a good stiff clay, hard sand or rock. This depth below the surface is excavated and filled in with concrete in various ways, or piled, according to the practice of the locality, or judgment of the engineer, so as to economise material without risking unequal settling of the structure, which cannot be too carefully guarded against, as it has often led to the failure of the chimney.

CHAPTER III.

DESCRIPTIONS OF CHIMNEYS.

The following account of some of the largest brick and stone chimneys that have been built may be off interest. The highest chimney is at Mr. Townshend's Works, Port Dundas, Glasgow; and it is, with the exception of the spire at Strasburg, the Great Pyramid, and the spire of St. Stephen's at Vienna, the loftiest building in the world. It is circular in section, and rises to a height of 451 feet from the ground. The foundations are laid 14 feet below the surface on a bed of stiff clay, mixed with pebbles, and consist first of six courses of hard brick on edge, covering a circular area of 47 feet in diameter, diminished by outward "scarcements " to a diameter of 44 ft. Over this solid substratum the foundation proper consists of twenty-six courses of brick also on edge, diminished by scarcements on both sides from 21' 9" to 8' 6" thick. At the surface the outside diameter is 32 ft., with walls 5' 3" thick (61 bricks). There is an inside lining of firebrick carried up 50 feet, with an air space round it. At the cope the outside diameter is 12" 8', with 14 inch walls, the total height of the building being 468 feet. The thickness diminishes by half a brick every step. There are six steps ; the first of 40 ft., then four of 80 ft., the top step being 91 ft. The outside batter is straight from bottom to top. The point of least stability is at top of second step from the ground.

In Glasgow there is also the celebrated chimney at the works of Messrs. Tennant and Co., St. Rollox. From the base of foundation to the top of chimney it measures 455 ½ feet. The section above ground consists of five steps, - 54, 60, 96, 140 and 85 ft. in height. Commencing from the ground the thickness of wall at ground is four bricks; and diminishing half a brick at each step, being two bricks thick at top. The outside diameter at ground level is 40 feet, and at summit 13' 6". The point of least stability is at top of third step. The foundation is 50 feet square, and 20 feet deep. The inner chimney is a cylinder 16 feet diam., and 260 ft. high. It is not connected with the outer one, but nearly touches it at the top. The weight of the chimney is estimated to be about 7,000 tons.

In Halifax, at Messrs. Crossley's, Dean Clough Mills, there is a large octagonal chimney of stone. Its height when built was 381 feet, but some of the top was removed on account of the immense weight at the foundation. The width at the bottom is 30 ft. Nearly 10,000 tons of brick and stone were used in the erection, being considerably more than the weight of Messrs. Townshend's chimney in Glasgow.

The tall brick chimney at the Edinburgh Gas Works is 341 ½ feet from the bottom of the foundation. The shaft is circular, 264 feet high, and built in steps of 35, 40, 48, 58 and 83 ft., commencing from the base, which is 3 ½ bricks thick, the highest being 1 brick. The inside diameter of the shaft is 20 ft. at base, and 11' 4" at top. The foundation and pedestal, which are square, are of stone, 77 ½ ft. high. The cost was nearly £5,000.

At Huddersfield there is a tall circular chimney at Messrs. Brooks's Fireclay Works. It measures from the foundation 321 ft., and rises 306 ft. clear above the ground. The base at the foundation is 36 ft. square. At the ground line it is 31 ft. and at the summit 11 ft. diameter. The flue is 14 ft. diam. at the bottom and 9 ft. at the top.

In Bradford, at Messrs. Mitchell Bros. factory, there, is an octagonal stone chimney, that rises 300 feet above the ground. The foundations consist of two courses of concrete 22 ft. and 21 ft. square by 12 inches thick each, resting upon the rock. It measures 20 ft. across at the foundation and 9 ft. at the summit.

At the West Cumberland Haematite Iron Works, there is a circular brick chimney, designed by Prof. Rankine, 251 feet above the ground, with 17 ft. foundation below. Inside diameter of basement is 18 ft 10 in. Inside diameter of the four circular archways for flues, 7 ft. 6 in. Outside diameter at top of cone, 15 ft. 3 in., and 1 ½ brick thick; outside diam. at 2 ft. above bottom, 25 ft. 7 in.; outside diam. of square basement, 30 ft. X 30 ft.; outside dimensions of foundation course, 31 ft. 6 in. ; outside dimensions of concrete, 31 ft. 6 in. sq. There are three steps, the two uppermost being 80 ft. each, whilst the bottom one is 88 ft. high. At summit the thickness is 1 ½ brick ; at bottom of lowest step, 2 ft. above the ground, 21 bricks, increasing by courses four bricks in order to spread the pressure. The cost was £1,560.

The well known chimney of the Shell Foundry at Woolwich Arsenal is 223 ft. 9 in. above the ground, with 16 ft. of brickwork below, making, 239 ft. 9 in. above the bed of concrete. The base above the ground is 20 ft. square, with plinth and cornice, 27 ft. high, on which the octagonal shaft is erected. It is 16 ft. 9 in. diam. at base, and 6 ft. 6 in. at top. The walls are 2 ft. 7 ½ in. thick at bottom of shaft, and are reduced by steps of 37 ½ , 4 ½ in., the top step of 26 ft. being 9 in. thick. The uppermost 9 ft. is bell-mouthed, and built in cement. The Portland stone cap weighs about 17 tons.

Perhaps the tallest square chimney in this country is that at the Camperdown Linen Works, near Dundee. This chimney, or rather tower, rises 282 feet from the ground. It is somewhat in the Italian style, and is built of variegated brick. At the summit of the tower, which is 230 ft. high, constructed in panels, there is a balcony, above which the shaft is octagonal. The thickness of the walls at the ground is 5 ft., at the balcony 2 ft. 6 in., and at the top 18 inches. This tower is quite distinct from the chimney proper, which is circular in form, 14 ft. 6 in. diam. at base, and 13 ft. 3 in. at top. Its thickness is 18 in. from ground to first panel; 14 in. from this to the balcony, and 9 in. from balcony to summit. The tower is 24 ft. 6 in. sq. at the ground, and 20 ft. sq. at the balcony, above which it tapers gradually to the summit. The weight is about 5,000 tons, and the cost is said to have been about £6,000.

At Messrs. Lister's, Manningham Mills, Bradford, there is a lofty square chimney, 249 ft. high, with panelled sides, and circular top. The inside of the square at ground is 10 ft., gradually increasing to 11 ft.

At Connah's Quay, Chester, there is another lofty square chimney, 245 feet high from the ground. The size inside is 17 ft. 6 in. at base, and 7 ft. at top. Its cost is said to have been but little over £2,000.

Besides the circular, octagonal, and square forms generally used, there are a few peculiar shapes sometimes met with.

With a view to get a parallel flue from bottom to top without using an inside shaft, the walls of a square chimney may be carried up in one thickness about 150 ft. high, and 14 inches thick. In order to obtain stability two buttresses are run up at each side, tapering to nothing at the summit. There is a chimney of this kind at the South Metropolitan Gas Works.

Some chimneys have been built, having a section like an eight-pointed star, the inside being octagonal or circular, with or without inside lining, or air space.

Iron chimneys are seldom used except for small boilers, and are usually of small dimensions, and made circular. A few, however, have been erected within the last ten years of considerable dimensions. There are two at the Creusot Works in France. One is 197 ft. high, 4 ft. 3 in. dia. at top, and 10 ft. at bottom. It is constructed with a hollow batter, and is firmly bolted to masonry work just clear of the ground. It was riveted together horizontally, and lifted into its place with a crane. The thickness of the plates is 3/32in. at top, and 7/16in. at bottom. Its weight is 28 tons.

There is a still larger chimney at Creusot, of which a description will be found in " Engineering," vol. xiii. It is 279 feet high, 7 ft. 6 ½ in. dia. at the top, 22 ft. 11 ½ in. at the bottom, and weighs 80 tons. It is built also with a hollow batter, and is held by bolts and a strong angle-iron ring to a mass of masonry weighing about 300 tons. Its cost was about £1,600.

The experience in such chimneys is not yet sufficient to enable us to judge with any degree of certainty of their durability. It is, however, certain that with some kinds of fuel, in spite of careful painting and partial lining with brickwork, their destruction will be very rapid, and the adoption of iron chimneys can only be recommended where the cost of brick and stone precludes their adoption.

A brick or stone chimney, substantially built, and with a fair margin of stability, will last many generations, whilst an iron chimney of moderate thickness cannot be depended upon, in many cases, to last more than forty or fifty years.

THE SHAPE OF CHIMNEY TOPS.

Although the position and surroundings of the furnaces, with respect to the direction of the wind, have a very decided influence upon the draught, and may outweigh any small advantage that one chimney top may have over another, yet it must be admitted that the shape of the top has a perceptible influence in promoting or retarding the draught when a strong wind is blowing

Some chimneys, which are not excessively wide, have a better draught in all high winds, which may be accounted for by the arrangement of the boiler-house, its surroundings producing a greater pressure of air at the furnaces when a gale is blowing. In some arrangements a gale from a certain quarter may have an exhausting action, tending to draw the air from the furnaces, whilst a gale from the opposite quarter tends to increase the pressure at the furnaces, and so improves the draught.

With a plain or “bluff " top, a strong wind may act partially as a damper, as may sometimes be seen by the action it has upon the column of smoke as it issues from the chimney. The wind not only flattens the escaping column, but it also tends to produce downward eddies, especially in very large chimneys. There is often evidence of the downward eddies on the outside in the blackened appearance of the masonry on the lee side of the chimney. But as the wind, striking against a bluff top tends to rise vertically in the first place, and clear the windward side of the chimney at a short distance above it, this shape is decidedly better than that of some of the tops so often seen, which are concave at the rim and convex at the orifice, and appear to be designed to guide the wind right into the chimney and check the draught. A better shape for an open top is concave towards the orifice, so as to give the wind an upward direction, whereby the tendency will be rather to promote than check the draught.

When there is both an inner and outer shaft, the design for deflecting the current of air upwards can be carried out to the greatest advantage. The top of the inner shaft should be stopped off a few feet below that of the outside shaft and surmounted by a concave deflecting cap, from which the currents of air admitted through suitable openings in the outer shaft are deflected upwards and not only prevent any downward eddying, but tend to induce an exhausting action in the inner shaft, and consequently to promote the draught. At the same time the top of the outside chimney should be surmounted with a concave deflecting cap.

It appears strange that in the endless variety of designs for chimney caps, in the case of a single shaft, advantage has never or very seldom been taken of the opportunity to make the cap hollow, and in such a manner as to cause an induced current upwards on the same principle as that just mentioned for a double shaft.

Some very shapely chimney caps are made concave below and convex above; were they made concave both above and below they would have the best form for splitting the current of wind, and so prevent it from interfering with the draught

Covered tops of a pyramidal shape, having vertical, tapered openings at the corners, have been used with decided advantage. By this arrangement the wind can blow into only one or at most two of the openings at once, leaving the others free to discharge fully. The sum of the areas of the openings should in this case be considered as the size of the orifice of the chimney. When practicable, the openings should be so arranged that when the wind is blowing from the chimney to the furnaces, it does not tend to blow down any of the openings, or, in other words, one side of the top should face the furnaces when these are behind the chimney.

The cost of chimneys varies within very wide limits. A few years ago chimneys up to 90 feet high could be built in the Midland Counties in a certain style for £1 per foot, but a more usual cost is from £2 to £2 10s. per foot, for chimneys up to 100 feet high. As much as £22 a foot has been paid for some of the ornamental tall towers with inside shafts.

CHAPTER IV.

CHIMNEYS - LIGHTNING CONDUCTORS.

THERE are many engineers at the present time who argue that lightning conductors are useless, or even worse than useless ; hence the number of tall chimneys seen unprovided with lightning rods in various parts of the country.

The destructive effects of lightning are much more frequent and ruinous than is generally supposed. Whilst some twenty cases could be quoted where lightning has fallen on unprotected powder magazines, and caused their explosion, killing thousands of people, and laying whole towns in ruins, it may be questioned whether a single case can be cited of a powder magazine being struck that was properly protected by a lightning conductor.

The causes of the widespread disbelief in conductors as a means of preserving chimneys and other lofty buildings against the destructive action of lightning are :

1. The opinion commonly held, and often where we should least expect to find it, viz., that metallic bodies, especially when pointed, attract lightning and are therefore dangerous. This opinion is probably due to the fact that during thunderstorms luminous points have often been seen on spires, vanes, ship's masts, and other elevated metallic bodies. The glowing appearance here spoken of is unattended by any heating effects, and is harmless. This phenomenon, like some other effects of atmospheric electricity, is due to the highly charged electrical condition of the clouds and atmosphere, and it is at once concluded that these bodies have a superior attractive force for electricity over all others. Now metallic bodies, whether pointed or not, have no more power of attracting or drawing the lightning to them than non-metallic bodies, and it is the confusing of the apparent with the actual attractive force, or erroneously concluding that metals are good attracters because they are good conductors, that has brought about the misunderstanding on this point. Now in no case can it be said that the conductor attracts the lightning in the active and adverse sense which is here implied, and in which this term is often used. On the other hand, the conductor acts, especially when its top is pointed, in preventing the prominence to which it is applied from becoming highly electrified by induction, and in so much actually prevents the structure from attracting the cloud that electrifies it. On an electrified cloud passing over a pointed conductor, the opposite and induced electricity of the earth is discharged from the point of the conductor, and the cloud and air are often thereby neutralised without producing lightning at all. But when a discharge does take place the duty of the conductor is entirely passive: by offering a line of comparatively small resistance, it determines the direction of the discharge, which is not, however, in the first place brought about by the presence of the conductor, or, what more often happens, the presence of the uninsulated pointed conductor, by its peculiar property, prepares the resisting air in such a manner that the current of electricity is discharged quietly and without violence or a flash of lightning. Should, however, the electrified clouds be driven to the erection by the winds in such masses that the opposite kind of electricity does not stream away from the point of the conductor in sufficient quantities to prevent a spark from passing, the spark, or flash of lightning, will pass from the cloud to the conductor in preference to any neighbouring point, since the electric density will be greater here, and the resistance least. Hence the duty of the conductor may be considered as being entirely passive, as, by offering a line of comparatively small resistance, it determines the direction of the discharge when it becomes inevitable, although it is not brought about by the presence of the conductor in the first place, but by the action of the clouds and a large area of ground. To use an oft repeated simile, the conductor no more attracts the lightning than a water-spout, on the side of a house, attracts the rain from the clouds which it leads to the drain in the ground.

The fact that so many well-known buildings, which were repeatedly struck by lightning before being furnished with rods, have escaped being struck after the lightning rods were applied, would appear to be conclusive evidence of the passive character of the pointed conductor with respect to the discharge, and that its presence averts a violent explosion by rapidly neutralising the electrical condition of the atmosphere.

The luminous appearance, accompanied by a whizzing noise, sometimes observed when a very dense discharge is received by the conductor, is of a perfectly harmless character, and is probably of the same nature as the "glow" discharge, so well known to those who have made and witnessed electrical experiments.

The disbelief in the efficacy of lightning conductors is sometimes due to the carelessly expressed opinions of many writers on electricity, to the effect that thoroughly efficient lightning conductors might discharge the electricity gradually and harmlessly into the ground, but would be a poor protection to the building in the event of its being struck by a flash. Now, in answer to this, there are many cases on record of ships and buildings having been struck by lightning. Those provided with efficient conductors have borne the shock unharmed, whilst those unprotected have suffered severely. Perhaps the most convincing evidence of the efficiency of good lightning conductors is that adduced by Sir W. S. Harris from the journals of H. M. ships. In 1861, he writes, "We had between the years 1810 and 1815, that is, within about five years, no less than 40 sail of the line, 20 frigates and 12 sloops and corvettes' placed hors de combat by lightning. In 250 such cases, 100 seamen were killed and 250, at least, severely hurt. In the merchant navy, within a comparatively small number of years, no less than 34 ships, most of them large vessels, with valuable cargoes, have been totally destroyed, being either burnt or sunk, to say nothing of a host of vessels partially destroyed or severely damaged. Damage to H. M's ships by lightning has happily ceased (since effective conductors were applied) ; it is now not known in the British navy." Damage to ships by lightning seldom occurs now, as most ships are fitted with wire ropes, which act as lightning conductors.

A more reasonable objection, at first sight, to the use of conductors, is that many buildings have been damaged in spite of the presence of lightning rods,
and when it is assumed that a conductor acts by attracting the lightning, which would not take place but for the conductor's presence, the doubt at once arises whether the amount of security afforded by the rod really outweighs the danger provoked by its supposed active influence in attracting and bringing down a large flash of lightning. In all cases where the matter has been properly investigated, it has been found that the conductors have been ignorantly and wrongly applied. Either the continuity of the conductor between its termination and the earth has been broken by the presence of rust at the joints, or by the iron connections under ground rusting away; by the rod itself being broken; or by the too common careless or ignorant mode of not bringing the end of the conductor properly to earth. It is the opinion of many that if the rod is merely buried a foot or two in the ground it is all that is required. We shall presently see this is by no means sufficient.

There is an opinion widely spread, and due in a great measure to the use of the terms "thunderbolt" and " electric fluid," by many writers on electricity, that a small rod of copper, from 12 inch to 1 inch diameter, is totally inadequate to carry off such a large quantity of "fluid " sufficiently rapidly and safely as is supposed to exist when a largo flash of lightning is observed.

Many persons point to buildings, such as the dome of St. Paul's cathedral, as not being provided with any external special conductor, yet which have escaped being struck in very severe storms. In most of these cases, by the arrangement of the materials of which it is constructed, the building itself is a first-rate conductor, in some cases for a certain height only and in others from the summit to the ground. St. Paul's is now fitted with copper rope conductors.

In applying a lightning conductor to a chimney or other similar structure, the principles to be kept in view are:

1. To use the best available material ; that is, which acts best as a conductor of electricity, and resists corrosion. This material is copper.

2. To provide an adequate sectional area to lead the electricity harmlessly a-way. This is best arrived at by experience. Harris, in 1861, after citing a number of cases of terrific tropical thunderstorms, concludes that a copper rod 3 in. diameter, or an equal quantity of copper under any other form, would resist the heating effect of any discharge of lightning which has yet come within the experience of mankind. Faraday considered a ½ in. copper rod sufficient, but of course approved of using 5/8 or 3/4 in. rods when the
expense was not too great.
3. It should be made in such a manner as to run the least risk of having its continuity interrupted in the event of its being fractured. With this object in view, rope is better than rods, since it can be made in the first place in one continuous length, whereby the risk of badly formed joints is avoided ; it can be readily coiled and carried to its destination without being cut; and, in the event of being roughly used, the breakage of one or more strands does not destroy the efficiency of the remainder. When joints are used they should be formed by screwing the ends right and left-handed, and bringing them in close contact by a screwed copper socket of ample strength. The conductor should be supported or suspended in such a manner as not to risk its fracture by settlement of the structure or disturbance of one or more of its supports or guides.

4. The upper extremity should project above the top of the chimney to a distance, say, equal to the diameter of the chimney top, and should terminate in a brush of three or four points arranged round the central terminal, and curved to project therefrom at an angle of about 45'.

5. As a rule the rod should be placed inside the structure in the case of a monument, where it is less liable to be damaged, and is in a better general position for protecting the building, besides being out of sight. But for a chimney the rod should always be outside, as the gases from some coals are liable to corrode copper and iron rapidly wherever they come in contact with these metals, especially in the presence of moisture.

6. To prevent lateral discharge the conductor should be in communication with all hoops or pieces of metal round the chimney, and the use of all insulated pieces of metal, especially arranged parallel to the conductor, should be avoided.

7. The rod should terminate in the ground in two or more branches, which should be carried into a well, and terminate in a large copper plate, or be connected with a water drain (made of metal, and not fireclay), or pump, water or gas pipes, or any other good conducting channel. Where this is not practicable, the several branches should be carried into earth that is permanently moist, and end in a cast-iron case filled with coke, or cinders ; or have a large copper plate terminal ; or where no moist earth exists permanently the branches under ground should have plenty of length, say 30 feet or more, according to the nature of the ground and size of rod. If no earth-plate is used, the wires of the copper rope should be unstranded and spread out.

When placed outside the chimney the rod may be brought down in contact with the stone or brick-work, and no insulating means are required. A flash of lightning has sufficient intensity to break through miles of air in some cases, hence an attempt at insulation a few inches or even feet in length can have no practical effect in preventing it from striking the brickwork in case the rod should prove insufficient to carry it safely away.

The earth terminal of the rod - unlike the rod itself - should expose as much surface to the soil as possible, because this surface is the measure of the section of solid earth employed to carry off the discharge. Many authorities have advocated the use of a ball instead of a point at the top of the rod, the former being considered best for attracting the lightning from the building; a point is, however, far better for drawing off the electricity, as it does so quietly, and without sparks, and would commence its neutralising effect long before a ball would act, and by so doing might be the means of preventing a violent discharge altogether. In considering the advantages claimed for the ball top, it must not be forgotten that in comparison with the vast area of most electrified thunder clouds, the largest ball, the use of which could be contemplated, must be, after all, a mere point. Although many cases of fusion of copper points are recorded, this material should be still used in preference to iron for the points of conductors, although iron has a higher point of fusion.

All combinations of copper and iron in contact should be avoided, especially, when the smaller part of the combination is of iron, to avoid the rapid destruction of the iron by galvanic action, which is but too likely to occur; for instance, iron nails, spikes, staples, &c., should not be brought in contact with the copper rod to support it.

Notwithstanding that water or gas pipes of iron, placed a few feet from a conductor where it reaches the ground, have been broken by the lightning springing to them from the conductor, it yet appears to be the safest plan to connect the conductor with such pipes. In some cases the breakage has been doubtless due to explosion of the steam formed by the intense heat of the lightning current on meeting with the comparatively greater resistance of the ground between the conductor and the pipes. Where there is no resistance offered to the passage of the current no heat or violent effect will ensue.

In a previous paragraph we have spoken of the importance of terminating the rod in "good earth." Upon this depends the value of the lightning rod. Most of the accidents which have taken place - where conductors have been on the building - can be accounted for by insufficient earth connection. A lightning rod when fixed should be tested by a galvanometer, and the earth should be tested at least once a year.

A very portable galvanometer for testing lightning conductors has recently been introduced by Mr. Richard Anderson, M.S.T.E., F.C.S., of 101, Leadenhall Street, London.

The following woodcut shows the arrangement of the battery, galvanometer, and resistance coils. The battery consists of three cells, and is a modification of the old manganese cell, in which the carbon and oxide of manganese occupy the outer and the zinc plate the inner or porous cell. By this arrangement (introduced a few years ago by Mr. H. Yeats, of Covent Garden), the surface of the negative element is greatly increased, and hence a more constant current is obtained, on account of the battery not polarising so rapidly as in the old form. Another advantage of this arrangement is that the cells can be almost entirely sealed up, the air openings being made within the porous cell. In the centre of the lid of the box is placed the galvanometer with a tangent scale. On the left are two terminals by which to connect the conductor to be examined. On the right hand end of the lid are placed five keys, marked respectively, L, B, 1, 2, 3. Under B is one pole of the battery, so that by depressing this key, as will be seen by following the connections in the diagram, the battery current is sent through the galvanometer direct. If, however, we depress key No. 1, we connect the battery with the galvanometer through a known resistance. Key No. 2 has a larger resistance, and. No. 3 still larger. The fifth key, L, closes the circuit within the limit of the instrument, but on being depressed opens it and includes the line or conductor placed between the two terminals at the other end. On pressing down L and B, it will be seen that the resistance of the line or conductor may be compared with the known resistance connected with any of the keys Nos. 1, 2, 3, or any of these resistances may be included with that of the line, so as to get a convenient deflection of the galvanometer needle.

In the case with the battery is a bobbin of insulated wire for connecting the instrument with the conductor and earth to be tested.

Transcribed by SCG/14 January 2007