with a central column, a prodigious weight superimposed. It is needless to say that, in such instances, the strongest material was necessary, and we always find it so employed. So in the columns, or rather pillars, of the naves in such edifices, the greatest care was usually taken to select the hardest stone. Generally speaking, the thickness of walls and piers should be proportioned rather to their height than to the weight they are to bear. hence often the employment of a better material, though more costly, is in truth the most economical. 1502. TABLE OF THE WEIGHT REQUIRED TO CRUSH CUBES OF STONE. 1502a. In the above list B stands for Bramah, and C for the Commissioners' Report, &c. It is of very great importance to notice that the size of the cubes experimented upon by the latter, was only two inches; those by Rennie were only one and a half inch cubes. A set of experiments on Portland stone, of the weight sustained up to the point of fracture, i.c. the crushing weight, by accurately cut cubes of two inch faces placed between perfectly smooth lead surfaces, were carried out with the well-known AmericaL mechanical testing machine, by Mr. Abel (Builder, 1863, p. 860):— 14,795-8 lbs. He also observes that "no definite conclusion can be drawn from the comparative properties of the specimens of stone from one and the same locality, quarried at different periods of time, regarding the influence exerted by exposure, after quarrying, upon the quality of the stone. On the whole, the evidence may be considered as a little in favour of the opinion that an improvement in the strength of the stone is effected, to some extent, by seasoning." 1502b. A very instructive set of experiments on the strength of Portland stone (brown bed), a material now so greatly employed in building, was made by a committee of the Institute, above-mentioned. 1502c. C. H. Smith has observed ( Transactions of the Institute of British Architects, 1860, page 174.), that "the stone which possesses the least cohesive strength, or that which will crush with less pressure than any other, is nevertheless strong enough, when well fixed in a building, for almost all practical purposes. No architectural members have to sustain greater pressure, in proportion to their size, than mullions of large Gothic windows. The tracery in the great north window of Westminster Hall is now executed in Bath stone, which is remarkable for having the least cohesive strength of all the specimens described as experimented upon in the Report on Stone, &c. Some of the mullions of that window are less than nine inches wide and more than forty feet high, sustaining not only their own weight, but also that of the whole of the tracery beneath the arch. The eastern window of Carlisle Cathedral, built with a friable red sandstone, is fifty feet high, the mullions are smaller, and the tracery much heavier than in that at Westminster, yet in neither of these examples are there any symptoms of crushing. The cohesive strength of stones is never more severely tested than during their conversion by workmen from the rough state to being fixed in their final situation in a building. During these operations, iron levers, jacks, lewises, and various other implements are applied. frequently with but little regard for the mechanical violence which a stone will safely bear; and it may, there fore, be considered a useful practical rule, that, however soft a stone 14ay be, if it resist the liability of damage until out of the masons' hands, there can be little doubt of its possessing sufficient cohesive strength for any kind of architectural work. If the foundation be insufficient, or any part of the edifice give way, so as to cause an unfair or unequal pressure, a soft stone will, of course, yield sooner than a hard one." 1502d."Unfortunately," writes Warr, Dynamics, 1851, "those experimental results which we possess were obtained without attention to the fact that the specimens should be of a certain height to show a proper compressive strength. The bulk of the examples are with cubes, a fault excusable with those experimenters who made their work public before those peculiarities were well known, but the same cannot be said of the investigations conducted by the Commissioners; these experiments, executed with singular minuteness on some points, would have been useful, from their variety and specification of the localities, but they were made on (2-inch) cubes, at a period when the laws of fracture were as public as at present, and are therefore of limited value." 1502e. Hodgkinson (Phil. Trans., 1840, p. 385), found that in small columns of one inch to one and three-quarters inch square, and from one to forty inches long, a great falling off occurred when the height was greater than twelve times the side of the base. Thus, when the length was 12 times the size of the base, the strength was 138 a little less 96 75 52 He also found that with pillars shorter than thirty times the thickness, fracture occurred by one of the ends failing, and as the longer columns deflected more than the shorter, they presented less of the base to resist the pressure, and therefore more readily gave way. Thus the practical view from these experiments points out an increase of area at the ends as being most economical, and that in proportion to the middle as 13,766 to 9,595 nearly. From the experiments it would appear that the Grecian columns, which seldom had their length more than about ten times the diameter, were nearly of the form capable of bearing the greatest weight when their shafts were uniform; and that columns, tapering from the bottom to the top, were only capable of bearing weights due to the smallest part of their section, though the larger end might serve to prevent lateral thrust. This last remark applies, too, to the Egyptian columns, the strength of the column being only that of the smallest part of the section. (British Association for the Advancement of Science, 15th Report, 1845, p. 27.) 1502f. It might be asked, how does this apply to those small shafts or colonettes so freely used with piers in pointed architecture, and which are generally in height upwards of thirty times their diameter. We would refer the student to the paragraph 1502c., respecting the mullions in windows, and to the circumstance that the small shafts are not pinned-in to the work, but are left free, so that they only apparently carry the weight imposed on their capitals. Where no attention has been paid to this necessary precaution, in modern work, the shaft has fractured when of soft, or shaky, stone. 1502g. TABLE OF THE STRENGTH OF SHAFTS 12 INCHES LONG, 3 INCHES DIAMETER, (Being experiments made by a committee of the Institute, as above-mentioned.) 1502h. Fairbairn, in a paper read at the Manchester Philosophical Society, and given in vol. xiv. of the Proceedings; and also in his Useful Information, &c., 2nd Series, has detailed the following results of his researches: 1502i. He further shows that the resistance of strong sandstone to crushing in a direc tion parallel to the layers, is only six-sevenths of the resistance to crushing in a direction perpendicular to the layers. The hardest stones alone give way to crushing at once, with. out previous warning. All others begin to crack or split under a load less than that which finally crushes them, in a proportion which ranges from a fraction little less than unity in the harder stones, down to about one half in the softest. The mode in which stone gives way to a crushing load is in general by shearing. The factor of safety in structures of stone should not be less than eight, in order to provide for variations in the strength of the material, as well as for other contingencies. In some structures which have stood it is less; but there can be no doubt that these err on the side of boldness, as urged by Rankine, Civil Engineering, page 361. A brick made by Beale's machine being placed on bearers seven inches apart, was broken in the middle by the weight of 2,625 lbs. A common hand-made brick was broken by 645 lbs. The hollow or frog formed in the underside of a brick necessarily lessens its resisting power. Young (Nat. Phil.) states that the cohesive strength of a square inch of brick is 300 lbs., but the quality is not stated. Other experiments give the following strength of bricks: Used at Edinburgh Gas Works, of fire clay and iron stone, 400 O 15021. Brickwork.—Brick piers 9 inches square, 2 feet 3 inches high, made of good sound Cowley stocks, set in cement, and proved two days afterwards : Cracked at · 25 tons Broke at 30 tons 30 " 35 Brick flat, compressed quarter of an inch Brick on edge, did not compress 1502m. Mr. L. Clarke's experiments for the works at the Britannia and Conway tubular bridges, on brickwork in cubes, showed that— 9 inches, cemented, No. 1 or best quality, set between deal boards, weighing 54 lbs., crushed with 19 tons 18 cwt. 2 qrs. 22 lbs. 9 inches, No. 1, set in cement, weighing 53 lbs., crushed with 22 tons 3 cwt. O qrs. 17 lbs. 551.3 lbs. per square inch. = 612.7 lbs. = 454 3 lbs. per square inch. 9 inches, No. 3, set in cement, weighing 52 lbs, crushed with 9 inches, No. 4, set between boards, weighing 54 lbs. crushed Mean 568.5 lbs. 417-0 lbs. The three last cubes continued to support the weight, although cracked in all directions; they fell to pieces when the load was removed. All began to show irregular cracks a considerable time before it gave way. The average weight supported by these bricks was 38-5 tons per square foot. equal to a column 583-69 feet high of such brickwork. (Fair. bairn. Application, &c., page 192) 1502a. To crush a mass of solid brickwork 1 foot square, requires 300,000 lbs. avoirdupois, or 134 tons 7 cwt. 15020. Besides compression, stone is subject to detrusion and a transverse strain, as when used in a lintel. Of these strengths in stone little is officially known, but we are perfectly aware of the danger of using any kind of stone for beams where there is much chance of serious or of irregular pressures. Its weakness in respect to this strain is manifest from all experimental evidence concerning it. Gauthey states the value of a constant S, for hard limestone = 78 lbs; for soft limestone = 69 lbs. Hodgkinson, taking the power of resisting a crushing force as = 1000, notices Tensile Transverse strain. 1502p. The danger above noticed is so great, that it becomes essentially necessary in all rough rubble work to insert over an opening either an iron or timber lintel, or a brick or stone arch, to carry the superincumbent weight, and thus prevent any pressure upon the stone. This must be done more especially when beams or lintels of soft stone are used; the harder stones, as Portland, may in ashlar work support themselves without much danger. In rubble masonry, the stone arch may be shown without hesitation in the face of the work; and also in domestic architecture, the brick arch may exhibit itself in the facework if thought desirable. Portland stone has been constantly used to extend over a comparatively wide opening. All blocks set upon it should have a clear bed along the middle of its length. Thus cills to windows should always be set with clear beds, or, as the new work settles, they are certain to be broken. Lintels over even small openings worked in Bath or some of the softer stones, are very likely to crack across by very slight settlements, especially when supported in their length by a mullion or small pier, as is often introduced. We need hardly add that where impact or collision is likely to occur, no lintel of stone should be used. 1502q. Marble mantles may sometimes be seen to have become bent by their own weight. Beams of marble have been employed in Grecian temples as much as 18 feet in the clear in the propylæa at Athens; and marble beams 2 feet wide and 13 inches deep were hollowed out, leaving 42 inches thickness at the sides and 3 inches at the bottom; these beams were about 13 feet in the clear in the north portico of the temple at Bassæ near Phigaleia. 1502r. The cohesive power of stone is seldom tested. The subject of crushing weights, or the compression of timber and metals, will be treated in a subsequent section (1631e. et seq.); and the strength of some other materials will be given in the chapter MATERIALS. OF THE STABILITY OF WALLS. 1503. In the construction of edifices there are three degrees of stability assignable to walls. I. One of undoubted stability; II. A mean between the last; and the III. The least thickness which they ought to possess. We are, 1504. The first case is that in which from many examples we find the thickness equal to one eighth part of the height: a mean stability is obtained when the thickness is one tenth part of the height; and the minimum of stability when one twelfth of its height. however, to recollect that in most buildings one wall becomes connected with another, so that stability may be obtained by considering them otherwise than as independent walls. 1505. That some idea may be formed of the difference between a wall entirely isolated and one connected with one or two others at right angles, we here give figs. 591, 592, and 593. It is obvious that in the first case (fig. 591.). a wall acted upon by the horizontal force MN, will have no resistance but from the breadth of its base; that in the second |