Page:An Encyclopædia of Cottage, Farm, and Villa Architecture and Furniture.djvu/1131

 ARCHITECTURAL FITNESS. 1107 resolution of forces, by which every point relative to the strength of a building may be tested. This problem was first given to the English reader by Emerson, in his Mechanics, and will be found familiarly explained to the young Architect in Tredgold's Carpentry, Gwilt's Rudiments, and other v^-orks. The practical object of the problem is, to enable the builder to determine, with ease and absolute certaintj', what parts of con- struction, whether of timber, stone, iron, or walling, act as ties, or binding parts; and what as struts, or parts supporting weight ; and what is the proportion or amount of tension or pressure on each. ]Iuch of the strength of all modern buildings, of several stories, depends on the walls being tied together by the different floors, and by the roof. De- prive the walls of the greater number of the dwelling-houses in the principal towns in Britain of these sources of strength, and they would be blown down by the first storm of wind which occurred, notwithstanding the durable nature of their materials. The durability of a building, no doubt, depends, in part, on its construction : but, essentially and principally, it depends on the durability of the materials of which it is composed ; and the tests for ascertaining this durability are derived from the principles of chemistry, and chiefly from the law of attraction. A very strong construction may be formed of timber ; but the duration of such an edifice could never be put in competition with that of one t)uilt of bricks or stones, united by mortar or cement. From the preceding observations on strength and durability, are drawn the following well known practical precepts : viz., to have solid and secure foundations ; to use materials of the best quality : to dispose of them in a manner suitable to their natures ; and, to employ only the best workmanship. The following observations are illustrative of some of these precepts. 2182. The Strength of Building Materials, such as timber, brick, stone, iron, &c., may readily be ascertained by subjecting them to pressure, tension, and torsion, by mechanical means, or by loading them with weights. 2183. The Durability of Building Materials is more commonly ascertained from the previous experience of builders, than by experiinents made on scientific principles; never- theless, there are some geological facts, relative to the stratification of rocks, which ought to influence the builder as to the position in which stones taken from such rocks ought to be placed in walls ; and there are some chemical facts, relative to the decomposition of the integral particles of stone, from which conclusions may be drawn as to its durability, previously to its being applied. The stones, or earths, which, when reduced to powder, are made into mortars, or cements, may always be subjected to experiments, so as to prove their value before employing them. With regard to timber, there are compositions which have been applied to it for the purpose of increasing its natural durability, and others for rendering it less destructible by fire. 2184. All stratified Stones used in Walls should have the plane of their layers or laminae declining towards the outside of the wall ; or, at all events, placed horizontally. The object of this position is, to present the edges of the lamina to the action of the weather, and not their broad surfaces. There are some exceptions to this rule ; but they are so few as not to require notice. When the face of the laminte forms the surface of the wall, the water which runs down it after rains, sinks into the interstice between the outside lamina and the next to it ; and in time, with the aid of frost, causes the outside surface to scale off: and this process many times repeated, will at last so far destroy the stone, as to undermine the superincumbent parts of the wall. On the other hand, when the edges of the lamina are presented to the action of the weather horizontally, and rather declining outwards, though the rain-water will still enter between them, yet, as it cannot penetrate far horizontally, the injury done by it will be comparatively trifling. 2185. Brand's Test for ascertaining the probable Action of Weather on Building Stone is as follows : — Boil two one-inch cubes of the stone to be tried, in a solution of sulphate of soda saturated at a common temperature, for half an hour : then expose the cubes to the air for evaporation. The salt crystallises, and has the effect of freezing on the stone. Then dip the stone in the cold solution until the crystals fall. After this, expose the stones to the air. This experiment repeated, during five days, will produce the same effect on the stone which exposure to the open air would do in many years. {Lit. Gaz. for 1829, p. 6.33.) 2186. The Art of mixing Earths so as to form Mortars which will set, or solidify', either by themselves, or in conjunction with stones or bricks, can only be scientifically understood through some knowledge of chemistry. All lime mortars depend for their strength on their quality of absorbing carbonic acid gas and water, and solidifying them. All cements, or rapidly solidifying mortars, though they depend for their strength on the same qualities as lime mortar, owe their power of rapid solidification to the presence of some metallic oxide, the value of which principally results from its capacity for absorbing oxygen. 2187. Tlie Solidity rf Walls depends on their homogeneousness, on the J)o^ition of