ON ARCHITECTURE AND CARPENTRY. 123 



it at different parts of its height, is usually much varied by the weight of 

 the floors and of the contents of the apartments. But sometimes the 

 obliquity of the surface of the wall may become inconvenient, by promoting 

 the growth of moss and weeds. In building a wall, the first precaution 

 that is required, is to dig deep enough to ascertain the nature of the ground ; 

 the next, to lay a sufficiently extensive and firm foundation ; and it has 

 been very properly recommended that where a well is wanted, it should be 

 dug before the foundations of the house are laid, in order to examine the 

 qualities of the different strata which are to support them. The disposition 

 of the stones or bricks, is not a matter of indifference ; the strength is obvi- 

 ously greatest when all the surfaces are either horizontal or vertical ; for if 

 they are oblique, they must have a tendency to slide away laterally, and the 

 wall must be very liable to crack : hence the reticulated walls, sometimes 

 employed by the ancients, of which all the joints were oblique, possessed 

 but little durability. If the materials are thrown together without order, 

 they press on the parts in contact with them ; but occasionally, as in the 

 case of piers or quays, this circumstance may be of some advantage in 

 opposing external pressure ; or at least the effect of such a pressure may 

 remove the inconvenience which would otherwise arise from the irregularity 

 of the structure. 



In some cases it is necessary to unite the stones of a building mechani- 

 cally, either by cramps of iron, fixed by means of melted lead, or by other 

 methods, similar to those which are more usually employed in carpentry. 

 Mr. Smeaton was obliged to fix the stones of his light house to the rock and 

 to each other, by dovetail joints, and to connect each horizontal tier with 

 the tier below it, by pins of wood passing through the stones, with wedges 

 driven in at each end, to make them expand, and tie the stones fast 

 together. But, in general, it is sufficient to employ mortar, made of lime 

 or terras and sand, of which the utility depends principally on the firmness 

 and cohesive strength that it acquires in consequence of its chemical pro- 

 perties. Sometimes the whole structure is composed of a mass which is at 

 first soft, but hardens as it dries ; in this manner mud walls are built ; and 

 the materials called pise are of a similar nature. (Plate XI. Fig. 151.) 



The wall or column, when raised, must in general help to support a 

 single lintel or beam, an arch, a dome, or a roof of carpentry. The strength 

 of the lintel depends more on the nature of the substance than on any art 

 employed in forming it, excepting the precaution to give it as much depth 

 as is convenient, especially towards the middle, if the depth be anywhere 

 unequal ; but the construction of an arch affords considerable scope for the 

 exertion of mechanical science. 



The simplest theory of the arch, supporting itself in equilibrium, is that 

 of Dr. Hooke,* the greatest of all philosophical mechanics. The arch, 

 when it has only its own weight to bear, may be considered as the inversion 

 of a chain suspended at each end ; for the chain Kangs in such a form that 

 the weight of each link is held in equilibrium by the result of the two forces 

 acting at its extremities ; and these forces or tensions are produced, the one 



* Hooke, De Potentia Restitutiva, 1678, p. 31. See Waller's Life of Hpoke, 

 prefixed to the edition of his posthumous Works, Lond. 1705, p. 21. 



