160 tECTURE XIV. 



surfaces are either horizontal or vertical; for if they are oblique, thej' 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 ma- 

 terials 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 circum- 

 stance 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 mechanically, 

 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 

 ufility depends principally on the firmness and cohesive strength that it ac- 

 quires in consequence of its chemical properties. 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 pis6 are of a si- 

 milar nature. (Plate XL 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 con- 

 venient, especially towards the middle, if the depth be any where 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 hangs in such a form, that the weight of 



