246 Mr Sang on the Construction of Circular Towers, 



Before proceeding to analyze the proper mode of construc- 

 tion, it may be useful to view the defects of the forms hitherto 

 adopted ; the utility of the inquiry will be seen, when it is 

 shewn that the common construction introduces into the build- 

 ing one element of self decay. 



All the building materials with which we are acquainted 

 consist of granular particles united by a cement, or of crystals 

 closely impacted among each other. The formation of sand- 

 stone, and the composition of granite, serve well to exemplify 

 the two classes. 



The grains of quartz, of which sandstone is composed, may 

 be regarded as possessing absolute strength, provided their 

 lateral spreading be resisted ; thus, if a quantity of sand be 

 put in the bottom of a cylinder, and if a piston be brought 

 down upon it, no conceivable pressure will, so long as the 

 bounding cylinder remains, destroy the particles of sand. Ima- 

 gine these grains to have been deposited from a gently agi- 

 tated fluid, and it will appear that each successive particle 

 seeks a position the most conformable with the contiguous 

 ones, so as to give nearly the greatest possible compactness 

 to the whole. After this deposition has taken place, other 

 matters is gradually accumulated above, until the body of sand 

 be subjected to an intense pressure. In this state of things 

 a slow infiltration of water, containing calcareous or siliceous 

 matter, goes on, and there is gradually formed around the 

 grains of sand, and between their contiguous surfaces, a calc 

 or silico-sinter, acting like glue in inducing a cohesion pre- 

 viously awanting. 



A slight attention to the circumstances under which this for- 

 mation takes place, shews that while the cement is formed under 

 the hydrostatic pressure due to the depth of the waters, the 

 sand particles, around which it is placed, are subjected to that 

 pressure, and also to the weight of the superincumbent solid 

 material. 



What, then, will be the consequence of removing the sand- 

 stone from the position in which it was originally formed ? 



So soon as the hydrostatic pressure is removed, both the 

 grains and the cement will expand, the expansion being pro- 



