408 Caution with refpeft to Aquedufts, &c. [Book VII, 



a tendency to difplace the two particles g b y the par- 

 ticle g cannot move any farther becaufe it is impeded 

 by the fide and bottom of the veflel ; but the particle b 

 may be raifed from the bafe towards the top, unlefs a 

 column equal to the column i k, or fomething equi- 

 valent, prefTes upon it to prevent it. 



It is upon this principle that the water, elevated by 

 the New River water-works, after having defcended 

 from a bafon in a vertical pipe, and then after having 

 flowed horizontally in a fucceffion of pipes under the 

 pavement, is raifed up again, through another pipe 4 

 as high as the fountain at the Temple Garden. It is 

 alfo upon this principle that a veflel may be filled 

 either at the mouth or at the bottom indifferently, 

 provided that it is done through a pipe, the top of 

 which is as high as the top of the vefTel to be filled. 

 Hence it follows, that when piers, aqueducts, refer- 

 voirs, or other hydraulic works for the retention of 

 water are to be conftructed, it becomes neceflary to 

 proportion their ftrength to the lateral preflure which 

 they are likely to fuftain, which becomes greater as 

 the height of the water is more confiderable. Nearly 

 the fame precautions are neceflary to be taken with 

 refpecl: to what fome philofophers call the grofler 

 fluids, which alfo have a propenfity to expand, as well 

 on account of the fmallnefs of their parts as from the 

 frrall degree of cohefion which exifts between them. 

 Walls defigned to fupport terraffes ought to be fuf- 

 ficiently ftrong to refill the lateral preflure of the earth 

 and rubbifh which they are to fuftain, fince this prefliire 

 will be greater as the particles of earth, and of the 

 other materials of which the terrafles are compofed, are 

 lefs bountl together, and in proportion as the terralies 

 are more elevated. 



All the parts of the fame fluid are in equili- 

 brium 



