156 OUTLINES OF NATURAL PHILOSOPHY. 



The true principles on which the resistance of walls 

 is to be estimated, are not sufficiently understood ; 

 and, in the mean time, a skilful and cautious engi- 

 neer, can do nothing better than compute the re- 

 sistance on different hypotheses, and take care that 

 the actual strength be greater than any thing that 

 theory points out as necessary. 



235. As the wedges made by extending the 

 voussoirs, according to the preceding theorems, 

 would be of an inconvenient length near the spring 

 of the arch, it will be best to substitute for them 

 such a resistance as does not press till it is pressed 

 upon. 



Thus, if on the pier immediately behind the arch, 

 there be raised a body of masonry formed of ho- 

 rizontal courses of large stones, the voussoirs being 

 continued no farther than to come in contact with 

 the ends of those courses, the purpose of equili- 

 brium will be answered, supposing the work of 

 sufficient strength, because any of the voussoirs pres- 

 sing against the horizontal courses, will have its own 

 defect of weight immediately supplied by their re- 

 action. Each voussoir will then receive, with mathe- 

 matical exactness, the precise degree of pressure re- 

 quired for the equilibrium, and can neither have 

 more nor less than that quantity. The upper part 

 of the arch is here supposed to be loaded in the 

 manner that strict theory requires ; to which there 

 does not appear to be any practical objection. If 

 the pier be an intermediate one between two arches, 

 the opposite pressure on the horizontal abutting 

 courses will render their effect more certain, 



On 



