MECHANICS. 157 



On the subject of Arches, see HUTTON'S Principles of 

 Bridges. BOSSUT, Recherches sur PEquilibre des 

 Voutes, Mem. de TAcad. des Sciences, 1774 and 

 1766; also Mecanique of the same author, edit. 

 1802, p. 383., &c. PRONY, Architecture Hydrau- 

 lique, torn. i. 358., &c. AT WOOD'S Treatise on the 

 Construction and Properties of Arches, published in 

 1801. This last work is particularly commend- 

 able, for the great accuracy of the investigation, 

 the minuteness with which the subject is consider- 

 ed, and the various experiments brought in support 

 of the theory. See also article Bridge, in the 

 Edinburgh Encyclopaedia. 



STRENGTH OF TIMBER. 



236. If a beam of wood, of which the section is 

 any given figure, having its area = a 2 , have one 

 end firmly fixed in a wall ; if s be the strength of 

 each fibre estimated laterally ; d the distance of 

 the centre of gravity of the section, from the edge 

 where the fracture terminates, and round which 

 the beam, when it is broken, turns as on a hinge ; 

 let / be the length of the beam, measured from 

 the wall from which it projects, to the end at 

 which a weight W is hung, just sufficient to break 



it; Wxl=.sa?xd 9 and Wzz p. 



The weights, therefore, required to break different 

 beams, are as the strength of the individual fibres, 



multiplied 



