CHxiPTEE VII. 



ox THE STRENGTH OP MATERIALS USED IN THE ARTS, AND 

 THEIR APPLICATION TO ARCHITECTURAL PURPOSES. 



SECTION I. 



ON THE STRENGTH OF MATERIALS. 



rponwhatdoes 258. When materials are employed for 

 I'^materi'^'def niechanical purj)oses, their power, or strength, 

 pend? f^^J. resisting external force, apart from the na- 



ture of the material, depends upon the shape of the 

 material, its bearing, or manner of support, and the nature 

 of the force applied to it. 



Under what cir- 259. A beam, or bar, will sustain the greatest 

 rbcanTsustTi!! appHcatiou of force, when the strain is in the 

 force f '"'''" direction of its length. 



260. The strongest of all metals for resisting tension, or a 

 BtrenRth o^f dif- direct pull, is iron in the condition of tempered steel. The 

 fereiit substan- strength of mctals is affected by their temperature, being 

 * ^ dimiuislied, in general, as their temperature is raised. Wood 



of the same kind is subjected to very great variations of strength. Trees 

 that grow in mountainous or windy places, have greater strength than 

 those which grow on plains ; and the different parts of a tree, such as the 

 root, trunk, and branches, possess different degrees of strength. Cords of 

 equal thickness are strong in proportion to the fineness of their strands, and 

 also to the fineness of the fibers of these strands. Ropes which are damp, 

 are stronger than those which are dry ; those which are tarred than the un- 

 tarred, the twisted than the spun, the unbleached than the bleached. Other 

 things being equal, a rope of silk is three times stronger than a rope of flax. 



How does the 261. Of two bodics of similar shape, but of 

 afffct'^ Hs''"'^^ different sizes, the larger is proportionably the 

 Ktr^ngth? weaker.* 



• A knowledge of the strength of various materials in resisting the action of forces ex- 

 erted in diflferent directions, is of prent importance in the arts. In the following tables 

 are collected the results of the most recent and extensive expcTiraents upon this subjf^ct. 

 The bodies subjected to czperimeat are supposed to be in the form of long rods, the cross- 



