ON PASSIVE STRENGTH AND FRICTION. 151 



kinds: but there is seldom occasion to determine their absolute strength in 

 resisting a force thus applied : if they are sufficiently stiif, their parts arc not 

 often separated by any violent efforts. 



In Older to investigate the strength of the various substances employed for 

 the purposes of the mechanical arts, it is most convenient to use a ma- 

 chine furnished with proper supports, and gripes, or vices, for holding the ma- 

 terials, and with steelyards for ascertaining the magnitude of the force ap- 

 plied, while the extension or compression is produced by a screw or a winch, 

 with the intervention of a wire, a chain, or a cord : provision ought also to 

 be made for varying the cHrection of the force, when the flexure of the ma- 

 terials renders such a change necessary, (Plate XI. Fig. 148.) 



According to the experiments of various -authors, the cohesive strength of 

 a square inch of razor steel is about 150 thousand pounds, of soft steel 120, 

 of wrought iron 80, of cast iron 50, of good rope 20, of oak, beech, and 

 willow wood, in the direction of their fibres 12, of fir 8, and of lead about 

 3 thousand pounds : the cohesive strength of a square inch of brick 300, and 

 of freestone 200. Teak wood, the tectona grandis, is said to be still stronger 

 than oak. 



The weight of the modulus of the elasticity of a square inch of steel, or 

 that weight which would be capable of compressing it to half its dimensions, 

 is about 3 million pounds; hence it follows, that when a square inch of steel 

 is torn asunder by a weight of 150000 pounds, its length is first increa.sed to- 

 one twentieth more than its natural dimensions. 



The strength of different materials, in resisting compression, is liable to 

 great variation. In steel, and in willow wood, the cohesive and repidsive 

 strength appear to be nearly equal. Oak will suspend much more than fir; 

 but fir will support twice as much as oak; probably on account of the.cuiva- 

 ture of the fibres of oak. Freestone has been found to support about 2000 

 pounds for each square inch, oak in some practical cases more than 4000. 



The strongest wood of each tree is neither at the centre nor at the circum- 

 ference, but in the middle between both; and in Europe it is generally thicker 



