134 OF WOOD IN GENERAL. 



strength of wood along the grain depends upon the strength of 

 the fibres ; that across the grain, upon their cohesion. This latter 

 or lateral strength is, in broad-leaved trees, from J to J of the 

 longitudinal strength ; but in coniferous woods it is only from 



o-Q- tO J--Q. 



One of the simplest and most instructive tests of timber is that 

 of transverse strength or breaking weight. Laslett, in his Wool- 

 wich experiments, took pieces 84 inches long, 2 inches wide, and 

 2 inches thick, placed upon supports 72 inches apart, and then 

 poured water gradually into a scale suspended from the middle, 

 noting the deflection with 390 Ibs. weight and at the breaking 

 point. The transverse strength (p) is calculated from the 



formula p = ^^, where w' = the breaking- weight in pounds. 



/ = the length between supports, b = the breadth, and ^ = the 

 thickness of the sample, or with the dimensions employed, 

 w'x72 =13 i lc > 

 x 2 x 4 



Mr. Gamble uses the formula , ,.,, where L = the length 



between supports in feet, 5 = the breadth of the bar in inches, 

 and 6? = its thickness in inches. Bauschinger employed for 

 bending tests beams 20 inches square arid 9 feet long, with 

 98 *4 inches between their supports; and Professor Lanza of the 

 Massachusetts Institute of Technology employed beams varying 

 from 4 to 20 feet in length, from 2 to 6 inches in width, and 

 from 2 to 12 inches in thickness. Then, W being the load at 

 the centre in tons, / the length in inches of the beam between 

 supports, b its breadth, and h its thickness, also in inches, /, the 

 greatest direct stress on the fibres, or coefficient of bending 

 strength, is obtained in tons per square inch from the formula 



Wl 

 f = f rirjj. If ^ = the deflection at the centre in bending in inches, 



the coefficient of elasticity (E) in tons per square inch is obtained 



W/ 3 

 from the formula E = \ -^n-z. Sir John Anderson has reduced the 



