116 OF WOOD m GENERAL 



modulus of longitudinal extensibility that bears Ms name. This 

 is generally called the modulus of elasticity, but incorrectly, since 

 there are others. It is constant for any material, being represented 

 by the letter E, and is, in fact, the ratio of the intensity of push or 

 pull to longitudinal strain. Thus, if Z= length and a?= change of 

 length under a stress, p, then dl : I: :p : E, or 



Obviously, a stress apphed to a transverse section of wood will 

 have to break the fibres across, while one applied to a longitudinal 

 section tends to separate the fibres from each other. Thus the 

 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 longi- 

 tudinal strength ; but in coniferous woods it is only from ^^ to ^. 



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

 of transverse strength or breaking weight. Two knife-edges, or 

 supports for the ends of the beam, a cradle to shng from the centre, 

 in which pig-iron or other weight can be placed, a stretched cord 

 and a 2-foot rule are practically all the apparatus required for such 

 testing. Laslett, in his Woolwich 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 sus- 

 pended from the middle, noting the deflection with 390 lbs. weight 

 and at the breaking point. The transverse strength {f) is calcu- 



w'l 

 lated from the formula f--q~^'> where tf;'=the breaking weight in 



pounds, Z= the length between supports, &= the breadth, and d= the 

 thickness of the sample, or with the dimensions employed, 



^ f x2x4 

 Mr. Gamble uses the formula -j^, where L=the length between 



supports in feet, & = the breadth of the bar in inches, and d-its 

 thickness in inches. Bauschinger employed for bending tests 

 beams 20 inches square and 9 feet long, with 984 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, I the length in inches 

 of the beam between supports, h 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 



