HOW FAILURE UNDER STRESS OCCURS IN TIMBER. 423 
One or two more illustrations are here given to show the features of rupture caused 
by stresses across the grain. 
Fig. 28 is that of two blocks of Oak crushed in a tangential direction, the one on 
the right to a greater degree than the other. The failure has been as in direct 
compression along a plane making an angle with the direction of stress, but that plane 
is composed of a series of steps or slips in each case along the lines of the medullary 
rays. Any severance that takes place along the zones of spring wood is the consequence 
of this initial slip. 
STRENGTHS. 
In order to obtain a comparison of the strengths of the timbers when cut with the 
greater dimensions of the cross-section radial or tangential, experiments were made in 
the determination of (1) modulus of elasticity, by cross-bending ; (2) rupture load by 
compression ; (3) by tension ; (4) by cross-breaking. 
Modulus of Elasticity by Cross-bending. 
1. Beams of Oak, Ash, and Box woods were cut of a length sufficient to allow 3 feet 
span, the approximate sections being: Oak, 3 inches by 14 inch; Ash, 83 inches by 
12 inch; Box, 2 inches by 1 inch to 22 inches by 14 inch; and there were as many 
test pieces as it was possible to cut from the section of a single trunk of each timber. 
The form of extensometer was that shown in fig. 36, and was devised by the author. 
To obtain the true deflection, shackles were fixed to the neutral axis of the beam at A, 
B, C by means of wood serews. The screws at A and C supported the carriers D, and 
D,, and to them in turn were attached scales H, and H,, and also screws F, and F,, the 
latter acting as fulcrums for the magnifying levers G, and G,, one being placed at either 
side of the beam. The screws of the shackle B, which is placed in the centre of the 
span immediately underneath the centre of the load W, bear on the end of tbe 
