G. — ^ENGINEERING . 1 27 



rungs are riven. Hurdles, hoops, and laths are other examples. Knees 

 in ships are chosen so that the grain follows the required outline. 



Owing to the enormous difference in strength in timber along and 

 across the grain, it is obviously important to get the grain in exactly 

 the right direction to bear the loads it has to carry. The most perfect 

 example I ever saw of building up a plywood structure to support all 

 the loads on it was the frame of the German Schutte-Lanz airship, 

 which was made entirely of wood. At the complex junctions of the 

 various girders and ties the wood, which was built up of very thin 

 veneers— hai-dly thicker than plane shavings — layers were put on most 

 ingeniously in the direction of every sti'ess. 



During the war I have had to reject numerous types of built-up 

 struts intended for aeroplanes, because the grain of the wood was in 

 the wrong direction to bear the load. The example shown — a McGruer 

 stmt — is one of the most elegant designs, using the gi'ain correctly. 



Many of the tests applied to timber are wrong in theory and conse- 

 quently misleading. For example, the common method of determining 

 Young's modulus for timber is to measure the elastic deflection of a 

 beam loaded in the middle and to calculate the modulus by the ordinary 

 theory, neglecting the deflection due to shear, which is legitimate in 

 isotropic materials ; but in timber the shear modulus is very small — for 

 example, in spruce it is only about one-sixtieth of Young's modulus — 

 and consequently the sliear deflection becomes quite appreciable, and 

 the results obtained on test pieces of the common proportions lead to 

 errors in the calculated Young's modulus of about 10 per cent. 



The lantern plates show thi-ee standai'd .tests : the first is supposed 

 lo give the shearing strength of the timber, but these test pieces fail 

 by tension across the grain — not by shearing. Professor Eobertson 

 has shown that the true shear strength of spruce is about three times 

 as great as the text-book figures, and has designed a test which gives 

 fairly reliable results. The second figure repi'esents a test intended to 

 give the mean strength across the grain, l^it the ooncentration of stress 

 at the gi'ooA'es is so great that such test pieces fail under loss than half 

 the pi'operload. This fact was shown in a striking manner by narrow- 

 ing a sample of this shape to half its width, when it actually bore a 

 greater total load — i.e., more llian double the stress borne by the 

 original sample. The third figure represents a test piece intended to 

 measure the I'athor vague quality, 'strength to resist splitting.' The 

 results actually depend on the tensile strength across the grain, on the 

 elastic constants, and on the accidental position of the bottom of the 

 groove relatively to the spring or autumn wood in the annular rings. 

 Unless the theory is understood, rational tests cannot be devised. 



There are some valuable tropical timbers whose structm'e is far 

 more complex than that of our ordinaiy northern woods. The grain in 

 tlieso timbers gro\^■s in alternating spirals — an arrangement which at 

 first sight is almost incredible. The most striking example of this type 

 of wood I have seen is the Indian ' Poon.' Tlie sample on the table 

 has been split in a series of tangential planes at varying distances from 

 the centre of the tree, and it will be seen that the grain at one depth 

 is growing in right-hand spiral round the trunk; a little farther out 



