STRENGTH. 93 



the molecules, is a very important factor, but specific weight 

 gives no indication of this. With these results before him, 

 Mayr concludes that to foretell the strength of wood by 

 means of its specific weight is like a forecast of approaching 

 weather, from no factor except the position of the barometric 

 column. \Ye must therefore try to produce the cleanest, 

 straightest and most cylindrical stems in the least possible 

 time, whether or no such timber is heavy or light. 



The strength of a beam depends on the manner in which it 

 is supported, and the point of action of a straining force ; 

 a beam supported at one end and laden at the other 

 possesses only one quarter of the strength of the same beam, 

 when supported at both ends and weighted in the middle. If 

 after the weight has been applied and removed there is a 

 change of form in the beam, the limits of elasticity have been 

 exceeded. It has been assumed that this limit is half that of 

 the breaking strain, that a beam which breaks with eight tons 

 loses its elasticity with four tons. In engineering structures 

 employing wood the limits of elasticity are never permitted to 

 be approached, especially as Jlaupt and Thurston have deter- 

 mined that the limit of elasticity is much lower when the load 

 is permanent. 



Another very important item in the strength is the trans- 

 verse shape of the beam, and the direction of the annual 

 zones with respect to the supports. The transverse strength 

 is greatest when the section is a rectangle with its sides in the 

 ratio 10:7, the beam resting with its narrower side on the 

 support. Such a beam exhibits the maximum strength (100) 

 when the annual zones are approximately perpendicular to 

 the supporting surface (Fig. 43 a). 



If such a beam rests on its broader side, its strength is u'O 

 (Fig. 43 <). A beam with a square section but of the same 

 area as (a) has a strength of 75 if the annual zones are nearly 

 perpendicular to the supporting surface (Fig. 43 I), 65 when the 

 annual zones are parallel to the supporting surface. A rect- 

 angular beam with the pith in its centre and placed on its 

 narrow side has a strength of 90 (Fig. 43 d) and if: square 

 of 70 (Fig. 34 <>). (Another test gave b = 84, c = 70.) 



The way in which a beam is cut out of a log also affects its 



