PROPERTIES OF WOOD. 51 



that weight and strength are dependent upon the number and com- 

 pactness of the fibers. 5 



THE STRENGTH OF WOOD. 



Strength is a factor of prime importance in wood. By strength 

 is meant the ability to resist stresses,, either of tension (pulling), or 

 of compression (pushing), or both together, cross stresses. When a 

 horizontal timber is subjected to a downward cross stress, the lower 

 half is under tension, the upper half is under compression and the 

 line between is called the neutral axis, Fig. 42. 



Htutral Ax/5 



Fig. 42. A Timber Under Cross Stress, Showing- Neutral Axis, and the Lines of 



Tension and Compression. A knot occurring in such a timber should be in the 



upper half, as at A. 



Wood is much stronger than is commonly supposed. A hickory 

 bar will stand more strain under tension than a wrought iron bar 

 of the same length and weight, and a block of long-leaf pine a greater 

 compression endwise than a block of wrought iron of the same height 

 and weight. It approaches the strength of cast iron under the same 

 conditions. 



Strength depends on two factors: the strength of the individual 

 fibers, and the adhesive power of the fibers to each other. So, when 

 a piece of wood is pulled apart, some of the fibers break and some are 

 pulled out from among their neighbors. Under compression, how- 

 ever, the fibers seem to act quite independently of each other, each 

 bending over like the strands of a rope when the ends are pushed 

 together. As a consequence, we find that wood is far stronger under 

 tension than under compression, varying from two to four times. 



5 For table of weights of different woods see Sargent, Jeswp Collection^ 

 pp. 153-157. 



