MR WITHERS. 



223 



exterior layers is prevented, by the adhesion to the 

 drier more filled up central column (which probably 

 had contracted a little dm'ing the formation of the 

 exterior sap-wood layers), the contractile force of the 

 exterior balancing equally around this central column. 

 Should this balance be destroyed by the stem being 

 cleft up the middle, the longitudinal contraction will 

 immediately take place, and the two halves will 

 bend outward, from the outside layers contracting 

 more than the inside layers. We have seen an ash 

 tree rend up the middle from the cross section above 

 the bulb, nearly to the top, on being cut across in 

 fj^lling, owing to the longitudinal contractile force 

 of the exterior existing even before drying. 



Should the dried stem of a tree, of considerable 

 length, be laid hollow, supported at each end, the 

 outside layer being stretched almost to breaking by 

 the longitudinal contraction being greatest in the 

 outermost part, a very small weight, aided by a slight: 

 jerk or concussion, may be sufficient to burst the out^ 

 side layer on the lower side, the outside layers on the 

 upper side not standing out as a support above, but 

 combining their contractile force with gravity ta 

 rend the lower. As the outer layer gives way, the 

 strain is thrown concentrated upon the next outers 

 most, which also gives way, and the beam is broken 



