40 



ANIMAL MECHANICS 



FIG. 7. 



parts (A, B, C) : the upper part A supports the 



weight by its solid- 

 ity and resistance 

 to compression ; the 

 lowest part B, on the 

 other hand, resists 

 by its toughness, or 

 adhesive quality. Be- 

 twixt the portions 

 acting in so different a manner there is an inter- 

 mediate neutral, or central part C, that may be 

 taken away without materially weakening the 

 beam, which shows that a hollow cylinder is the 

 form of strength. The writer lately observed a 

 good demonstration of this : A large tree was 

 blown down, and lay upon the ground ; to the wind- 

 ward, the broken part gaped; it had been torn 

 asunder like the snapping of a rope : to the lee- 

 ward side of the tree, the fibres of the stein were 

 crushed into one another and splintered ; whilst 

 the central part remained entire. This, we pre- 

 sume, must be always the case, more or less ; and 

 here we take the opportunity of noticing why the 

 arch is the form of strength. If this transverse 



o 



piece of timber were in the form of an arch, and 

 supported at the extremities, then its whole thick- 

 ness, its centre, as well as the upper and lower 

 parts, would support weight by resisting compres- 

 sion. But the demonstration may be carried 



