ON PRUNING. 73 



various curves and shapes of the stem, roots, 

 and limbs, are the best possible for possessing 

 the greatest strength. The direction of the 

 forces the different parts of the tree have to 

 resist, are not easily determined, or it would 

 be easy to prove, by the common rules for the 

 comparative strength of beams of timber, that 

 the curves at the joinings of the roots and 

 limbs to the stem, and the shape of the stem, 

 roots, and limbs, are the best that could be de- 

 vised for strength, A piece of timber strained 

 with any force in one direction, should be 

 deeper in the direction of the force than it is 

 wide, in order to have the greater strength in 

 the same piece ; on this account joists for floors 

 are much deeper than they are wide, because 

 the force or pressure they have to resist is 

 always in one direction ; but if the pressure 

 be applied not always in one direction, but at 

 different times in different directions, the 

 strongest shape would be that of a circle or 

 cylinder : thus we see the trunks and boughs 

 of trees round, the force they have to resist 



