1 88 PRACTICAL TREE REPAIR 



The way in which the metal is applied to the 

 opening of the cavity is governed by the nature 

 of the strains on the metal, after it is in place. 

 Those strains have two origins. The pressure 

 of the ingrowing callus upon the tin produces one 

 kind of strain, a strain which falls first on the 



if 



Before-and-after views of a typical job of metal work 



nails and then on the metal itself. This strain 

 is not often the source of trouble, for as soon as 

 the callus grows over the nails they are so 

 anchored that they cannot give way. The other 

 strain is far more serious. It results from the 

 tearing and shearing forces produced by the bend- 

 ing and twisting of the tree. If the opening is a 

 large one, in a storm its sides will shorten and 



