TREATMENT WITHOUT FILLING 191 



not so easy as the simplicity of the above direc- 

 tions would indicate. A great deal of ingenuity 

 is required to fit the metal neatly and closely. 

 For instance, in order to make it conform to a 

 " valley " it must sometimes be slit in from the 

 edges and overlapped. 



The treatment of the outer surface of the metal 

 is the last but by no means the least important 

 process in tinning cavities. A very heavy dress- 

 ing should be put on, especially around the edges. 

 Not only is this last necessary as a precaution 

 against the entrance of water, but also against 

 rust, especially of the nails. A heavy dressing 

 also tends to prevent the starting of the nails in 

 zero weather. 



Tin work requires a certain amount of after 

 care, like every other sort of work in trees. The 

 surface dressing may need renewal after a few 

 years. There is one objection to tinning which 

 may have to be remedied if it develops. That is 

 the tendency of the tin, or such other metal as 

 may be used, when there is a high breeze, to 

 " snap," a noise which means that the tin is being 

 forcibly, though slightly, wrinkled. After a year 

 or two the tin gives way in one or two places and 

 the noise stops. As soon as a piece of tin begins 

 to snap one or more horizontal slits must be cut 

 in it with a can opener or a sharp cold-chisel, so as 

 to relieve the tendency to buckle. The slits must 



