TREATMENT WITHOUT FILLING 189 



lengthen, spread apart and come together. If 

 this strain is too great it rules out the use of metal. 

 A moderate amount, however, can be accommo- 

 dated in various ways. 



The first thing that can be done is to reduce 

 the amount of strain on the metal by bracing the 

 sides of the cavity together, exactly as is done in 

 preparing a cavity for a concrete filling. If 



Cross section showing how zinc can be bowed in to allow for 

 spreading of the sides of a cavity 



there is a good, strong nailing edge at each side, 

 a few narrow strips of sheetiron can be nailed 

 across the opening to help keep it rigid. The 

 strain can also be met by dividing the metal up 

 into sections of greater or less size, according to 

 the amount of sway. When that is done the work 

 must begin at the bottom, each successive section 

 being lapped over the one below it an inch or two. 

 Under certain conditions the tendency of the 

 nails to tear through the metal under stress can 

 be lessened by bowing the metal in a little so that 



