GENERAL METHODS 139 



If we have, then, a simple cavity for which 

 concrete is a suitable filling we know in advance 

 that it will be useless to try to make of the filling 

 a rigid mass. Our bracing has also been done 

 with that principle in mind. Knowing the plan 

 we are going to work on, we are ready to proceed 

 with the work of filling the tree. There are, as 

 has already been suggested, two ways of putting 

 concrete into a cavity — the dry method and the 

 wet method. 



CONCRETE: DRY METHOD 



The dry method is the commoner one. It looks 

 simple but it is not an easy job for a beginner to 

 set concrete up in an open cavity in such a way 

 that it will be well tamped and compacted, and yet 

 avoid " slumping " of the face of the filling. It 

 is well to begin with a job which slopes back a 

 little. It is not practicable, in this method, to 

 use a very large aggregate. Mix a mortar of 

 one part cement to four parts of natural gravel 

 which has passed through a half-inch screen, mak- 

 ing it so dry that a vertical wall can be cut in it 

 without slumping, and yet so wet that a handful 

 of it squeezed tightly will not fracture when the 

 hand is opened. 



Set a pail of this mortar and a pail of wet 

 stones near the tree. After laying down a layer 

 of stones in the back of the cavity and out to 



