ii6 PRACTICAL TREE REPAIR 



ture, for therein lies the real solution of the prob- 

 lem. The writer has tried several schemes to 

 overcome the mechanical difficulties presented by 

 the wet mixture. The most successful of them, 

 and also the best way of handling the dry mixture, 

 will be described later under the general descrip- 

 tion of the method of filling cavities. 



Little need be said concerning the details of 

 mixing concrete and the like, for such matters are 

 familiar to almost every one, and they are de- 

 scribed fully in pamphlets which can be secured 

 free of charge from the Government or the manu- 

 facturers, and in cheap and easily obtainable hand- 

 books. 



Something ought, however, to be said about 

 quantities and costs. As soon as a job has pro- 

 gressed far enough to make possible an estimate 

 of the cubical contents of the cavity, the amount of 

 materials required to fill it should be figured out 

 and provision made for obtaining them. To do 

 this it is necessary to know how much cement and 

 aggregates is needed to make a certain amount 

 of concrete. Suppose the aggregate is a natural 

 mixture of sand and gravel. It will be a little 

 shy of gravel and the mixed sizes will not bulk as 

 large as if they were separated, so you will have 

 to use one part of cement to four parts of gravel 

 to make the equivalent of a 1:2 :4 concrete. At 

 the rate of one to four, a bag of cement, three and 



