246 GREENHOUSES 



This is the formula commonly .used for walls 

 above ground and for bridges and similar 

 work. For sidewalks, floors, etc., which are 

 supported on a firm foundation and are not 

 subjected to heav}^ strain, a weaker mixture 

 of i part of cement, 2^A parts of sand and 5 

 parts of stone or gravel, is sometimes used. 

 For plastering the outside of walls and for 

 similar purposes a mixture of cement and 

 sand alone in the proportion of i to i is used, 

 as it is easily worked and leaves a smooth 

 surface. 



Mixing. For small jobs concrete is 

 usually mixed by hand. The essentials are: 

 (i) A tight platform or mixing board of suf- 

 ficient size; (2) a convenient measuring box; 

 (3) suitable shovels; and (4) a supply of 

 water. Quite commonly the sand and gravel 

 is measured in the wheelbarrows in which 

 it is hauled, a little experience, secured by 

 carefully measuring the amount for a few 

 times, being all that is necessary to insure 

 sufficiently accurate measuring. The bar- 

 row loads are checked up from time to time, 

 however, to see that they are not over-run- 

 ning or falling short. 



