FO UNDA TJONS. 233 



cement-mortar of various proportions, depending upon the 

 ratio of voids of the material. In practice, a good concrete 

 can be made with one part of cement, two parts sand, and 

 four parts broken material. In foundation-work, a good grade 

 of Portland cement, sharp sand, and clean stone should be in- 

 sisted upon. The volume of water used to incorporate the 

 mass is the subject of never-ceasing discussion amongst the 

 engineering fraternity, but in the author's practice a good 

 concrete has been made by so dampening the mixture that 

 after being deposited and rammed, a slight appearance of 

 water upon the surface is all that is necessary. Concrete for 

 small foundations is usually mixed by hand, upon a 12 X 

 12-ft. frame or light platform, the ingredients being placed 

 conveniently. A proportion of sand, by measure, is first 

 spread over the board into which is dumped the specified 

 proportion of cement, and the two components thoroughly in- 

 corporated by the workmen with their shovels ; spreading this 

 mixture so that it shall be somewhat higher along the outer 

 edges of the mixing-board, water is sprayed from a small hose 

 upon the mass, which is quickly turned with shovels until every 

 particle has been completely incorporated. Into this liquid 

 paste the proper proportions of stone, after a drenching, are 

 added, and quickly turned by the laborers until each particle 

 of stone has been coated with the mortar. The concrete is 

 then carefully deposited by the shovels of the workmen, in 

 layers from 3 to 6 ins. thick, into the foundations. Such 

 mixing and spreading by hand will cost approximately 60 cts. 

 per cubic yard ; the cost of the concrete will depend upon 

 varying conditions, and will range from $4.00 to $6.00 per 

 cubic yard in place. 



Maximum Pressures. The action of the wind upon the 

 cylindrical surface of a tank and the application of that force 

 as pressure upon the base has been previously explained. 

 The normal pressure due to the load is the weight divided by 



