136 SMALL WATER SUPPLIES. 



of cement or to grade the aggregate depends upon 

 the relative local prices of cement and of aggregate, 

 and the size of the work. These are the proved 

 methods that are free from doubt as to enduring 

 efficiency and strength, and are of reasonable cost. 



It should be remembered that trowelling on a 

 plaster coat of mortar and washing with thin grout 

 are but modifications of use of rich proportions of 

 Portland cement. 



The use of finer sand than is usually accepted for 

 ordinary concrete work may prove to be of special 

 value. Its advantage, if successful, will consist 

 simply in cheapening the process of grading ordinary 

 materials to an ideal analysis. Its effect on reducing 

 strength is well known, and its use will in some 

 cases be limited by strength requirements. 



Puzzolan and sand cement are superior to Port- 

 lands in securing impermeability, but they are 

 somewhat inferior in strength. Colloidal clay as 

 a substitute for 5 per cent or 10 per cent of the 

 sand, or the substitution of i per cent or 2^ per 

 cent solutions of alum sulphate, or possibly other 

 electrolytes for the mixing water, may prove cheap 

 and effective processes. 



The degree of imperviousness which may be ex- 

 pected from the methods described is such as to meet 

 ordinary conditions, as the demand for dry basements, 

 roofs, and walls, and the storage of water or its con- 

 veyance under moderate heads without objectionable 

 loss or resulting damage from its escape. 



Two questions of great importance, the solutions 

 of which are being sought, are whether it is neces- 

 sary to secure a greater degree of protection for rein- 



