XXVIII. J. M. SMAIL. 



minimum depth for working conditions; if the source is a 

 polluted one, a greater depth should be retained, because 

 at times the surface of the sand gets broken, and polluted 

 water passes down to the underdrains. The greater the 

 deptli of sand, the more support there is for the surface 

 layer of slime, which, as is generally known, is the most 

 useful part of the filter. The quality and size of grains of 

 sand is of importance. There is considerable diversity of 

 opinion on this subject. Some engineers favour the use of 

 sand so fine that GO > of the particles would pass through 

 a sieve of 1,900 meshes to the square inch; it is stated that 

 excellent results have been obtained from same. On the 

 other hand it is contended that such line sand is neither 

 necessary or desirable. The method adopted in one case 

 in selecting sand for filters appears to have some merit. 

 The plan adopted is to have sieves of different mesh, and 

 to select by trial the sieve which will remove 10f u of the 

 finest sand from a sample known weight. A sieve of 900 

 meshes to the square inch is then used, and all particles 

 which will not pass the mesh are removed; the balance of 

 sand left after removing the fine and the coarse is approxi- 

 mately of uniform size, and may be looked upon as the most 

 important of the bulk. The fraction represented by the 

 weight of the residue, divided by the total weight of the 

 original sample, gives the measure of the uniformity of 

 size of the sand, and the nearer this fraction is to unity 

 the more suitable the sand is for filtration purposes. This 

 method is adopted by Mr. Silcock, m. inet. cm The uniformity 

 co-efficient of sand used in 27 cities in Great Britain and 

 on the Continent ranges from 4*7 to 1*5, the latter being 

 that recommended in American practice. 



The rate of filtration on the Continent and in Great 

 Britain averages about 1*5 gallons per square foot per 

 hour. As a result of careful observation, it has been found 



