50 THE MECHANICAL ANALYSIS OF SOILS [chap. 



washed in a stream of water till all the fine earth is 

 gone, dried, picked over to free it from roots and 

 stubble, and weighed as "stones." To get the pro- 

 portion borne by the stones to the soil, the fine earth 

 is also weighed, an addition being made of the weight 

 lost by the stones in washing. 



Of course the figure obtained for the proportion of 

 stones is only approximate, for if the stones are of any 

 size they will be very irregularly caught by the auger 

 or even by the 6-inch square tool. The material passing 

 the sieve is again spread out in a thin layer in an 

 ordinary room, until the surface maintains the same 

 colour as the lower layers; it is then bottled up as 

 " air-dry fine earth " for analysis. 



The Mechanical Analysis of a Soil 



The mechanical analysis that follows consists in 

 dividing the fine earth into a series of fractions con- 

 sisting of particles of known size ; we can use sieves to 

 sort out the coarser grades, but the finer ones must be 

 separated by their relative powers of remaining sus- 

 pended in water. 



The methods in use depend on two principles: 

 in one, the hydraulic method (Hilgard, Schoene, Nobel), 

 soil is washed by successive currents of water of veloci- 

 ties calculated to carry particles of the required size 

 according to the table on p. 33 : in the other, the sedi- 

 mentation method of Osborne, Knop, and Schloesing, 

 the soil is suspended in water and allowed to stand, the 

 separation being effected either by the times required 

 for the particles to settle down through a fixed distance, 

 or by the distances fallen in a given time. The method 

 to be described is based upon the latter principle. The 

 hydraulic method requires special apparatus, and is only 

 suited to laboratories entirely devoted to soil analysis. 



