54 THE MECHANICAL ANALYSIS OF SOILS [chap. 



is measured under the microscope until the right time 

 is hit off to secure the desired range of size in the 

 sediment. The relationship between the time of settle- 

 ment, the height of the liquid column, and the diameter 

 of the particles, is governed by the formula : — 



v = 2ga* O-/)) 



9 V 



where <r is the density of the particle, a its radius, p the 

 density, and tj the coefficient of viscosity of the liquid. 

 The application of the formula, however, requires to be 

 checked by observation with the microscope, because 

 the particles are not spheres. 



The hygroscopic moisture and the loss on ignition 

 also require determination, which is described under 

 the chemical analysis of a soil. 



Interpretation of Results. 



It is as yet impossible to predict the behaviour of 

 a soil under cultivation from a consideration of its 

 mechanical analysis; in a general way we can see 

 whether a soil is heavy, whether it is likely to dry 

 "steely," or whether it will crumble readily under 

 proper cultivation, and whether it is more suitable for 

 market gardening or wheat growing, but the more 

 refined points of difference connected with the manage- 

 ment of given soils, which become known by experience 

 to a good practical farmer, cannot as yet be deduced 

 from the analysis. It is necessary to accumulate more 

 data, until we possess the mechanical analysis of a 

 large number of soils whose texture and amenability 

 to cultivation have been ascertained by long practice; 

 then we shall be able to assign any soil by its mechanical 

 analysis to a known type. 



The power of a soil to retain moisture and resist 

 moderate drought depends on a predominance of the 



