88 



MECHANICAL ANALYSIS OF SOIL 



The size of the various fractions, expressed as 

 diameters in millimetres, is as follows : — 



A resume of the operations involved in the mechani- 

 cal analysis of soil by the process of sedimentation 

 in water is given on page 87. 



67. Mechanical Analysis by the Hydraulic Method. 



The principle of this method depends on the fact 

 that particles in a liquid are carried upward, if the 

 liquid is given an upward motion and its velocity is 

 greater than the rate of subsidence of the particles in 

 the still liquid. Within limits this is proportional to 

 the size of the particles, in the case of soil. Various 

 methods have been adopted for obtaining definite 

 speeds of flow corresponding to the limiting sizes of 

 the various grades of particles. The most satisfactory 

 apparatus in use is probably Mayer's modification of 

 Schone's elutriator. 



Apparatus. — The apparatus is shown in Fig. 15. 

 It consists of a glass vessel fitted with a tap at the 

 lower extremity for admitting water. For a distance 

 upwards of 20 cm. the sides of the tube are parallel, 

 and the bore about i cm. diameter. For a distance 

 of 50 cm. the tube then expands conically until the 

 internal diameter reaches 5 cm. For a distance of 

 10 cm. the vessel is again strictly cylindrical, after 

 which it narrows rapidly until at the top it is 2 cm. 

 in diameter. The top is closed with a stopper carrying 



