III.] 



WEIGHT OF SOIL 



7 



Woburn soil is a loose, coarse-grained sand, containing 

 only a little stone derived from the rock below. It will 

 be seen that, if the stones are excluded, the density 

 increases with the depth, because of the greater consoli- 

 dation caused by the weight above and to some extent 

 by the washing down of the finest particles, but the 

 increase does not continue much below the depth of 3 

 feet, the limit of these measurements : 



From the data thus obtained as to density and 

 pore space, together with a mechanical analysis to 

 show the proportion of particles of various sizes, it is 

 possible to calculate for any given soil both the number 

 of soil particles and the area of the surface they expose, 

 on the assumption that the particles are spherical. 

 Approximately with grains 1 mm. in diameter, there 

 would be 700 grains in 1 gram of the soil, and the 

 number of grains to the gram will vary inversely as 

 the third power of the diameter, i.e., if the diameter be 

 divided by 10 and become o-i mm., there will then be 

 700,000 grains to the gram. The surface possessed by 

 all the soil grains can be similarly deduced by calcula- 

 tion, and will be found to vary inversely as the diameter 

 of the individual grains ; a sphere 1 inch in diameter 

 will have only half the surface of the eight spheres of 



