2. 0.071–0.114│ 10 2.194 1.724 1.425 „ „ │ 20 2.898 2.012 1.578 „ „ │ 30 3.602 2.300 1.731 „ „ │ 40 4.306 2.588 1.884 „ „ │ 50 5.010 2.876 2.037 „ „ │ 60 5.714 3.164 2.190 „ „ │ 70 6.418 3.452 2.343 „ „ │ 80 7.122 3.740 2.496 „ „ │ 90 7.826 4.028 2.649 „ „ │ 100 8.530 4.316 2.802 Similar sets of data have been collected with powdered limestone, clay and humus. The general conclusions from the experiments are as follows: 1. Clay (kaolin) and humus (peat) are almost impermeable for water, and fine quartz and limestone dust are also very impermeable. 2. The permeability of a soil for water increases as the particles of the soil increase in size, and when particles of different sizes are mixed together the permeability approaches that of the finer particles. 3. The quantity of water passing through a given thickness of soil increases with the water pressure but is not proportional thereto, increasing less rapidly than the pressure. 4. The quantity of water passing under a given pressure is inversely proportional to the thickness of the soil layer when the particles are very fine and the pressure high. =177. Method of Whitney.=—To determine the permeability of the soil or subsoil to water or air, in its natural position in the field, the following method, due to Whitney, can be recommended: