500 



REPORT — 1879. 



complete cessation of percolation in April. There is no such uniformity in the 

 wettest periods of equal duration, and in consequence percolation does not recom- 

 mence till the close of the longer wettest periods of thirty, sixty, and ninety days. 

 Mr. Evans's soil gauge frequently leaves off recording percolation a month earlier, 

 and hegins to record it again a month later, than Mr. Greaves's. Mr. Evans's 

 gauge is filled with a mixture of gravel, loam, and mould, and Mr. Greaves's of 

 gravel, loam, and sand, which probably accounts for the difference in this respect. 



Characters of the Soil. — The calibre of the constituent grains of the soil, and 

 the percentage of grains of various calibres in the natural admixture, should be 

 known for every soil on which percolation experiments are or have been carried 

 out. In a collection of grains of given calibre spheres will occupy more space than 

 any other shape, or the absorbent capacity is least when the grains are spherical. 

 The space occupied by any number of spheres, from one upwards, which exactly 

 lie in a cubic foot, is •5236 cubic feet as long as the arrangement is cubical, the 

 retentive power increasing with the fineness of the grains. The natural arrange- 

 ment is, however, pyramidal, in which one sphere rests in the hollow between four. 

 As more spheres will thus go into a cubic foot the space occupied is somewhat 

 greater than - 5236, and the absorbent capacity somewhat less than -4764. The 

 absorbent capacity (pyramidal") decreases with the diminishing calibre of the grains. 



The absorbent capacity is the space available for holding water between the grains 

 of the soil. The retentive power, the quantity which the soil can hold by capil- 

 larity, increases with the fineness of the soil. The percolative capacity presents 

 three cases : — 1. Natural percolation from rainfall. 2. The percolative capacity at 

 retentive point. 3. The percolative capacity under pressure. 



Natural percolation depends upon the amount of and difference between the 

 absorbent and retentive power, and so upon the calibre of the soil and the shape of 

 the grains ; the quantity of rain falling ; the humidity of the soil at the time of 

 the fall ; the temperature of the soil, the percolating rain, and the air ; and the 

 humidity of the air. 



Percolation commences when the degree of humidity of the soil just exceeds 

 the retentive point. The absorbent capacity of a cubic foot of sand of less than 

 •035 in calibre was 7308 cubic inches, or -422 cubic feet, which equals a depth of 

 5 inches of rain on the square foot ; its retentive power 461 cubic inches, which 

 equals a depth of 3-208 inches of rain on the square foot. Such falls of rain in 24 

 hours are excessively rare, so that the soil is rarely or never saturated. It gene- 

 rally exists in a degree of humidity far below retentive point, so that only the 

 excess over this deposit can percolate. The percolative capacity at saturation point 

 requires to be determined experimentally ; also a degree of humidity corresponding 

 to the observed annual average natural percolation. Mr. Evans gives this as 8'227 

 inches for the 25 years, 1835-1860, which equals a daily transit of 3-24 cubic 

 inches through each square foot. We do not know the average humidity represented 

 by this percolation. Even when the humidity exceeds retention point, percolation 

 sets in and a waterline is formed. The degree of percolation represents the mini- 

 mum waterline, and saturation point the maximum waterline. Therefore the 

 degree of humidity, the quantity percolating, and the height of the ivaterline can be 

 expressed in terms of each other. 



