DRAINAGE. 91 



it lowers the temperature of that soil 10. If the soil has 

 the means of discharging that 1 Ib. of water by filtration, 

 no effect is produced beyond what is due to the relative 

 temperatures of the rain and of the soil. Mr. Dickenson, 

 the eminent paper-maker, who has several mills and a 

 considerable landed estate in Hertfordshire, has deduced 

 from a series of observations, which are, we believe, entitled 

 to great confidence, that of an annual fall of 26 inches of 

 rain, about 11 are filtered through a porous soil. The 

 whole of this 1 1 inches (and probably more) must be got 

 rid of by a retentive soil, either by evaporation or by 

 superficial discharge. The proportions in which each of 

 these means will operate will vary in every case, but this 

 will be an universal feature that these 11 inches will 

 maintain in undrained retentive soils, at all except some 

 accidental periods of excessive drought, a permanent sup- 

 ply of water of drainage, which will be in constant course 

 of evaporation, and will constantly produce the cold con- 

 sequent thereon. Retentive soils never can be so warm 

 as porous, for a simple reason. Every one knows, or may 

 know, that if into two flower-pots, with holes in the bottom, 

 are put respectively equal portions of gravel and clay, 

 equally heated to any point short of torrefaction, and if 

 equal quantities of water are administered to the surface 

 of each, water (water of drainage) will run from the gravel 

 long before it begins to run from the clay. Gravel can 

 hold by attraction much less water than clay can. At the 

 time when each is saturated by water of attraction, and 

 neither holds any water of drainage, evaporation will begin 

 to act upon the water in each, and will act most strongly 

 in the vegetative period of the year. The cold produced 

 will be in proportion to the quantities of water evaporated 

 respectively, and will of course be greatest in the retentive 

 soil. We will reserve a further cause of coolness in reten- 

 tive soils, which is also connected with evaporation, till we 

 have spoken of the depths of drains. 



2nd. The temperature of retentive soils is very much 



