REMARKS ON DRAINING. 



117 



soil. Air, and even carbonic acid, the great source of the 

 food of plants, must be driven from their recesses by a fluid ex- 

 erting- such pressure as that above mentioned. 



It has been ascertained that the specific gravity of water is 

 greatest at a temperature between 39° and 40° Fahrenheit : there- 

 fore in any quantity of water not of uniform temperature, that 

 which is 39° or 40°, or, if hotter or colder, that which is nearest 

 to the above, will always be at the bottom ; it will not be dis- 

 placed by the addition of a quantity of the temperature of 60°, 

 nor will the two mix so as to acquire a common temperature 

 without mechanical force. Hence if a retentive soil is saturated 

 with snow-water, or that from rain of about 40°, such water will 

 not be displaced by lighter, warmer rains. The water derived 

 from the latter must run off by the surface, or stand exposed to 

 the cooling effects of evaporation : in either case its heat is lost 

 without benefiting the soil. The only remedy is to drain 

 deeply ; the coldest water, because the heaviest, will then be 

 the first to move, and the pores of the soil which it previously 

 occupied will be filled with air, except when the latter is par- 

 tially displaced by the descent of rain-water, which can then 

 pass freely through spaces no longer exclusively occupied by 

 colder, denser water. 



It is shown by Mr. Parkes, in his 1 Essay on the Philosophy of 

 Drainage,' that in draining the Red Moss, near Bolton-le-Moors, 

 the thermometer in the drained land rose in June, 1837, to 

 66° at 7 inches below the surface, while in the neighbouring 

 water-logged land it never rose above 47° at any time throughout 

 the year. From within 7 inches of the surface to the depth of 

 30 feet, the bog in its natural state maintained, in winter and in 

 summer, an invariable temperature of 47 °. This, it may be 

 remarked, is about the mean atmospheric temperature of the 

 year in that locality, and the difference between the hottest and 

 coldest months is there 24° or 25° ; but it must be inferred from 

 the uniformity of temperature in the saturated undrained land, 

 that summer rains are ineffectual as regards the communication of 

 heat downwards, and that on such land the intensity of the sun's 

 rays is lost. 



The fact that heat cannot be transmitted downwards through 

 water is well known ; but that it is as impossible to heat saturated 

 soil downwards should be also familiar to every one engaged in 

 either horticultural or agricultural pursuits. Some experiments 

 are detailed in the Gardener's Chronicle, No. 3, 1849, p. 35, 

 by which it was proved that a mixture of peat and water, 

 constituting an artificial bog, could not be heated at a foot- 

 below its surface by pouring a quantity of boiling water upon 

 it. I have recently made a similar experiment with saturated 



