Evaporation is the great generator of cold, and one cause, no doubt, why soils saturated with moisture have a languid vegetation in the vernal months: but will it explain, says Mr. E., why the comparative powers of supporting vegetables should be reversed in the autumnal ones? Vegetation at the spring of the year on limestone and sandy soils is at least a fortnight earlier than on clayey or even rich deep soils. But the case is reversed in autumn: for without an unusual degree of temperature, vegetation on limestone, and sandy soils nearly ceases in November, whereas on clayey or rich deep soils it continues to the end of January; and frequently after the first or second fall of snow (provided the soil has not yet been cooled down by previous frost) exhibits at its dissolution the verdure of April.
If the presence of a greater quantity of moisture on argillaceous soils and the consequent generation of cold be alone sufficient to account for the effect, since the same disproportion exists in spring, the effect ought to be the same at both seasons.
Mr. Egremont, in his ingenious pamphlet,[1] endeavours to solve this apparent paradox by a reference to the different capacities for heat in different earths: the solution, however, as he does not appear to have subjected any of these earths to actual experiment, is merely an inference derived from reasoning on the difficulty. To this relative power in different earths of retaining and transmitting heat he has recourse for explaining why the presence of a greater quantity of moisture in clayey and deep soils, than in calcareous and sandy ones, should at one part of the year (independent of the cold generated by evaporation) prevent so great an accumulation of heat on their surface, when at another it is the means of retarding sudden refrigeration; and why comparative dryness should at the spring of the year be favorable to the quicker accumulation of heat and consequent earlier vegetation, when in autumn and at the approach of winter it should be favorable to the loss of heat and produce a consequent earlier decline of vegetation. The subject is curious, and not so unconnected with that which is before us, as it may at first sight appear; I shall therefore steal a leaf out of Mr. Egremont's book, and present it to the reader as worthy of his attention:—
"Land or earth, particularly when dry, according to Kirwan, receives heat from light very readily, but transmits or conducts it through its own substance very slowly. Dr. Hales, found the air and surface of the earth in the month of August to be at the temperature of 88°, when a thermometer placed at 16 inches deep stood at 70°. In the month of October, when the air and surface were at 35°, at 16 inches deep the temperature was 48°, and at 24 inches 50°,
- ↑ Observations on the Mildew.
