nearly the mean annual heat. Van Swieten also observed that the greatest cold, if it lasted only a few days, could not penetrate 20 inches.
"If heat be generated on land in proportion to the continued absorption and fixation of the rays of light, its accumulation on the surface will be retarded by the power which the soil has of transmitting it downwards, a power which may vary according to the nature of the substances of which the soil is composed, and to its comparative dryness. But generally I believe it will be found that land is heated or cooled sooner, and to a higher degree, than water: whence it will have a greater range of temperature, and the air incumbent on it, as well as other bodies in contact with it, will be hotter in summer and cooler in winter than that over the sea.
"This is agreeable to general experience as well as to the particular experiments of Dr. Raymond, who found land in the neighbourhood of Marseilles often heated in summer to 160° of temperature, while the sea did not exceed 77°; and in winter, when the land was cooled down to 14° or 15°, the sea was never lower than 44°.
"On the other hand, when the rays of light fall on water, fewer rays are combined, less heat is the result, and that which is produced being more readily mixed by undulatory motion, or transmitted downwards in a manner similar to its progress in solids, it cannot be accumulated either so soon or in the same degree, at the surface; hence, as before observed, air incumbent on the sea is cooler in summer than that on the land.
"From the absence of the sun’s rays in winter, sufficient heat is not generated to keep up the temperature of the atmosphere, reduced by cold winds from the poles, by evaporation, and other causes. At this time the equilibrium is in some degree restored, or at least the severity tempered by heat given back from the earth and its waters.
"The heat from the surface of the earth then is much sooner reduced to the temperature of the incumbent air than the surface of the sea, the surface having but a slow and imperfect supply of heat from the interior, while the sea, from the change of specific gravity and other causes of motion, is constantly presenting a warmer surface to the air; consequently that which is incumbent over the sea will be warmed more than that over the land.
"By these facts, and by a similar mode of reasoning, as much as by the principle of evaporation, I would explain why clayey and deep soils which are most retentive of moisture should be the longest in heating or cooling; why the returning sun-beams cannot so soon accumulate heat on the surface of those soils; and why, under such circumstances, vegetation at the spring is so much more
