33 



daily supply of heat to water, then, may be said to float within a moderate depth of the 

 surface, forming a kind of reservoir of heat. On the other hand, water is a good radiant, 

 and as such is continually, both day and night, giving off radiant caloric, which is absorbed 

 by traversing the air, and thereby tends to raise the temperature of the latter medium. 

 Hence, it is most probable that much of the heat so radiated oif is detained in the 

 lower strata of air. Meanwhile a balance is struck in the water itself of the quanti- 

 ties received and parted with, by the preponderance of one or the other of which it gairi= or 

 loses in average temperature in the twenty-four hours. Thus, in the warm season, when 

 days are long and nights short, the general temperature of the air is slowly rising above 

 its annual average, and vice versa in the opposite season. Below a certain depth, however, 

 the temperature of the ocean would appear to be determined by other causes, and to 

 be very little dependent on its superficial amount or fluctuations. It results from the ob- 

 servations of Kotzebue, Beechy, and Sir James C. Ross as a general fact ascertained by 

 thermometric soundings that the deep sea water below a certain level, determined by 

 the latitude, is of invariable temperature throughout the globe, and that a very low one ; 

 the calculations of Lenz founded on Kotzebue, results, giving 36° F., and those of Ross, 

 39"'.5 (which last is the temperature at which pure water attains its maximum of density). 

 The depth at which the fixed temperature is attained, is about 7,200 feet at the equator, 

 diminishing to latitude 56' on either side of that Une, where it attains the surface, and 

 the sea (superficial currents apart) is of equal temperature at all depths. Thence, again, 

 the upper surface of this uniform substratum descends, and at 70° of latitude has already 

 attained a depth of 4,500 feet. Thus the ocean is divided into three great regions ; two 

 polar basins in which the surface temperature is below 39°, and one medial zone above it, 

 attaining 82° at the equator, and at the poles of course the freezing point of sea- water. 

 It is within these respective regions only then, that superficial currents can act as trans- 

 porters of meteorological temperature. 



" The habitudes of dry land with relation to incident heat are very different. There 

 is no mobility of parts, and the communication of heat downwards is therefore entirely 

 a process of conduction. But what is most influential, is the fact that the absorption is 

 performed strictly on the exposed surface, which therefore in the instant of absorption 

 fixes upon itself within a very minute depth all the heat which, falling upon water, would 

 in the same instant be disseminated through many feet or yards of its substance. The 

 mere superficial film then becomes much more heated, and since it is a law of radiation 

 that its intensity increases rapidly with the temperature of the radiant surface, it radiates 

 out on the very instant a much larger fraction of the total incident heat, than in the case 

 of water, besides imparting to the air, by contact communication a proportionally greater 

 amount. In water, the absorbed heat is for the most part withdrawn from the radiant 

 action, enveloped and husbanded. In dry land it is instantly and wholly exposed to such 

 action in its most intense form. It is no uncommon thing in dry and light (i. e. badly 

 conducting) soils, in hot climates to find a superficial temperature of 120° to 140° F.,or 

 even more. 



" That portion of the heat which enters the soil is conducted downwards, and so long 

 as the surface is gaining in temperature a wave of heat is continuously propagated down- 

 wards into the earth. When the surface, however, by the decline of the sun, begins to 

 lose heat, this ceases, and (the radiation still continuing) what may be called a wave of 

 cold (less comparative heat) begins to be propagated, and so on alternately during the day 

 and night. These waves as they run on spread forwards and backwards, and so by degrees 

 neutralize and destroy each other. Thus the diurnal fiuctuations of temperature beneath 

 the surface grow continually less as the depth increases, the rate of diminution depending 

 on the " conductibility " of the soil. In ordinary soils, the difference between the diurnal 

 and nocturnal extremes becomes imperceptible at four feet below the surface. In like 

 manner the general increase of heat due to the summer season, and of cold during winter 

 are propagated in similar, but larger and fullAr annual waves, which, in their turn neu- 

 tralize each other at more considerable depths and become imperceptible at forty or fifty 

 feet. Prof. Forbes has shown in an elaborate memoir on this subject that at depths varying 

 from fifty-seven to ninety-nine feet according to the nature of the soil, the annual 

 variation does not exceed 0°.01 C. 



