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temperature at three inches below the surface has fallen to 26°.5 in loose sandy soils, and at a depth of twelve inches the freezing point has only once been observed. But in clay soils, at three inches the lowest is 28°, whilst at twelve inches the temperature often falls to freezing, and even at twenty-two inches 32° has once been recorded.
171. Damp air is a much better conductor of heat than dry air. Damp air consequently feels colder than dry air of the same temperature, in the same way as a marble mantelpiece feels colder than the carpet, though both are at the same temperature, because the heat is conveyed away from our bodies more rapidly. Thus at the breaking of the severe frost which prevailed in Great Britain in December 1860, when the temperature had risen to 32°, and the air become moister, the weather felt more disagreeably chilly than when the temperature was below zero.
172. Snow being composed of crystals, with a very large quantity of air entangled among their interstices, is on that account one of the worst conductors of heat. It thus serves to protect the soil in winter in two ways—(1) it prevents the escape of heat from the earth to the air; and (2) it sets a limit to the depth to which severe frosts penetrate, thus protecting the roots of plants from injury.
173. Convection.---Though fluids and gases are bad conductors of heat, yet they may be quickly heated by a process of circulation of their particles called convection. When heat is applied to the bottom of a vessel containing water, the particles at the bottom, being heated, become lighter and rise to the surface, and other particles descend to take their place. Thus two currents are formed, the hotter ascending through the centre of the vessel, and the colder flowing down the sides. This circulation continues until the whole of the water attains the same temperature.
174. The communication of heat by convection is seen on the most extensive scale over the globe in the winds and in the currents of the ocean. We see it in the ascending and descending currents of the atmosphere everywhere, which are caused by the daily fluctuations of temperature on the surface of the earth; for when the surface is heated by the sun, the air immediately resting on it becomes heated and ascends, and colder particles descend to occupy its place. Under the tropics the air becomes highly heated, ascends, and flows off towards the poles, whilst colder currents flow from the poles to the equator, giving rise to the polar and equatorial currents of the atmosphere. The same cause puts in circulation the waters of the ocean. The great and beneficial effect resulting from these currents is a more equal distribution of temperature, thereby mitigating the rigours of the polar cold, and moderating the scorching heat of the tropics.
175. Radiation.—An interchange of heat is constantly going on among bodies freely exposed to each other, whether their temperature be the same as, or different from, that of the bodies which surround them. If we stand before a fire we feel the influence of its heat, though at some distance from it. This mode by which heat is communicated is called radiation. Radiant heat proceeds in straight lines, diverging in all directions from its source; is only to a limited extent affected by the air through which it passes ; and is not diverted from its straight course by the wind. Its intensity is proportioned to the temperature of the source, is inversely as the square of the distance from the source, and is greater according to the degree of inclination of the surface on which the rays fall.
176. If a body be placed in the presence of other bodies, some colder and some hotter than itself, it will from this mutual interchange of temperature receive more heat from the hotter bodies than it radiates to them, and thus become warmer; but it will receive less heat from colder bodies than it radiates to them, and consequently will become colder. This is the condition in which the earth is placed. When its surface is turned towards the sun, it receives more heat than is radiated from it; but when it is turned from the sun towards the cold regions of space, it gives out more heat than it receives. These two conditions under which the earth is placed are so essentially distinct that the subject of radiation
falls naturally under two heads-viz., solar radiation and terrestrial radiation.
177. SOLAR RADIATION.—(1.) On Land.-Of the solar heat which arrives at the surface of the earth, the part which falls on land is wholly absorbed by the thin superficial layer exposed to the heating rays; and since there is no mobility in the particles of the land, the heat can be communicated downwards only by conduction. While the temperature of the surface increases, a wave of heat continues to be propagated downwards through the soil. The intensity of the daily wave of temperature rapidly diminishes with the depth-the rate of diminution depending on the conducting power of the soil—until at about four feet from the surface it ceases to be perceptible.
178. As regards the effect of the solar heat on the temperature of the air, it is the temperature of the extreme upper layer of the land which requires almost exclusively to be considered, seeing it is chiefly by contact with the surface that the temperature of the air is increased or diminished. Hence it is badly conducting surfaces which have the greatest influence in raising the temperature of the air. For this reason, and on account of the absence of vegetation by which part of the heat would be spent in vaporising the sap, the surface temperature of the sandy deserts of the tropics frequently rises to 120°, 140°, and, more rarely, 200°. When these hot particles of dust are lifting into the air by the winds, the temperature of the air itself has been known to rise to 125° in the shade ; and it is this which gives the sirocco and simoom of the desert their dreaded name. It is in the deserts of Africa and Arabia where the highest temperature on the globe occurs, their mean summer temperature ranging between 92° and 95°. The surface of loam and clay soils is not heated to so high a degree as sandy soils, and the surface of rocks to a less degree, because, being better conductors, the heat is not . left to accumulate on the surface, but is more quickly conveyed downwards.
179. When the earth is covered with vegetation, the whole of the solar heat falls on the vegetable covering; and as none of it falls directly on the soil, its temperature does not rise so high as that of land without any covering to protect it. The temperature of plants exposed to the sun's rays is not so high as that of the soil under the same circumstances, partly because a portion of the solar heat is lost in the process of evaporation from the pores of the plants, and partly because the heat is not allowed to accumulate on the surface of the plants as it does on the soil. For the leaves being thin, and the greater part of their substance being on that account in immediate contact with the air which envelops them, they are quickly robbed of their superfluous heat; and heated air being lighter, it is constantly flowing off and giving way to colder air to supply its place. Hence one chief difference between the climates of two countries, the one covered with vegetation and the other not, consists in the heat being more distributed over the twenty-four hours in the former case, and being less intense during the hottest part of the day.
180. Influence of Forests on Climate.—The effect of vegetation in changing the hours of the distribution of the highest and lowest daily temperatures is most strikingly illustrated in the case of forests. Trees are heated and cooled by solar and nocturnal radiation in the same manner as other bodies. They do not, however, acquire their maximum temperature tin a little after sunset. This occurs in summer at 9 P.M., while in the air the maximum temperature occurs between 2 and 3 P.M. Hence trees may be conceived as reservoirs in which the heat of the day is stored up against the cold of the night. Changes of temperature take place very slowly in the tree, but in the air they are very rapid. Hence the effect of forests on the daily temperature is to make the nights warmer and the days colder, or to give to the climate of countries clad with trees something of the character of an insular climate. Evaporation goes on slowly from damp ground under the trees, being screened by them from the sun's heat. But since the air among the trees is little agitated or put into circulation by the wind, the vapour which arises from the soil is mostly left to accumulate above it among the trees. Hence, though exact observations are wanting to settle the question, it is probable that forests diminish the evaporation and increase the humidity. It also follows that forests lower the summer temperature, and maintain the winter temperature higher than it would otherwise be. This enables us to understand how forests increase the rainfall; for suppose an extensive forest to have its temperature 2o lower than that of the surrounding district, this lower temperature will have the same effect on the rain-bringing winds, as if a low range of hills opposed their course, and condensed their vapour into rain.
181. Thus it is evident that in two large districts, one wooded and the other bare, the distribution of the rainfall during the months of the year and during the hours of the day will differ materially from each other. For the answer to this question, which affects to a great extent the interests of large sections of the human race, especially those dependent on the culture of the sugar-cane, it is to be regretted that there is little else than speculation to guide us. No trustworthy answer can be given till we have a series of hourly observations of the rainfall from two stations under similar meteorological conditions, except that the one is in the centre of an extensive wooded district, and the other in the centre of an extensive district which is clear, or nearly clear, of trees.
182. The valley of Aragua, in Venezuela, is shut in on all sides, and the rivers which water it, having no outlet to the sea, unite and form Lake Tacarigua. This lake during the last thirty years of the past century showed a gradual drying up, for which no cause could be assigned. In the beginning of the present century the valley became the theatre of * deadly feuds during the war of independence, which lasted twenty-two years. During that time, land remained uncultivated, and forests, which grow so rapidly in the tropics, soon covered a great part of the country. In 1822 Boussingault observed that the waters of the lake had risen, and that much land formerly cultivated was at that time under water. The