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certain temperatures that exercise a powerful influence on vegetation and on health.
166. The most important question in connection with low temperature is the occurrence of frost. Both to the farmer and to the physician it is of the utmost moment to know how often and with what severity frost may be expected to occur, and when for all practical purposes it ceases to occur, or happens so seldom as to cause no alarm, and call for no precaution on the part of those whose interests may be affected by it. On the other hand, the question to be inquired into in an investigation of high temperatures as affecting agriculture, is not so much injury received as advantage gained by their occurrence. The growth and maturing of crops depend chiefly on the heat they receive from the sun. And in countries such as Scotland, whose mean summer temperature but barely exceeds the minimum heat required for the proper ripening of the staple objects of agriculture, the inquiry becomes invested with a peculiar interest, especially in examining places and localities differing in latitude, proximity to the sea, exposure, and elevation. As regards Great Britain, if the day temperature rises occasionally to 65°, the degree of heat thus received by the grain crops may be looked upon as sufficient for their growth up to the period of flowering; but after this a higher temperature is required, and a frequent day temperature of 70° is necessary to produce the finer qualities of wheat and barley. For other countries different temperatures would require to be investigated-namely, those essential to the successful cultivation of the staple products of the respective countries.
167. In order to render tables of this description practically as well as popularly useful, the occurrence of the critical temperatures should be given separately for every week of the year. Little has yet been done in preparing such tables, owing to the heavy tedious labour in compiling them. Perhaps Scotland is the only country whose climate has been thus examined; it is to be hoped that observers in other countries may be induced to analyse their observations and publish the results. In this way a storehouse of the most valuable information would be collected, by which agriculturists might arrive at a knowledge of the character of the climate they have to deal with; and by which physicians might reason with more certainty than at present regarding the spread of diseases, the rates of mortality peculiar to different countries, and the places to which invalids may be sent, so as to enjoy the greatest safety or receive the greatest advantage that can be procured from a change of climate.
TEMPERATURE-SOLAR AND TERRESTRIAL RADIATION.
168. The interchange of temperature among bodies takes place by conduction, convection, and radiation.
169. Conduction. The communication of heat by conduction proceeds from particle to particle, and implies contact with, or very near approach to, a hotter body. As a class, metals are the best conductors ; solids are better conductors than liquids; and liquids better than gases, which are the worst conductors.
170. The most important illustrations of conduction in meteorology, are the propagation of the changes of temperature downwards through the earth's strata from the surface as it is heated during the day or cooled during the night; and the communication of the same changes of temperature to the lowest stratum of the atmosphere which rests on the surface. As regards the relative conducting powers of different substances, dense soils, or soils having their particles closely packed together, are much better conductors of heat than loose porous soils, because the latter imprison large quantities of air in the interstices between the particles, thus diminishing the conducting power of the soil. From this it follows that light loose soils are subject to higher temperatures, and to a greater degree of frost, near the surface, than dense heavy soils ; but, on the other hand, that frosts and extreme temperatures do not penetrate so far down into light soils as into heavy soils. In Scotland, during the past nine years, the 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