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PDF - Wallace Online

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224 TROPICAL NATUREof humidity, as measured by the comparative saturation of theair, is as great as that of Batavia or even greater.A registerkept at Clifton during the years 1853-1862 shows a meanhumidity in January of 90, while the highest monthly meanfor the four years at Batavia was 88 ;and while the lowestof the monthly means at Clifton was 79 '1, the lowest atBatavia was 78 '9. These figures, however, represent animmense difference in the quantity of vapour in every cubicfoot of air. In January at Clifton, with a temperature of 35to 40 Fahr., there would be only about 4 to 4| grains ofvapour per cubic foot of air, while at Batavia, with a temperaturefrom 80 to 90 Fahr., there would be about 20grains in the same quantity of air. The most important fact,however, is, that the capacity of air for holding vapour insuspension increases more rapidly than temperature increases,so that a fall of ten degrees at 50Fahr. will lead to the condensationof about 1^ grain of vapour per cubic foot, while asimilar fall at 90 Fahr. will set free 6 grains. We can thusunderstand how it is that the very moderate fall of the thermometerduring a tropical night causes heavier dews and agreater amount of sensible moisture than are ever experiencedduring much greater variations of temperature in the temperatezone. It is this large quantity of vapour in theequatorial atmosphere that keeps up a genial warmththroughout the night by preventing the radiation into spaceof the heat absorbed by the surface soil during the day.That this is really the case is strikingly proved by what occursin the plains of Northern India, where the dailymaximum ofheat is far beyond anything experienced near the equator,yet, owing to the extreme dryness of the atmosphere, the clearnights are very cold, radiation being sometimes so rapid thatwater placed in shallow pans becomes frozen over.As the heated earth, and everything upon its surface, doesnot cool so fast when surrounded by moist as by dry air, itfollows that even if the quantity and intensity of the solarrays falling upon two given portions of the earth's surface areexactly equal, yet the sensible and effective heat produced inthe two localitiesmay be very different according as theatmosphere contains much or little vapour. In the one casethe heat is absorbed more rapidly than it can escape by radia-

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