OCR Text |
Show 110 STEI'PES AND DESERTS. of heat in America; it will of course be understood that the question respects the gene1·al hygrometric state of the atmosphere, and the temperature of the New Continent as a whole. Single districts, such as the island of Margarita, the Coasts of Cumana and Coro, are as hot and as dry as any part of Africa. It must also be remarked that the maximum of heat at certain hours of a summer's day has been found, on a series of years, to be almost equal at very different parts of the earth's surface, on the Neva, the Senegal, the Ganges, and the Orinoco; being approximately between 27° and 32° Reaumur (93° and 104° Fahrenheit), and generally not higher-providing the observation be made in the shade, at a distance from all solid bodies which could radiate heat to the thermometer, not in an air filled with hot particles of dust or sand, and not with spirit thermometers, which absorb the light. It is probably to fine grains of sand floating in the air, and forming centres of radiant heat, that we must ascribe the dreadful temperature of 40° to 44°.8 Reaumur (122° to 133° Fah.) in the shade, to which my unhappy friend Ritchie, who perished there, and Captain Lyon, were exposed for weeks in the Oasis of Mourzouk. The most remarkable instance of very high temperature, in an air probably free from dust, has been recorded by an observer who knew well how to place and to correct all his instruments with the greatest degree of accuracy. Riippell found 37°.6 Reaumur (110°.6 Fahrenheit), at Ambukol in Abyssinia, with a clouded sky, strong south-west wind, and an approaching thunderstorm. The mean annual temperat.ure of the tropics, or of the proper climate of palms, is, on land, between 20°.5 and 23°.8 Reaumur (or 78°.2 and 85°.5 Fahrenheit), without any considerable difference between the observations collected in Senegal, Pondicherry, and Surinam. (Humboldt, Memoire sur les !ignes isothermes, 1817, p. 54. Asie Centrale, t. iii. Mahlmann, Table iv.) The great coolness, I might almost say cold, which prevails for a considerable part of the year within the tropics on the coast of Peru, causing the thermometer to sink to 12° Reaumur (59° Fahrenheit), is, as I have noticed elsewhere, by no means to be ascribed to the vicinity of the snow-covered Andes, but rather to the fogs (garua) which veil the solar disk, and to a cold sea current which, commencing in the antarctic regions and coming from the south-west, strikes |