METEOROLOGY. — TEMPERATURE. 47 1 



111 the state of stability to which the surface of the globe appears 

 actually to have attained, the sun must be considered as the agent 

 which most directly iniiueiicos the temperature of our atmosphere. 

 The length of the day, the number of hours during which the sun is 

 above the horizon, coupled with the height to which he ascends, 

 such is the cause with which the temperature of each particular lati- 

 tude is primarily connected ; and, in looking at the subject practi- 

 cally, it is found to be so precisely ; not only is the mean tempera- 

 ture of the year dependent on the length of the days, and the meridian 

 altitude of the sun, but the mean temperature of each month in the 

 year is essentially connected with the same circumstances. In the 

 northern hemisphere, the temperature rises from about the middle 

 of January, slowly at first, more rapidly in April and May, to reach 

 its maximum point in July and August, when it begins to fall again 

 until mid-January, when it is at its minimum. 



The highest mean annual temperature is, of course, observed in 

 the neighborhood of the equator ; between 0° and 10° or 12° of lati- 

 tude on either side, at the level of the sea, where besides the equal- 

 ity of day and night, the sun, always elevated, passes the zenith 

 twice a year. The observations that have been made up to this 

 time, lead us to conclude that this temperature oscillates between 

 260 and 29" cent. ; 78.8o and 84.2° Fahr. _ 



Did the earth present unvarying uniformity of surface, not only 

 with reference to elevation but to constitution, so that the power of 

 absorbing and of radiating heat should be everywhere alike, the cli- 

 mate of a place would depend almost entirely on its geographical 

 position ; the points of equal temperature would be found on the 

 same parallels of latitude, or, to employ the happy expression intro- 

 duced by M. de Humboldt, the isothermal lines would all be parallel 

 with the equator. But the surface of our planet is covered with un- 

 dulations and asperities, which cause its outline to vary to infinity ; 

 and then the soil is dry, or swampy ; it is a moving desert of sand, 

 or covered with umbrageous and impenetrable forests ; and all this 

 causes corresponding varieties in climate, for the surface becomes 

 heated in different degrees as it is in one or other of these condi- 

 tions. Another very important consideration is, that the surface is 

 a continent, or an island in the ocean : the climate of a country, or 

 a district, is vastly influenced by its proximity to or distance from 

 the sea. The difficulty, the slowness, with which such a mass of 

 liquid as the ocean becomes either heated or cooled, is the cause of 

 the temperate character both of the summers and winters of the 

 shores it bathes, and the islands of moderate dimensions it surrounds. 

 As we penetrate great continents from the sea-board, we find that 

 the temperature both of summer and winter becomes extreme, and 

 the difference between the mean summer and mean winter tempera- 

 ture is great ; and again we find, that places which have considera- 

 bly different latitudes, have still very nearly the same mean annual 

 temperature. The mean temperature of Paris, in latitude 48° 50 , 

 is about 51.4° F. j that of London, in lat 51° 31', is 50.7° F. j that 



