THE ATMOSPHERE. 275 



an ascending current, the heated and expanded air over some 

 spot rising in a vertical column. Dense and colder air flows 

 towards that point producing the horizontal cur/ent which 

 is remarked by an observer on the earth's surface. Some 

 winds are of a very limited range, and depend upon local 

 circumstances ; such are the sea and land breeze experienced 

 upon the coasts of tropical countries. From its low con- 

 ducting power, the surface of the land is more -quickly heated 

 than the sea, so that soon after sunrise the expanded air 

 over the former begins to ascend, and is replaced by the colder 

 air from the sea, forming the sea breeze. But after sunset, the 

 earth's heat being less in quantity, is more quickly dissipated 

 by radiation than that of the sea, and the air over the land 

 becomes dense and flows outwards, displacing the air over the 

 sea, and producing the land breeze. It is obvious that these 

 inferior currents must be attended by a superior current in an 

 opposite direction, or that the air in these winds is carried in a 

 perpendicular vortex of no great extent, of which the motion is 

 reversed twice every twenty-four hours. A grand movement 

 of a similar nature is produced in the atmosphere, from the 

 high temperature of the equatorial compared with the polar 

 regions of the globe ; the air over the former constantly ascend- 

 ing, and having its place supplied by horizontal currents from 

 the latter, within the lower region of the atmosphere. Hence, 

 if the earth were at rest, the wind would constantly blow at its 

 surface, from the poles to the equator, and in the opposite direc- 

 tion in the upper strata of the atmosphere. But the earth, accom- 

 panied by its atmosphere makes a diurnal revolution upon its axis, 

 in which any point on its surface is always passing to a point in 

 space previously to the east of it, and with a velocity propor- 

 tional to its circle of latitude on the globe ; a velocity which is 

 consequently nothing at the poles, and attains its maximum at 

 the equator. The result of this is, that the lower current or 

 polar stream, in tending to the equator, is constantly passing 

 over parallels of latitude which have a greater degree of ve- 

 locity of rotation to the east, than the stream itself, which comes 

 thus to be felt as a resistance from the east ; and instead of 

 appearing as a wind directly from the north as it really is, this 

 stream appears, as a wind from the east with a certain northerly 

 declination, which diminishes as the stream approaches the 

 equator, where it flows directly from the east, constituting the 



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