INTRODUCTORY ESSAY. 75 



grand elements which give the character to the climate, react 

 naturally upon one another/ so that it is not easy to deter- 

 mine which is the cause and which the eflfect. 



For all practical purposes we may regard the sun as the 

 sole source of the temperature of the surface of the globe. 

 If the surface of our planet were uniform, the sun^s heating 

 power would be directly proportional to his altitude, and the 

 mean temperature would diminish equably in receding from 

 the equator. A variety of circumstances disturb this regular 

 gradation of temperature. These are — 1. The more rapid 

 heating and cooling of land than sea, which arises in a great 

 measure from the heat being gradually diffused throughout 

 the ocean (by means of oceanic currents), the hot water from 

 the tropics being thus carried into temperate regions, while 

 the cold water of the Arctic seas occupies its place. Proxi- 

 mity to the ocean, therefore, promotes uniformity of tem- 

 perature. — 3. The elevation of the land above the level of 

 the sea. The sun's heating power is rather augmented at 

 great elevations; but a diminution of temperature at high 

 levels is caused by the rarefaction of the air, and is a conse- 

 quence of the law according to which, the specific heat of the 

 atmosphere increasing inversely with its density, its sensible 

 heat becomes absorbed as it expands. As this law is imiversal, 

 it follows, that when a current of air ascends or descends, its 

 temperature is changed to an amount exactly proportional to 

 the change of level; and it is only when such a current is 

 hotter than the normal temperature of the place whence it 

 ascends, that it is a warm wind at a higher level. — 3. The 

 presence or absence of clouds. These intercept the solar rays 

 during the day, and tend to keep the ground cool. During 

 the night, on the contrary, clouds intercept the radiation of 

 the heat accumulated in the earth during the day, and tend 

 to keep the ground warm. A cloudy climate is hence an 

 equable one, having comparatively cool days and warm nights, 

 cool summers and mild winters. 



When the sky is clear, the air in contact with the earth 

 becomes warmed by radiation from its heated surface; and 



