THE WEATHER. 



215 



snow-clad mountain should still be warm and temperate. 

 The truth is, that there is just as much heat combined 

 with an ounce of air on the mountain top as in the valley ; 

 but above, the heat is diffused through a space perhaps 

 twice as great as when below, and therefore is less sen- 

 sible. It may be the same air which sweeps over a warm 

 plain at the side of a mountain , which then rises and 

 freezes water on the summit, and which in an hour 

 after, or less, is playing among the flowers of another 

 valley, as a warm and gentle breeze.' 



2. CURRENTS OF THE AIR. 



(a) Great Current from East to West. 



It is well known that near the equator there is a con- 

 stant wind blowing from east to west. This is called 

 the Trade Wind ; and its regularity is such as is difficult 

 for us to conceive, who live in a country where the 

 variableness of the wind is proverbial. Through the 

 Indian, Atlantic, and Pacific Oceans, this wind blows 

 almost unceasingly. It is interrupted by the land in 

 many places, and in different seasons it blows with 

 greater or less degrees of violence. North and south of 

 the equator, too, it changes from an east to a northeast 

 and southeast wind. 



The cause of this wind is the following. Let N S 

 represent the poles, and 

 E Q, the equator. Now 

 a portion of air at a, re- 

 volving with the earth in 

 the direction of the arrow, 

 moves at the rate of about 

 ten miles an hour; where- E 

 as another portion at 6, 

 upon the equator, moves 

 at the rate of fifteen 

 miles an hour, on account 

 of its revolving in a 

 greater circle. Now in 

 consequence of the heat upon the equator, the air is 

 rarefied and rises, and the air from a, comes down to 

 supply the vacancy. It has however an eastward velocity 

 of only ten miles, while that part of the earth to which it 



