WINDS. 283 



_^ , . ^, The trade-windg are caused bv the movements of vast cur- 



What IS the . 



cause of the rents of au* which are continually flowing between the poles 

 trade-winds ? ^j^^j ^^iq equator. Thus the air which has been greatly heated 

 by the sun in regions near to the equator, rises and runs over toward either 

 pole in two grand upper currents, under which there flows from north and 

 south two other currents of colder air to occupy the space vacated, and to re- 

 store the equiUbrium. 



,^ , . 625. In the northern hemisphere the trade-winds blow from 



What occasions , . , , , . , ^ , 



the direction of the north-east, and m the southern hemisphere from the south- 



the trade-winds ? 



east. 



The reason they do not blow from the direct north and south is ovsang to 

 the revolution of the earth. The circumference of the earth being larger at 

 the equator than at the poles, every spot of the equatorial surface mast move 

 much faster than the corresponding one at the poles : when, therefore, a cur- 

 rent of air from the poles flows toward the equator, it comes to a part of the 

 earth's surface which is moving faster than itself; in consequence of which 

 it is left behind, and thus produces the effect of a current moving in the op- 

 posite direction. 



The region over which the trade-winds prevail extends for about 25 degrees 

 of latitude, on each side of the equator, in the Atlantic and Pacific oceans. 



The reason the trade-winds do not blow uninterruptedly from the equator 

 to each pole is owing to the change which takes place in their temperature as 

 they move north and south. Thus in the northern hemisphere the hot air 

 that ascends from the equator and passes north, gradually cools, and becomes 

 denser and heavier, running as it does over the cold current below. The 

 cold air from the pole, too, gradually becomes warmer and hghter as it passes 

 eouth, so that in the temperate climates there is a constant struggle as to 

 which shall have the u^er and which the lower position. In these regions, 

 consequently, there are no uniform winds.* 



What are mon- ^26. MoDsoons arG periodical cuiTeiits of aiF 

 soons? which in the Arabian, Indian, and China seas 

 blow for nearly six months of the year in one direction, 

 and for the other six in a contrary direction. 



They are called monsoons from an Arabic word signifying season ; they are 

 also called periodical winds, to distinguish them from the trade-winds which 

 are constant. 



. . Tlie theory of the monsoons is as follows: — During six 



theory of the months of the year, from April to October, the air of Arabia, 

 monsoons ? Persia, India, and China^ is so rarefied by the enormous heat 



of their summer sun, that the cold air from the south rushes toward these 



• The existence of a great current of air in the upper regions of the atmosphere, flow- 

 Ing in an nearly contrary direction to the trade-winds, has been confirmed by the ob- 

 servations of travelers who have ascended the Peak of Teneriffe, or some of the high 

 mountains in the islands of the Southern Pacific Ocean. At a height of about 12,000 feet 

 a wind is encountered, blowing constantly in au opposite direction to that which prevails 

 «t the level of the sea below. 



