124 ROTATION OF WINDS. SECT. XV. 



and when his intense heat expands the air over the Great Gobi 

 and other arid Asiatic deserts, it ascends ; the N.E. trade-wind is 

 drawn in to fill the vacuum and ascends with it ; then the S.E. 

 trade- wind, being no longer met and balanced by the N.E. trade, 

 passes into the northern hemisphere, and as. ij: proceeds north- 

 ward from the equator it is deflected to the west" by the rotation 

 of the earth, combined with the indraught over the heated 

 deserts, and becomes the S.W. monsoon, which blows while the 

 sun is north of the equator, but as soon as he goes south, and no 

 longer rarefies the air over the Indian deserts, the S.E. trade- 

 wind resumes its usual course, and is then known as the S.E. 

 monsoon. The influence of the heated deserts is perceptible to 

 the distance of 1000 miles from the shore ; the monsoons prevail 

 with great steadiness over the Arabian Gulf, the Indian Ocean, 

 and part of the China Sea. At the change, torrents of rain and 

 violent thunderstorms accompany the conflict between the con- 

 tending winds. 



The Sahara desert in North Africa, and those of Utah, Texas, 

 and New Mexico, occasion the monsoons which prevail in the 

 North Atlantic and on both sides of Central America, and the 

 monsoons which blow to the north of Australia show the sterility 

 of the interior, even if other proofs were wanting. From the 

 powerful effect of the land in drawing off the winds from their 

 course, it may be seen why the N.E. trade-winds are narrower 

 than the S.E. trades. 



In the extra-tropical winds in the North Atlantic, which blow 

 from the 40th parallel to the pole, the north-westerly are to the 

 easterly as 2 to 1 : hence there would be an accumulation of air at 

 the pole at the expense of the equator, did not a current rise at 

 the pole and return to the equator through the high regions of 

 the atmosphere, which confirms the theory of the rotation of the 

 wind. 



There are many proofs of the existence of the counter-currents 

 above the trade-winds. On the Peak of Teneriffe the prevailing 

 winds are from the west. Light clouds have frequently been seen 

 moving rapidly from west to east at a very great height above 

 the trade-winds, which were sweeping along the surface of the 

 ocean in a contrary direction. Rains, clouds, and nearly all the 

 other atmospheric phenomena, occur below the height of 18,000 

 feet, and generally much nearer to the surface of the earth. 



