The sun is perpetually raising enormous quantities of vapour from the ocean which the trade-winds carry to the equator: it is condensed when it rises with the air into the higher strata, and forms a ring of clouds along the southern side of the belt of equatorial calms that surrounds the earth, which, during the day, is perpetually pouring down torrents of rain, while the sun continually beating upon its upper surface dissolves the clouds into invisible vapour which is carried by the winds and condensed into rain on the extra-tropical regions. The whole system of trade-winds, equatorial and tropical calms, with the cloud ring, follow the sun in declination; consequently in its journeys back and forwards it annually travels over 1000 miles of latitude, and regulates the dry and rainy season in the tropical parts of the earth. The monsoons, which are periodic winds in the Indian Ocean, in part depend upon this movement. For when the sun is in the northern hemisphere the trade-winds come northward with him; 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 it proceeds northward 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 contending 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.