Raphael Semmes, Memoirs of Service Afloat During the War Between the States

[727] it may have evaporated from the lakes and rivers over which it has passed. When it reaches the extensive plains between the Himalayas and Ghauts, which are the great cotton region of Hindostan, it has not a drop of water with which to nourish vegetation; and if it were to prevail all the year round, those plains would speedily become parched and waste deserts.

Let us see, now, how this catastrophe is avoided. When the sun is in the southern hemisphere, that is, during the winter season, the north-east monsoon prevails in Hindostan. When he is in the northern hemisphere, the south-west monsoon, which is the rainy monsoon, or crop monsoon, prevails. This change of monsoons is produced as follows: Soon after the sun crosses the equator into the northern hemisphere, he begins to pour down his fierce rays upon Hindostan, and, passing farther and farther to the north, in the latter part of April, or the beginning of May, he is nearly perpendicularly over the Great Deserts marked in the sketch. These deserts are interminable wastes of sand, in which there is not so much as a blade of grass to be found. They absorb heat very rapidly, and in a short time become like so many fiery furnaces. The air above them rarefies and ascends, a comparative vacuum of great extent is formed, and a great change begins now to take place in the atmospheric phenomena. This vacuum being in the rear of the arrow A, or the north-east monsoon blowing over Hindostan, first slackens the force of this wind—drawing it back, as it were. It becomes weaker and weaker, as the furnaces become hotter and hotter. Calms ensue, and after a long struggle, the wind is finally turned back, and the south-west monsoon has set in.

If the reader will cast his eye on the series of arrows, B, C, D, E, and F, he will see how this gradual change is effected. I say gradual, for it is not effected per saltum, but occupies several weeks. The arrow F represents the south-east trade-wind, blowing toward the equator. As this wind nears the equator, it begins to feel the influence of the deserts spoken of. The calm which I have described as beginning at the arrow A, is gradually extended to the equator. As the south-east wind approaches that great circle, it finds nothing to oppose its passage. Pretty soon, it not only finds nothing to oppose its passage, but something to invite it over; for the calm begins