Again (§ 476-7):

“The agents which produce monsoons reside on the land. These winds are caused by the rarefaction of the air over large districts of country situated on the polar edge, or near the polar edge, of the trade-winds. Thus, the monsoons of the Indian Ocean are caused by the intense heat which the rays of a cloudless sun produce, during the summer time, upon the Desert of Cobi and the burning plains of Central Asia. When the sun is north of the equator, the force of his rays, beating down upon these wide and thirsty plains, is such as to cause the vast superincumbent body of air to expand and ascend. There is, consequently, a rush of air, especially from toward the equator, to restore the equilibrium; and, in this case, the force which tends to draw the north-east trade-winds back becomes greater than the force which is acting to propel them forward. Consequently, they obey the stronger power, turn back, and become the famous south-west monsoons of the Indian Ocean, which blow from May to September inclusive.

“Of course, the vast plains of Asia are not brought up to monsoon heat per saltum, or in a day. They require time both to be heated up to this point and to be cooled down again. Hence, there is a conflict for a few weeks about the change of the monsoon, when neither the trade wind nor the monsoon force has fairly lost or gained the ascendency. This debatable period amounts to about a month at each change. So that the monsoons of the Indian Ocean prevail really for about five months each way, viz.: from May to September, from the south-west, in obedience to the influence of the over-heated plains, and from November to March inclusive from the north-east, in obedience to the trade-wind force.”

What the “trade-wind force” is, Lieutenant Maury tells us in another paragraph, viz.: “Calorific action of the sun and diurnal rotation of the earth”—the received calorific theory. I have already shown, I think, conclusively, that there is no expansion and ascent in the supposed region of calms, which induces, or can induce, the trades; and that, in point of fact, the air on the land is cooler under the belt of rains. But as Lieutenant Maury, whose reputation is national, adopts the theory, I shall be pardoned for copying the following table, showing the difference of temperature at two cities of India, before, after, and while the belt of inter-tropical rains is over them. It will be seen that the temperature is actually less when the belt is there, viz., in July and August, than in April and May. This should be conclusive upon that point.

Months.Anjarakandy.Calcutta.
Rain.Temp.Rain.Temp.
M.M. M.M.
January,2,2626°,50,018°,4
February,2,2627°,767,6821°,5
March,6,7728°,424,8225°,6
April,29,3329°,8130,8428°,5
May,175,9628°,616,2429°,7
June,794,0526°,6575,2429°,3
July,807,5925°,8338,3828,°1
August,572,9826°,0311,3128,°3
September,311,3126°,4254,9128,°0
October,157,9126°,842,8627°,2
November,65,4226°,920,3023°,0
December,29,3326°,50,019°,2
Year,2955,1427°,21928,7426°,4

Anjarakandy is on the Malabar coast, between 12° and 13° north latitude. Calcutta in an angle of the Bay of Bengal, at 22° 30′ north latitude. The former is in and near the focus of the monsoons, and has a temperature in July (when 18 inches of rain fall), about as low as in December.

In the foregoing table from Kaemptz, the rain is in millimetres, about twenty-five of which make an inch, and the temperature is centigrade, which may be raised to Fahrenheit by adding four fifths of the quantity and also 32°—thus, if the height of the centigrade thermometer be 25°, add to this four fifths of 25°, which is 20°, and also 32°, the result is 77°. Twenty-five centigrade is therefore equal to seventy-seven Fahrenheit.

Lieutenant Maury is not, and should not be a theorist. He occupies the position, in some sort, of a national investigator, and, of course, of national instructor. Opinions which emanate from him, or which are endorsed by him, should be accurate. Sooner or later that which he has adopted in relation to the monsoons, and some others, must be abandoned. In addition to what has already been said, I wish to call his, and the reader’s attention, to several other facts and considerations in relation to the monsoons, and particularly those of India.

1st. The deserts of Cobi and Bucharia, which constitute the “burning plains” of Central Asia, north-east of the Indian Ocean, lie between 38° and 45° of north latitude, and under the zone of extra-tropical rains. They are not wholly rainless. They partake of that saline character which affects so much of Asia and the western part of this continent. South of them, running nearly east and west, are the lofty ranges of the Himmalaya and Kuenlun Mountains, and the table lands of Thibet. To their saline character, in part, but mainly to the interposition of these mountain ranges, depriving the counter-trade of moisture, they owe their comparative sterility. If bountifully supplied with rains, this salt would doubtless ere this have been washed to the ocean, as it has been from other countries, once as salt as they. But they have some rain, and more or less vegetation, and are not intensely hot. They lie too far north, and are too elevated. Their temperature is not materially different from that of the western, and comparatively desert portions of our own country, and they are utterly incapable of creating a monsoon at the Indian Ocean, and especially from the long line of Malabar coast, where the south-west monsoons are found in most strength. The sterile portions of Utah, New Mexico, and Texas are alike incapable of such effect upon the atmosphere of Central America and Mexico. These monsoons commence in May, and prevail until October, and the temperature of the air where they blow ranges with considerable regularity between 76° at night, and 84° at mid-day, on the Malabar coast, and a trifle lower in Central America.