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BOOK height;) so that the coast of Coromandel, and that of Malabar, have always their dry and their rainy seasons, at opposite periods of the year.

According to the preceding description, it is the southwest monsoon alone which presents any phenomena directly contrary to the general movement of the atmosphere; for the north-east monsoon is in conformity with it, and the north-west wind south of the line seems not to be altogether constant, and may perhaps arise from nothing more than a compound movement, or a higher current of air. What then is the origin of this half-yearly wind, which in summer blows from south and south-west, over all the Indian ocean? The sagacity of physical geographers has long been exercised by this question. We give the explanation of which Halley laid the ground-work, and which appears to us the most plausible.

General The monsoons always change some time after the equiexplanation of the noxes; they constantly blow towards that hemisphere in monsoons, which the sun is found. The action of this luminary on the atmosphere, is, therefore, plainly one of their causes. When its rays, reflected from the mountains of Thibet, scorching the plains of Bengal, and the valleys of the kingdom of Siam, rarefy and dissipate the atmosphere, the cold air becomes violently attracted from the regions about the south pole. The sun's action is seconded by the marine current, which proceeds from the south polar seas to those of India. This current must bring with it a column of vapours, continually disengaging themselves from its surface. The absence of a northern marine current must farther be added; we can even imagine, that the mountains of Thibet, and the whole central platform of Asia, may arrest and preserve the cold air, which would otherwise proceed from Siberia towards India.

But why does not this polar wind prevail south of the equator also? For the same reason which renders the

Deluc, Modifications de l'Atmosphère, No. 730. Muschenbroek, Essai de Physique, ii. 879.

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aquatic polar current inconsiderable there. The general BOOK movement of the ocean being here opposed by no obstacle, has too much force to be modified by the polar current. A similar result happens in the atmosphere, at all times intimately connected with the ocean, which feeds and modifies it. But on leaving New Holland between us and the Pacific Ocean, the general movement of the Indian sea must evidently be more and more abandoned to its individual force, and that force must soon be overcome by the polar current, which, after being long deflected or concealed by the general movement of the ocean, now re-appears in all its energy. The polar column of water now fills the atmosphere with cold particles, which, by their gravity, determine the whole atmospheric mass to flow towards the equator, more strongly and more directly than it would have flowed otherwise. It is possible, moreover, that higher currents may exist in the atmosphere, and descend towards the earth at the time when the monsoons commence.

ticular cir

On the west side, the mountains of Lupata in Africa, Explanaand those of Madagascar, may, or rather must concur in tion of parfurnishing their cloudy and stormy air to the south-west cumstanmonsoon, which, from this circumstance, begins here, in the ces. channel of Mozambique. It is possible that mountains in the interior of New Holland may exert a similar influence on the east side.

When the sun has passed into the southern hemisphere, the monsoon alters its direction; the mass of air which had been accumulated during summer on the central platform of Asia, now bestirs itself, and moves towards the regions south of the equator, where the atmosphere has been dilated and dissipated by the solar heat. Over most part of the Indian Ocean, this monsoon proceeds from the north-east, because the central platform lies to the north-east. On the other hand, as the seas of China, of Borneo, of New Guinea, of Java, have the centre of Asia to the north and north-west, the monsoon comes to them from those points. It arrives in a slow progression, in consequence of the many

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BOOK islands whose elevated summits arrest and turn it aside. The north-east monsoon is mild and agreeable, because the mass of air, condensed on the central platform of Asia during summer, having originally passed through the torrid zone, and afterwards remained exposed to the sun's action about the time of solstice, it has thereby lost the cold and the cloudiness which otherwise it might have acquired from contact with the Siberian atmosphere. It seems possible that this north-east monsoon may fall in with a remnant of the preceding monsoon, about the second or third degree of south latitude; a remnant, which is perhaps maintained in existence by the mountains of Africa, Madagascar, and New Holland, since the cold air of those mountains is not solicited to flow towards the south pole, and has no outlet but towards the equator. From this direct rencounter of the old and the new monsoon, a compound movement would result, which might produce those north-west winds so common between the equator and the tenth southern parallel, during the whole continuance of the north-east monsoon.

General conclusion.

Both monsoons are more powerful in the Arabic Gulf, because this strait and shallow piece of water has no currents in it but such as are superficial, and therefore unable to resist the action of the winds.

All the irregularities displayed by winds pertaining to a place or a region of the globe, are thus nothing more than combined effects resulting from the general atmospheric currents, from partial interruptions of them, from the sun's apparent motion, and the arrangement of mountains.

The reader who has followed us through those wide but indispensable details, may perhaps desire that we should recompense him by again pourtraying the useful or agreeable effects, of which the winds, whose paths we have just been tracing, are productive. Shall we stop to repeat obUtility and servations so familiar? The winds, it is well known, purify pleasures our atmosphere, by keeping up a perpetual agitation in it; from winds. they dissipate the miasmata exhaled from marshes and stagnant water; they raise and transport the clouds destined to fertilize the ground by means of rain. Millions

derived

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of seeds, furnished with their little pinions, ride upon the BOOK wings of the wind, and spread afar the empire of vegetation. The ingenuity of man has made a lever of the winds, which, when applied to machinery, spares him an immensity of toil. If the ocean is the highway of our globe, winds are the indefatigable coursers which rapidly transport our ships from pole to pole. Considering winds merely in a picturesque point of view, how many enjoyments do they procure to a lover of the great spectacle of nature—above all, to the inhabitant of mountains! Sometimes they spread over every valley a curtain of clouds, which shews the summits of the far distant Alps like so many islands scattered on the surface of an ocean; sometimes partially drawing this curtain aside, they open to us all at once the most astonishing prospects, in which the brightest sunshine forms a happy contrast with the contiguous shades. It is to storms of wind, that the painter and the traveller are indebted for the most extraordinary scenes which can meet their view. In the evenings of summer, and still more of autumn, it is the winds which, accumulating and marshalling their long trains of clouds, create and destroy before us those fugitive landscapes, those aërial mountains, which are tinged by the fires of the setting sun.

The atmosphere undergoes various other modifications in respect of its local temperature, or of physical climate. They will form the subject of our next book.

Physical climate.

Its causes.

BOOK XVII.

Continuation of the Theory of Geography. Of the Local Temperature of the Atmosphere, or of Physical Climates.

In no department of geography have authority and preconceived opinions been permitted to prevail more generally than in that which treats of the causes of physical climates. The sun was long considered as the only source of those varieties of temperature which we experience in the different regions of the earth. The influence ascribed to winds was determined only according to some local observations which had been made by Hippocrates in Greece, or in the neighbouring countries. That this branch of physics may be thoroughly investigated, it is requisite for civilized nations not only to despatch travellers to the neighbourhood of the poles and the equator, but also to form permanent establishments in these quarters of the globe. It is by uniting together and arranging, under general points of view, the results of particular local observations, that we shall endeavour to trace a sketch of climatology, in some measure corresponding to the present state of the other sciences.

Physical climate comprehends the degree of heat and cold, the drought, the humidity, and the salubrity, which occur in any given region of the earth.

The causes of physical climate are nine in number: 1st, The action of the sun upon the atmosphere. 2d, The interior temperature of the globe. 3d, The elevation of the earth above the level of the ocean. 4th, The general inclination of the surface, and its local exposure. 5th, The position of its mountains relatively to the cardinal