The Botanic Garden, a Poem in Two Parts. Part 1: the Economy of Vegetation

by Erasmus Darwin

SOUTH-WEST WINDS.

The velocity of the surface of the earth in moving round its axis diminishes from the equator to the poles. Whence if a region of air in this country should be suddenly removed a few degrees towards the north it must constitute a western wind, because from the velocity it had previously acquired in this climate by its friction with the earth it would for a time move quicker than the surface of the country it was removed to; the contrary must ensue when a region of air is transported from this country a few degrees southward, because the velocity it had acquired in this climate would be less than that of the earth's surface where it was removed to, whence it would appear to constitute a wind from the east, while in reality the eminent parts of the earth would be carried against the too slow air. But if this transportation of air from south to north be performed gradually, the motion of the wind will blow in the diagonal between south and west. And on the contrary if a region of air be gradually removed from north to south it would also blow diagonally between the north and east, from whence we may safely conclude that all our winds in this country which blow from the north or east, or any point between them, consist of regions of air brought from the north; and that all our winds blowing from the south or west, or from any point between them, are regions of air brought from the south.

It frequently happens during the vernal months that after a north-east wind has passed over us for several weeks, during which time the barometer has flood at above 301/2 inches, it becomes suddenly succeeded by a south-west wind, which also continues several weeks, and the barometer sinks to nearly 281/2 inches. Now as two inches of the mercury in the barometer balance one-fifteenth part of the whole atmosphere, an important question here presents itself, what is become of all this air.

1. This great quantity of air can not be carried in a superior current towards the line, while the inferior current slows towards the poles, because then it would equally affect the barometer, which should not therefore subside from 301/2 inches to 281/2 for six weeks together.

2. It cannot be owing to the air having lost all the moisture which was previously dissolved in it, because these warm south-west winds are replete with moisture, and the cold north-east winds, which weigh up the mercury in the barometer to 31 inches, consist of dry air.

3. It can not be carried over the polar regions and be accumulated on the meridian, opposite to us in its passage towards the line, as such an accumulation would equal one-fifteenth of the whole atmosphere, and can not be supposed to remain in that situation for six weeks together.

4. It can not depend on the existence of tides in the atmosphere, since it must then correspond to lunar periods. Nor to accumulations of air from the specific levity of the upper regions of the atmosphere, since its degree of fluidity must correspond with its tenuity, and consequently such great mountains of air can not be supposed to exist for so many weeks together as the south west winds sometimes continue.

5. It remains therefore that there must be at this time a great and sudden absorption of air in the polar circle by some unknown operation of nature, and that the south wind runs in to supply the deficiency. Now as this south wind consists of air brought from a part of the earth's surface which moves faster than it does in this climate it must have at the same time a direction from the west by retaining part of the velocity it had previously acquired. These south-west winds coming from a warmer country, and becoming colder by their contact with the earth of this climate, and by their expansion, (so great a part of the superincumbent atmosphere having vanished,) precipitate their moisture; and as they continue for several weeks to be absorbed in the polar circle would seem to receive a perpetual supply from the tropical regions, especially over the line, as will hereafter be spoken of.

It may sometimes happen that a north-east wind having passed over us may be bent down and driven back before it has acquired any heat from the climate, and may thus for a few hours or a day have a south-west direction, and from its descending from a higher region of the atmosphere may possess a greater degree of cold than an inferior north east current of air.

The extreme cold of Jan. 13, 1709, at Paris came on with a gentle south wind, and was diminished when the wind changed to the north, which is accounted for by Mr. Homberg from a reflux of air which had been flowing for some time from the north. Chemical Essays by R. Watson, Vol. V. p. 182.

It may happen that a north-east current may for a day or two pass over us and produce incessant rain by mixing with the inferior south-west current; but this as well as the former is of short duration, as its friction will soon carry the inferior current along with it, and dry or frosty weather will then succeed.