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

by Erasmus Darwin

2. If a current of north wind is passing over us but a few miles high, without any easterly direction; and is bent down upon us, it must immediately possess a westerly direction, because it will now move faster than the surface of the earth where it arrives; and thus becomes changed from a north-east to a north-west wind. This descent of a north- east current of air producing a north-west wind may continue some days with clear or freezing weather, as it may be simply owing to a vertical eddy of north-east air, as will be spoken of below. It may otherwise be forced down by a current of south-west wind passing over it, and in this case it will be attended with rain for a few days by the mixture of the two airs of different degrees of heat; and will prognosticate a change of wind from north-east to south-west if the wind was previously in the north-east quarter.

3. On the eastern coast of North America the north-west winds bring frost, as the north-east winds do in this country, as appears from variety of testimony. This seems to happen from a vertical spiral eddy made in the atmosphere between the shore and the ridge of mountains which form the spine or back-bone of that continent. If a current of water runs along the hypothenuse of a triangle an eddy will be made in the included angle, which will turn round like a water-wheel as the stream passes in contact with one edge of it. The same must happen when a sheet of air flowing along from the north-east rises from the shore in a straight line to the summit of the Apalachian mountains, a part of the stream of north-east air will flow over the mountains, another part will revert and circulate spirally between the summit of the country and the eastern shore, continuing to move toward the south; and thus be changed from a north-east to a north-west wind.

This vertical spiral eddy having been in contact with the cold summits of these mountains, and descending from higher parts of the atmosphere will lose part of its heat, and thus constitute one cause of the greater coldness of the eastern sides of North America than of the European shores opposite to them, which is said to be equal to twelve degrees of north latitude, which is a wonderful fact, not otherwise easy to be explained, since the heat of the springs at Philadelphia is said to be 50, which is greater than the medium heat of the earth in this country.

The existence of vertical eddies, or great cylinders of air rolling on the surface of the earth, is agreeable to the observations of the constructors of windmills; who on this idea place the area of the sails leaning backwards, inclined to the horizon; and believe that then they have greater power than when they are placed quite perpendicularly. The same kind of rolling cylinders of water obtain in rivers owing to the friction of the water against the earth at their bottoms; as is known by bodies having been observed to float upon their surfaces quicker than when immersed to a certain depth. These vertical eddies of air probably exist all over the earth's surface, but particularly at the bottom or sides of mountains; and more so probably in the course of the south-west than of the north-east winds; because the former fall from an eminence, as it were, on a part of the earth where there is a deficiency of the quantity of air; as is shewn by the sinking of the barometer: whereas the latter are pushed or squeezed forward by an addition to the atmosphere behind them, as appears by the rising of the barometer.

TRADE-WINDS.

A column of heated air becomes lighter than before, and will therefore ascend, by the pressure of the cold air which surrounds it, like a cork in water, or like heated smoke in a chimney.

Now as the sun passes twice over the equator for once over either tropic, the equator has not time to become cool; and on this account it is in general hotter at the line than at the tropics; and therefore the air over the line, except in some few instances hereafter to be mentioned, continues to ascend at all seasons of the year, pressed upwards by regions of air brought from the tropics.

This air thus brought from the tropics to the equator, would constitute a north wind on one side of the equator, and a south wind on the other; but as the surface of the earth at the equator moves quicker than the surface of the earth at the tropics, it is evident that a region of air brought from either tropic to the equator, and which had previously only acquired the velocity of the earth's surface at the tropics, will now move too slow for the earth's surface at the equator, and will thence appear to move in a direction contrary to the motion of the earth. Hence the trade-winds, though they consist of regions of air brought from the north on one side of the line, and from the south on the other, will appear to have the diagonal direction of north-east and south-west winds.

Now it is commonly believed that there are superior currents of air passing over these north-east and south-west currents in a contrary direction, and which descending near the tropics produce vertical whirlpools of air. An important question here again presents itself, What becomes of the moisture which this heated air ought to deposit, as it cools in the upper regions of the atmosphere in its journey to the tropics? It has been shewn by Dr. Priestley and Mr. Ingenhouz that the green matter at the bottom of cisterns, and the fresh leaves of plants immersed in water, give out considerable quantities of vital air in the sun-shine; that is, the perspirable matter of plants (which is water much divided in its egress from their minute pores) becomes decomposed by the sun's light, and converted into two kinds of air, the vital and inflammable airs. The moisture contained or dissolved in the ascending heated air at the line must exist in great tenuity; and by being exposed to the great light of the sun in that climate, the water may be decomposed, and the new airs spread on the atmosphere from the line to the poles.

1. From there being no constant deposition of rains in the usual course of the trade-winds, it would appear that the water rising at the line is decomposed in its ascent.