_Mrs. B._ Since so large a portion of the atmosphere, as is over the torrid zone, is in continued agitation, these agitations in an elastic fluid, which yields to the slightest impression, must extend every way, to a great distance; the air, therefore, in all climates, will suffer more or less perturbation, according to the situation of the country, the position of mountains, valleys, and a variety of other causes: hence it is easy to conceive, that almost every climate, must be liable to variable winds; this is particularly the case in high latitudes, where the earth is less powerfully affected by the sun's rays, than near the equator. _Caroline._ I have observed, that the wind, whichever way it blows, almost always falls about sun-set. _Mrs. B._ Because the rarefaction of air in the particular spot which produces the wind, diminishes as the sun declines, and consequently the velocity of the wind, abates. _Emily._ Since the air is a gravitating fluid, is it not affected by the attraction of the moon and the sun, in the same manner as the waters? _Mrs. B._ Undoubtedly; but the ærial tides are as much greater than those of water, as the density of water exceeds that of air, which, as you may recollect, we found to be about 800 to 1. _Caroline._ What a prodigious protuberance that must occasion! How much the weight of such a column of air, must raise the mercury in the barometer! _Emily._ As this enormous tide of air is drawn up and supported, as it were, by the moon, its weight and pressure, I should suppose, would be rather diminished than increased? _Mrs. B._ The weight of the atmosphere is neither increased nor diminished by the ærial tides. The moon's attraction augments the bulk, as much as it diminishes the weight, of the column of air; these effects, therefore, counterbalancing each other, the ærial tides do not affect the barometer. _Caroline._ I do not quite understand that. _Mrs. B._ Let us suppose that the additional bulk of air at high tide, raises the barometer one inch; and on the other hand, that the support which the moon's attraction affords the air, diminishes its weight or pressure, so as to occasion the mercury to fall one inch; under these circumstances the mercury must remain stationary. Thus, you see, that we can never be sensible of ærial tides by the barometer, on account of the equality of pressure of the atmosphere, whatever be its height. The existence of ærial tides is not, however, hypothetical; it is proved by the effect they produce on the apparent position of the heavenly bodies; but this I cannot explain to you, till you understand the properties of light. _Emily._ And when shall we learn them? _Mrs. B._ I shall first explain to you the nature of sound, which is intimately connected with that of air; and I think at our next meeting, we may enter upon the subject of optics.