235

PNEUMATICS

OF THE PHENOMENA OF THE ATMOSPHERE.

"Diffusing gently its enlivening power,

The genial air, we all around us feel

Cheering, though unexplored by human sight."

Mrs. Barbauld.

THE word phenomena, which stands at the head of this lesson, is one of very frequent occurrence in scientific compositions; it means, simply, appearance. It is derived from the Greek verb PHAINOMAI, which signifies to appear; but it is gene rally used to indicate any striking or remarkable appearance. The atmosphere, as the student will find in the following lessons, means that mass of air which surrounds this globe. Various conjectures have been made with respect to the height of the atmosphere: and as we know to a certainty the relative weight of a column of the atmosphere by the height to which its pressure will raise water or mercury in any empty tube, so different calculations have been founded on these data, to ascertain its extent as well as its density at different heights. If the air of our atmosphere was, indeed, every where of a uniform density, the problem would be very easily solved. We should in that case have nothing more to do than to find out the proportion between the height of a short pillar of air, and a small pillar of water of equal weight; and having compared the proportion the heights of these bear to each other in the small, the same proportion will be sure to hold in the great, between a pillar of water 33 feet high, and a pillar of air that reaches to the top of the atmosphere, whose height I want to know. Thus for instance, we find that a certain weight of water reaches 1 inch high, and a similar weight of air reaches 72 feet high: this then is the proportion two such pillars bear to each other in the small. Now if 1 inch of water is equal to 72 feet of air, to how much air will 32 feet of water be equal? By the common rule of .proportion I readily find that 32 feet or 384 inches of water will be equal to 331,776 inches, which makes something more than 5 miles; which would be the height of the atmosphere, was its density every where the same as at the earth, where 72 feet of air were equal to 1 inch of water.

But this is not really the case; for the air's density is not every where the same, but decreases as the pressure upon it decreases; so that the air becomes lighter and lighter the higher we ascend; and at the upper part of the atmosphere, where the pressure is scarce any thing at all, the air dilating in proportion, must be expanded to a surprising degree; and therefore, the height of the atmosphere must be much greater than has appeared by our calculation, in which its density is supposed to be every where as great as at the surface of the earth. In order, therefore, to determine the height of the atmosphere more exactly, geometricians have endeavoured to determine the density of the air at different distances from the earth. The following sketch will give an idea of the method which some geometricians have taken to determine this density, which is preparatory to finding out the height of the atmosphere more exactly.

Let us suppose a pillar of air to reach from the top of the atmosphere down to the earth's surface; and let us also suppose it marked like a standard by inches from the top to the bottom; let us still further suppose, that each inch of air, if not at all compressed, would weigh one grain. The topmost inch then weighs one grain, as it suffers no compressure whatever; the second inch is pressed by the topmost with a weight of one grain; and thus added to its own natural weight or density of one grain, now makes its density, which is ever equal to the pressure, two grains. The third inch is pressed down by the weight of the two inches above it, whose weights united, make three grains, and these added to its natural weight, give it a density of four grains. The fourth inch is pressed by the united weight of the three above it, which together make seven grains, and this added to its natural weight give it a density of eight grains. The fifth inch being pressed by all the former fifteen, and its own weight added, gives it a density of sixteen grains, and so on, descending downwards to the bottom. The first inch has a density of one, the second inch a density of two, the third inch a density of four, the fourth inch of eight, the fifth of sixteen, and so on. Thus the inches of air increase in density as they descend from the top, at the rate of one, two, four, eight, sixteen, thirty-two, sixty-four, and so on, which is called geometrical progression. Or if we have a mind to take this backwards, and begin at the bottom, we may say, that the density of each of these inches grows less upwards in a geometrical progression. If, instead of inches, wes uppose the parts into which a pillar of air is divided to be ex

tremely small, like those of air, the rule will hold good in these as well as in those. So that we may generally assert, that the density of the air from the surface of the earth decreases in a geometrical proportion.

This being understood, should I now desire to know the density of the air at any certain height, I have only first to find out how much the density of the air is diminished to a certain standard height, and thence proceed to tell how much it will be diminished, at the greatest heights that can be imagined. At small heights the diminution of its density is by fractional or broken numbers. We will suppose at once then, for greater ease, that at the height of five miles, or a Dutch league, the air is twice less dense than at the surface of the earth: then at two leagues high, it must be four times thinner and less dense; and at three leagues, eight times thinner, and so on. Instead of Dutch leagues, suppose we took a German league of seven miles, and that it was four times less dense at the height of the first German league, then it would decrease in the same proportion, and be four times less dense than the first, at the second league, that is, sixteen times; and four times less dense than the second, at the third league, that is, sixty-four times; and four times less dense than the third at the fourth league, that is, two hundred and fifty-six times less dense than at the surface. In short, whatever decrease it received in the first step, it will continue to have in the same proportion in the second, third, and so on; and this, as I have said, is called geometrical progression.

Upon the same principle it was attempted to calculate the height of the atmosphere. By carrying a barometer to the top of a high mountain, the density of the air at two or three different stations was easily ascertained. But alas! so feeble are human efforts in endeavouring to comprehend and measure the works of the Great Creator, that this theory was soon demolished. It was found that the barometrical observations by no means corresponded with the density which by other experiments, the air ought to have had; and it was therefore suspected, that the upper parts of the atmosphere were not subject to the same laws or the same proportions as those which were nearer the surface of the earth. Another still more ingenious method was therefore devised.

Astronomers know, to the greatest exactness, the place of the heavens in which the sun is at any one moment of time: they know, for instance, the moment in which it will set, and also the precise time in which it is about to rise. However, upon awaiting his appearance any morning, they always see

the light of the sun before its body, and they see the sun itself some minutes sooner above the mountain top, than it ought to appear from their calculations. Twilight they see long before the sun appears, and that at a time when they know that it is eighteen degrees lower than the verge of the sky. There is then in this case something that deceives our sight; for we cannot suppose the sun to be so irregular in his motions as to vary every morning; this would disturb the regularity of nature. The deception actually exists in the atmosphere. By looking through this dense, transparent substance, every celestial object that lies beyond it is seemingly raised up, in some such manner as we see a piece of money look as if raised higher in a basin filled with water. From hence it is plain, that if the atmosphere was away, the sun's light would not be brought to view so long in the morning before the sun itself actually appears. The sun without the atmosphere, would appear all blazing in light the instant it rose, and leave us in total darkness the instant of its setting. The length of the twilight, therefore, is in proportion to the height of the atmosphere; or let me invert this and say, that the height of the atmosphere is in proportion to the length of the twilight. It is generally found by this means to be about forty-five miles high, so that it was hence concluded either that, that was the actual limit of the atmosphere, or that it must be of an extreme rarity, indeed, at that height.

The density of the air, however, depends not merely on the pressure it sustains, but on other circumstances; so that it varies even at the same height in different parts, and even in the same place at different times, as is seen by the mercury in the barometer rising to different heights according to the state of the weather. Heat in particular is a very powerful cause in rarefying the air. From this circumstance arises one of the most striking and formidable of the atmospherical phenomena, I mean the wind. Wind is nothing but a strong current or stream of air. Whenever, therefore, the air is heated by the sun, or by any other means, it will be rarefied, and less able to resist the pressure of the adjacent air, which will consequently rush in to restore the equilibrium, to speak in the technical language of philosophy, or in plain terms, to reduce the rarefied part to a uniform density with the other. This current of air is sensibly felt near the door of a glasshouse, or wherever there is a large fire. A current of air is also to be perceived rushing through the key-hole, or any chink or crevice, into a heated room. This may serve to give the student a general idea of the causes of winds.

LESSON THE FIRST.

AIR AND ITS PROPERTIES.

1. The science of pneumatics treats of the mechanical properties of elastic fluids, such as their weight, density, compressibility, and elasticity.

2. The air which we breathe, and in which we live, surrounds the earth, and extends to a very considerable height above it on all sides.

3. The air together with the clouds and vapours floating in it, is call the atmosphere. The air is invisible because it is transparent.

4. The motion of the hand or of a stick, or whip, proves the existence of air as a resisting medium.

5. The air, like other bodies, has weight: for if a quart bottle be emptied of air and weighed, it will be found 16 or 17 grains lighter than it was before exhaustion air is therefore between eight and nine hundred times lighter than water.

6. The weight of the air is variable; its changes and relative weights are ascertained by means of the barometer; for as the mercury is higher or lower, the air is heavier or lighter.

7. The air is heavier near the earth's surface, and lighter as we ascend, till at length, in the higher regions of the atmosphere, it is almost nothing at all.

8. The air being a heavy body, presses like other fluids in every direction upon whatever is immersed in it, and its pressure is equal to that of a column of quicksilver, about 29 or 30 inches in height.

9. The pressure upon a common 'sized man is equal to more than 30,000 lb. weight, because the surface of his body is about 14 square feet, or 2016 inches, and a column of quicksilver 29 inches high and 1 inch square, weighs 15lb., therefore the whole pressure will be 2016 × 15 = 30240 lb.

10. The external pressure of the atmosphere would