• 100 cubic inches near the earth weighed 31 grains, while the same quantity taken at four and a half miles up in the air weighed onlv 12 grains, or two-fifths of the weight And now comes another very interesting question. If the air gets less and less dense as it is farther from the earth, where does it stop altogether ? We cannot go up to find out, because we should die long before we reached the hmit ; and for a long time we had to guess about how high the atmosphere probably was, and it was generally supposed not to be more than fifty miles. But lately, some curious bodies, which we should have never suspected would be useful to us in this way, have let us into the secret of the height of the atmosphere. Tliese bodies are the meteors^ or falling stars.

Most people, at one time or another, have seen what looks like a star shoot right across the sky, and disappear. On a clear starlight night you may often see one or more of these bright lights flash through the air ; for one falls on an average in every twenty minutes, and on the nights of August 9th and November 13th there are numbers in one part of the sky. These bodies are not really stars ; they are simply stones or lumps of metal flying through the air, and taking fire by clashing against the atoms of oxygen in it. There are great numbers of these masses moving round and round the sun, and when our earth comes across their path, as it does especially in August and November, they dash with such tremendous force through the atmosphere that they grow white-hot, and give out light, and then disappear, melted into vapour. Every now and then one falls to the earth before it is all melted away, and thus we learn that these stones contain tin, iroa. sulphur, phosphorus, and other substances.

is while these bodies are burning that they look to us like falling stars, and when we see them we know that they must be dashing against our atmosphere. Now if two people stand a certain known distance, say fifty miles, apart on the earth, and observe these meteors and the direction in which they each see them fall, they can calculate (by means of the angle between the two directions) how high they are above them when they first see them, and at that moment they must have struck against the atmosphere, and even travelled some way through it, to become white-hot. In this way we have learnt that meteors burst into light at least loo miles above the surface of the earth, and so the atmosphere must be more than lOO miles high.

Our next question is as to the weight of our aerial ocean. You will easily understand that all this air weighing down upon the earth must be very heavy, even though it grows lighter as it ascends. The atmosphere does, in fact, weigh down upon land at the level of the sea as much as if a is-pound weight were put upon every square inch oi land. This little piece of linen paper, which I am holding up, mea- A square inch of sures exactly a square inch, and paper, as shown in the as it lies on the table, it is bearing lecture. a weight of 15 lbs. on its surface. But how, then, comes it that I can lift it so easily ? Why am I not conscious of the weight?

To umJerstand this you must give all your attention, for it is^important and at first not very easy to grasp. You must remember, in the first place, that the air is heavy because it is attracted to the earth, and in the second place, that since air is elastic all the atoms of ;t are pushing upwards against this gravitation. And so, at any point in air, as for instance the place where the paper now is as I hold it up, I feel no pressure, because exactly as much as gravitation is pulling the air down, so much elasticity is resisting and pushing it up. So the pressure is equal upwards, downwards, and on all sides, and I can move the paper with equal ease any way.

Even if I lay the paper on the table this is still true, because there is always some air under it. If, however, I could get the air quite away from one side of the paper, then the pressure on the other side would show itself. I can do this by simply wetting the paper and letting it fall on the table, and the water will prevent any air from getting under it. Now see ! if I try to lift it by the thread in the middle, I have great difficulty, because the whole 15 pounds' weight of the atmosphere is pressing it down. A still better way of making the experiment is with a piece of leather, such as the boys often amuse themselves with in the streets. This piece of leather has been well soaked. I drop it on the floor, and see! it requires all my strength to pull it up.*

In fastening the string to the leather the hole must be very small and the knot as flat as possible, and it is even well to put a small piece of kid under the knot. When I first made this experiment, not having taken these precautions, it did not succeed well, owing to air getting in through the hole.

I now drop it on this stone weight, and so heavily is it pressed down upon it by the atmosphere that I can lift the^weight without its breaking away from it. Have you ever tried to pick limpets off a rock ? If so, you know how tight they cling. The limpet clings to the rock just in the same way as this leather does to the stone

Soaked leather lifting a stone paper- the little animal exweight. hausts the air inside its shell, and then it is pressed against the rock by the whole weight of the air above.

Perhaps you will wonder how it is that if we have a weight of 15 lbs. pressing on every square inch of our bodies, it does not crush us. And, indeed, it amounts on the whole to a weight of about 15 tons upon the body of a grown man. It would crush us if it were not that there are gases and fluids inside our bodies which press outwards and balance the weight so that we do not feel it at all.

This is why Mr. Glaisher's veins swelled and he grew giddy in thin air. The gases and fluids inside his body were pressing outwards as much as when he was below, but the air outside did not press so heavily, and so all the natural condition of his body was disturbed.

I hope we now realize how heavily the air presse s down upon our earth, but it is equally necessary to

THE AERIAL ocean IN WHICH WE LIVE, understand how, being elastic, it also presses upwards ; and we can prove this by a simple experiment I fill this tumbler with water, and keeping a piece of card firmly pressed against it, I turn the whole upsidedown. When I now take my hand away you would naturally expect the card to fall, and the water to be spilt. But no ! the card remains as if glued to the tumbler, kept there entirely by the air pressing upwards against it.* Inverted tumbler of water with card kept against it by atmospheric pressure.

And now we are almost prepared to understand how we can weigh the invisible air. One more experiment first. I have here {Fig. 15, p. 64) what is called a U tube, because it is shaped like a large U. I pour some water in it till it is about half full, and you will notice that the water stands at the same height in both arms of the tube (A, Fig. 15), because the air presses on both surfaces alike. Putting my thumb on one end I tilt the tube carefully, so as to make the water run up to the end of one arm, and then turn it back again (B, Fig. 15). But the water does not now return to its even position, it remains

Tlie engraver has drawn the tumbler only half full of water. The experiment will succeed quite as well in this way if the tumbler be turned over quickly, so that part of the air escapes between the tumbler and the card, and therefore the space above the water is occupied by mr less dense than that outside.

up in the arm on which my thumb rests. Why is this ? Because my thumb keeps back the air from pressing at that end, and the whole weight of the atmosphere rests on the water at c. And so we learn that not only has the atmosphere real weight, but we can see the effects of this weight by making it balance a column of water or any other liquid. In the case of the wetted leather we felt the weight of the air, here we

A, Water in a U tube under natural pressure effects of air.

B, Water kept in one arm of the tube by Now when we pressure of the air being at the open end only wish tO See the weight of the air we consult a barometer^ which works really just in the same way as the water in this tube. An ordinary upright barometer is simply a straight tube of glass filled with mercury or quicksilver, and turned upside-down in a small cup of mercury (see B, Fig. l6). The tube is a little more than 30 inches long, and though it is quite full of mercury before it is turned up (A), yet directly it stands in the cup the mercury falls, till there is a height of about 30 inches between the surface of the mercury in the cup C, and th at of the mercury in the tube B. As it falls it leaves an empty space above the mercury at B which is called a vacuum^ because it has no air in it. Now, the mercury is under the same conditions as the water was in the U tube, there is no pressure upon it at B, while there

is a pressure of 15 lbs. upon it In the bowl, and there fore it remains held up in the tube.

But why will it not remain more than 30 inches high in the tube ? You must remember it is only kept up in the tube at all by the air which presses on the mercury in the cup. Fig. 16. And that column of mercury C B now balances the pressure of the air outside, and presses down on the mercury in the cup at its mouth just as much as the air docs on the rest. So this cup and tube act exactly like a pair of scales. The air outside is the thlnrf to be weighed at one end as it presses on the mercury, the column C B answers to the leaden weight at the other end which tells you how heavy the air is. Now if the bore of this tube is made an inch square, then the 30 inches Tube of mercury inverted in a basin of mercury. of mercury in it weigh exactly 15 lbs., and so we know that the weight of the air is 15 lbs. upon every square inch, but if the bore of the tube is only half a square inch, and therefore the 30 inches of mercury only weigh y\ lbs,, instead of 15 lbs., the pressure of the atmosphere

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