It is while these bodies are burning that they look to us like falling stars, and when we see them we know that hey 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 100 miles above the surface of the earth, and so the atmosphere must be more than 100 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 15-pound weight were put upon every square inch of land.  This little piece of linen paper, which I am holding up, measures exactly a square inch, and as it lies on the table, it is bearing 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 understand 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 it 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 know 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.