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76 All these things write their own story on this piece of paper in front. I wonder if you can guess what is the use of that ping-pong ball tied to a thread? It tells the force of the wind, because the wind blows the ball away from the kite, so that it pulls the string, and the more it pulls the more it writes on this diagram. Then presently the kite is hauled down, and this diagram is read; and from this we find out what has been going on in the upper air. When these kites are up miles high, the pull on the wire (they use wire and not string) is tremendous, and special machines are necessary for winding the wire.

But lately it has been found better to use balloons instead of kites; balloons filled with hydrogen, which is so light that it takes them up for miles and miles before they burst. They always burst at last, because the pressure of the air outside keeps getting less and less as we go up (you remember how we illustrated that with the totem-post pictures), but the hydrogen inside does not get less, and so swells the balloon out more and more. I told you that if there were no air outside us, we might burst; and that is actually what happens to the balloon when it gets so high that the air outside it scarcely presses it at all. And then down it comes like a parachute, bringing the precious records, like those attached to the kite. But of course the kite is pulled down gently and smoothly by means of its wire, whereas the balloon-case drops from a great height, and it is important to prevent a jar on reaching the ground which might damage the instruments which make the records. They are costly, and must be used again and again if possible. Accordingly the