remarkable to those who have never thought about it before. Here it is all around us. It does not seem to be exerting any pressure and yet it is pressing on us all with tremendous force—15 pounds to every square inch. We are all supporting that great pressure, both inside and outside. If we had not the pressure outside, I suppose we should burst; and if we had not the pressure inside we should go flat. We can easily prove there is that kind of pressure by one of those old experiments which I think every generation ought to see, though it has been performed many times, and was first done many years ago: viz. the experiment of the "Magdeburg Hemispheres." We take two cups or hemispheres that are easily pulled apart when there is air both inside and outside. But if we fit them together, and if Mr. Heath kindly connects up the exhaust and takes all the air from inside, they do not, indeed, go flat, because they are made of good solid brass, but we shall find it very difficult to pull them apart. Now the air is sufficiently exhausted, and I want two very strong men from the front row. Mr. Heath and I will stand behind them in case by some accident the air is not all out and they come apart too easily. But you see it is all out, and though these strong giants are pulling as hard as they can, they cannot separate them. Even when Mr. Heath and I help them we cannot do it. We will now let the air in; and you will see it is as easy to pull them apart as it was at first.
We need not always use an air-pump to get rid of air. Sometimes one can get the air out from between one's hands by squeezing them tightly
