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72 like birds. I am sure you have all heard of a "vol plané," which means a soaring flight. And these two different ways of using the word may be illustrated in this way. A piece of paper like this is a plane: and we might put it at the end of a mechanical arm, and find the amount of pressure on it when it is moved about. Several such arms or sails put together would make a toy windmill: and, although that is scarcely a piece of scientific apparatus as it is usually made, a very little alteration would make it into one. In the toy, all the sails are set at the same angle, and they blow round merrily. But suppose we set some one way and some another, so that they want to go round in opposite directions, we could balance them so that the machine would turn neither way, however strong the wind was, and now we have a piece of scientific apparatus. Lord Rayleigh made many experiments with a simple machine of this kind, like a toy windmill, of which he could put the vanes at different angles. I think there were six vanes, and he set two of them in one direction and four in the other, but at a different angle, which he chose, so that the two would balance the four, and then the machine would not go round at all. By varying these angles he learnt a great deal: and the plane sails of his apparatus were aeroplanes. But I can take the same piece of paper and make it into another kind of aeroplane—a thing which soars; you probably know how to make this kind of dart, which soars about. That is a different kind of aeroplane, like those in which people fly. Here is a beautiful little model of an aeroplane that has been kindly lent from the Grahame-White Company. It was the