THE LENGTH OF OUR VOYAGE
it and even show it to you on the screen; and the method is this: we join two wires of different metals at both ends, and put one junction at the end A of the spinning bar and the other at the end B. Now it is a well-known fact that if one of these junctions be heated more than the other, an electric current will flow in the wires, which we can detect by using this galvanometer. You see that spot of light on the screen? Well, when an electric current flows it will move to right or left; and we want to know which way it moves when the end B is heated. Perhaps one of the ladies in the front row will kindly put her cheek against the end B for a moment. There goes the spot to the right, showing quite a warm cheek! (Perhaps that is better than a cool cheek?) Accordingly we know that if the end B gets warm, the spot will move to the right. Now we will buzz the arm round as quickly as we can, and there goes the spot to the right again, showing that the junction B is heated by its rushing through the air. The end A, you see, is so near the centre that it remains practically stationary.
If we could whirl the arm round ever so much more quickly we might heat the wire so much that it would shine like a meteor, and perhaps be burnt up; but you will easily understand why I cannot do this.
Some meteors, however, are too big to get burnt up. They come right through the air and bury themselves in the ground, as you remember the leaden ball buried itself when Galileo dropped it from the roof. I hope you will never have the ill-luck to be hit by a meteor, for it would hurt much worse than any bullet. Once a monk was sleeping