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VISITS TO THE MOON AND PLANETS

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quick, no less than 188,000 miles a second, so that you might think the difference would be very small. But you must remember that instead of the single mile between dwelling and office, we have now the enormous distance between one side of the Earth's orbit and the other, about 186,000,000 miles, or nearly one thousand times the distance travelled by light in one second. Hence Roemer found a difference of nearly 1000 seconds or about 16 minutes, and in this way the velocity of light was found for the first time.

Now, you may say, that is very interesting as a story, but you do not see any particular good can come of knowing that light takes time to travel; such a piece of knowledge is quite remote from our practical everyday life. But very often scientific discoveries of this kind lead to practical results of immense importance. The great man whose statue is in the entrance hall, Michael Faraday, made discoveries as to the behaviour of little bits of wire and glass which seemed equally unpractical; and yet they led to our electric railways, and motor-cars, and aeroplanes, which could never have existed but for Faraday's discoveries made with little bits of wire in this Royal Institution. Another great man following Faraday, James Clerk Maxwell, noticed that the velocity of light, first detected by Roemer, was the same as a measurement made with the electro-magnetic apparatus due to Faraday; and he concluded that light was electro-magnetic in its action. And then another great man, Hertz, said, "If light consists of waves, and is electro-magnetic, then we ought to be able to get electric waves," and he found