TEXT-BOOK ON WIRELESS TELEGRAPHY CHAPTER I THE EARTH, THE ATMOSPHERE AND THE ETHER IN its first state the earth was a mass of gaseous matter or "nebula" at a very high temperature, revolving round the sun. Through the sons of time it was gradually cooling down, until about 200,000,000 years ago, as calculated by Lord Kelvin, it began to be solid on the outer surface, just as the surface of water turns into ice when cooled. This solidification of the earth's surface continued until it became an irregular solid mass, in the aggregate shaped nearly like a sphere; but the surface is all uneven with high ridges and points called mountains, and deep depressions where the seas and oceans have collected. The cooling of the earth is going on continuously, and the surface is cooler than the interior, as can be easily proved by taking the temperature when one descends a mine. The lower we go the more the temperature rises, and the middle of the earth is still at a very high temperature, as shown by volcanic eruptions. The earth is a ball 8000 miles in diameter, and is surrounded by a mixture of gases called the atmosphere. The principal gases in the atmosphere are nitrogen and oxygen. This atmosphere covers the earth like the chamois skin of a tennis ball, and forms a layer probably about 200 miles thick. It is most dense at the surface of the earth, and gets lighter and lighter until the gases which compose it fade away into nothing; just as a vertical column of smoke which is dense at the bottom gradually spreads out and lightens more and more as it ascends. In the atmosphere of the earth float clouds and vapours formed from dust, water vapour, and gases which have risen from the earth's surface; but the highest of these is never much farther than 4 or 5 miles from the earth's surface. B The earth and its atmospheric envelope is always spinning round on its axis and at the same time is travelling round the sun in a big circle, whose radius is about 93 millions of miles. These rotations of the earth are similar to those of a spinning top, which spins on the floor and at the same time travels round and round in a circle on the floor. It is the spinning of the earth on its axis which causes night and day, and the travel of the earth round the sun which causes winter and summer. Now we must not forget that the earth is only a very small portion of the universe; there are lots of other planets travelling in circles or in elliptical orbits round the sun. We have to ask ourselves: what fills all the space of the universe in which these planets move? Is there anything in that space? The fact that we can see nothing in it does not justify us in assuming that there is nothing. We cannot see the air, i.e. the atmosphere, but we know it is there, and we know many facts concerning it. There is indeed another medium which pervades the whole universe. Evidence of the existence of this medium will accumulate as we proceed, but for the present we will consider one or two very elementary facts which show that such a medium does exist. We know that light and heat come to us from the sun; they travel millions of miles before they reach our atmosphere, and then they travel through the atmosphere until they affect our eyes or sense of touch. We know that light and heat travel in the form of lines or rays: if we let the light come through a hole in a shutter into a darkened room, we see it in the form of what are called rays of light, traced out in the dust particles of the room. Now every student of science knows that light and heat are forms of energy, and energy implies movement of something, so that the light and heat which come to us from the sun across space imply movement of something in that space. It cannot be the atmosphere, for the atmosphere extends a comparatively short distance into that space. There must therefore be some other medium in that space, whose motion conveys these forms of energy to us. It is easy to prove that atmosphere or air has nothing to do with the conveyance of light and heat. If we put an electric bell into a glass globe we can hear it ringing, but if we exhaust all the air out of the globe we can no longer hear the bell, so that the air is necessary for the conveyance of sound. But if we put a lamp into a glass globe from which all the air or atmosphere has been exhausted we shall still see the light of the lamp, and feel the heat radiated from it. It is thus evident that air is not the medium by which light and radiant heat are conveyed. |