Page:Popular Science Monthly Volume 20.djvu/694

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miles. Again, we have energy transferred from the sun to the earth by that thin, almost immaterial, subtile medium called the ether, by means of which we have motion transferred from the sun to the earth, which gives us at one moment light, at another moment heat, and at a third moment those effects that result in photographs. Energy is also transferred through matter in the shape of heat. I touch the lamps before me and feel them warm because the heat of the incandescent material which they contain has thrown the molecules of the covering glass into vibration, and that vibration imparts a vibration to my hand that gives the sensation of heat; and if I were to put a poker in the fire—which, no doubt, all of you have done—it would be found in a very short time that the heat of the fire had been transferred through the poker to your hand. Electric currents are a form of energy which is driven through matter. Electricity is transferred, principally, through metals; some of them transfer it with great alacrity—for instance, silver and copper. There are, on the other hand, materials, such as dry wood, glass, gutta-percha, India-rubber, and such like, that scarcely will allow any electrical energy whatever to pass through them; and the result is that bodies are divided into two classes, the one called conductors, that conduct electricity away, and the other called insulators, which prevent electricity from being conducted away. If you have noticed the telegraph-wires passing through the country, you will have seen that they are suspended on little earthenware knobs (there are specimens of the various kinds in use on the table for inspection), which are employed because they resist the passage of the electrical energy through them, and consequently the electrical energy must flow through the wire which they suspend to the distant end, where it is wanted to do work and produce desired effects.

We have now to consider the motive agency. I ought to have said at the first starting, as every boy fresh from school will know, ex nihilo nihil fit, from nothing nothing is created, and therefore there can be no effect without its cause There can be no wind without a difference of pressure in the atmosphere to cause motion of the air. At the present moment I can feel a draught, which is due to a higher pressure above and a lower pressure below on the other side of the door, and the air is circulating over my head from the higher to the lower pressure. The chimney-draughts are produced in a similar manner. Why are meteorologists able to tell us that there is a gale coming? Because they know that there is a higher pressure at one spot than there is on another; and where there is a difference of pressure there must be motion of air. So with water at different levels. The tendency is to produce a common level, and the passage of water from the higher to the lower level produces motion and energy. Tides are simply the result of the effort to produce equilibrium in the sea after it has been attracted in different directions by the force of the sun or the moon. Rivers flow because the water high up in the mountains is at a higher