Page:Popular Science Monthly Volume 20.djvu/693

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energy to it, and it sets the whole air of this room in motion, and every tympanum of every ear present is struck by the little bullets of air that are thrown into vibration, and the result is the effect called sound. As another illustration, I will take coal. I take a small piece of coal and warm it in an iron cup, so as to cause it to give out a little kinetic energy in the shape of heat, and, on placing it in this jar of oxygen [experiment performed], you see that the mere insertion of coal, slightly heated, in an atmosphere of oxygen, produces a most brilliant effect, which is the result of the conversion of the potential energy existing in the carbon, or coal, into the kinetic energy seen in the shape of motion, heat, and light. We have even more brilliant effect of this conversion of potential into kinetic energy. Here is a piece of magnesium wire which, when heated [light applied], has its potential energy converted into kinetic energy, and a most brilliant light is the result. But we need not go to light, and we need not go to heat, for instances of energy. I have shown you that energy is a form of motion; that it is so found in sound, heat, and light; but we also have energy as magnetism. There is a little toy-duck floating on the basin of water before me, and I merely hold in front of it a magnet, when you will see that a sort of affection is set up between the duck and the magnet, by the duck following the motion of the magnet round the basin. That is another form of energy. The last form of energy that I wish to draw your special attention to is that due to electricity. You have all seen the vivid flashes of lightning and have heard the dreadful roar of thunder; in those two effects we have energy. It is in its potential form when it exists in a charged cloud, and in its kinetic form when that charged cloud gives its charge up to the earth, and in flying to the earth produces that dreadful effect that we call lightning, with its accompaniment, the roar of thunder. Thus, you have illustrations of energy in its different forms, electrical, magnetical, chemical, heat, light, and motion.

Now comes the question, How is this energy transferred from place to place? You may have all stood upon the sea-beach and have seen the waves dashing upon the rocks, and sent up in the air in white and brilliant spray. You have seen the waves rolling over and over again in glorious breakers upon the sands. You have probably seen wrecks dashed to pieces by the force of waves, all which effects are the result of energy which has been imparted to the water by a storm miles and miles away, the energy of that storm having been transferred by the waves of the water till it meets with the resistance of the shore and produces those effects I have mentioned. Again, energy is transferred by air-vibrations. The explosion of a gun can be heard to a distance of twenty to twenty-five miles; and instances are known where the bombardment of a town has been heard at a distance of one hundred miles. It is well known that the roar of the cannon at Waterloo was heard on the English coasts, at a distance of over one hundred