674 THE POPULAR SCIENCE MONTHLY. 



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 j>hotographs. 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 earthen- 

 ware knobs (there are specimens of the various kinds in use on the 

 table for inspection), which are employed because they resist the pas- 

 sage of the electrical energy through them, and consecpiently the elec- 

 trical 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 dif- 

 ference 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 press- 

 ure 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 ? Be- 

 cause 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 



