Scientific Lectures. 127 



rotation, the body to be vibrated is brought in contact with the rotat- 

 ing teeth. 



I have here such an arrangement in the form of a barrel, with 

 various numbers of teeth at different parts, so that if a card is held 

 against one part, it will be struck twenty-four times in each revolution 

 of the barrel, at another part twenty-seven times, at another thirty 

 times, and so on. By means of a system of band wheels, we can give 

 this barrel a rapid and yet steady rotation. If, while it is thus rotating, 

 I bring this card in contact with the part where the thirty-two teeth 

 are placed, you hear a loud sound, and as I move it successively to the 

 points where it is struck more and more frequently by the more closely 

 arranged teeth, you perceive that the tone becomes more and more 

 shrill, and in fact runs up through the notes of the natural scale or 

 gamut. 



This apparatus is named from its inventor, a renowned investigator 

 in the subject of sound, Savart's Wheel. (See Fig. 7.) 



We will now see if, by simply exalting the rate of the vibratory 

 motions which in this case have developed sound, we can in exactly 

 the same manner produce the other forces, heat and light. 



We must not, however, forget that the gap between the rate of 

 sound vibrations and of those constituting heat is a very great one, 

 i. e., from about 10,000 to 200,000,000,000,000 in a second, and that 

 we must therefore take means greatly to exalt the rate of vibrations 

 which we wish to produce. Remembering that the finer and smaller 

 and closer the teeth, the higher the note or the more rapid the vibra- 

 tions, we substitute for the toothed wheel a brass tube with a smooth 

 surface, knowing that, under the microscope, a polished surface is like 

 a file or a sheet of sand paper. By a more powerful combination of 

 wheels and belts we give this tube as rapid a motion as we can, and 

 then press against it pieces of wood. Moreover, that it may be evi- 

 dent to all of you that heat is developed, I pour some cold water into 

 the tube and cork it tightly. In less than a minute, you see the cork 

 is blown out with a loud report, and is followed by a puff of steam, 

 showing that the water in the tube was actually made to boil by the 

 heat vibrations developed by the impact of the asperities of the tube 

 against the wooden pieces. The expulsion of the cork gives us, by 

 the way, an illustration of the opposite conversion, i. e., of the heat 

 vibrations into sensible motion, or, as we call it, mechanical force. 



We will now advance one step further and attempt the production 

 of light in just such a method as that which we have already used for 

 sound and heat. 



