Energy and Ether. 1025 



that a jar is communicated to the air when a sound is made. Mention 

 has been made of the transfer of vibrations through the air to piano 

 strings, resonators, and the like. Prof. Tyndall exhibited an interest- 

 ing experiment as follows. A sort of drum is made by stretching tis- 

 sue paper over a hoop, and on its surface is sprinkled a light la} T er of 

 sand. A second instrument is made by taking a stick a yard long, and 

 attaching each end of it to the center of a thin bronze disc, the whole 

 resembling two wheels joined by an axle. Now holding the stick in 

 one hand in such position that one of the discs faces the paper drum, 

 rub it up and down with a rosined rag. The shudder given to the 

 stick is communicated to the discs, and thence to the air, and finally to 

 the paper drum which starts into musical vibrations, accompanied by a 

 a dancing movement of the sand which is soon shaken into concentric 

 rings occupying the positions of no vibration. Again, if a cannon be 

 fired in the neighborhood of a house, the concussion of the air is apt to 

 break out the glass on the side next to the gun. Sounds are most com- 

 monly, though not always, originated by a stroke or blow made by one 

 ponderable body upon another. The vibration of one or both the strik- 

 ing bodies certainly takes place, and may be communicated to a third 

 body, provided it possesses a like fundamental. But it is important to 

 observe that when the fundamental of the third body is not the same 

 it will still convey the sound through its molecules, although it will not 

 vibrate; while if it have the same fundamental it will vibrate and 

 "reinforce" the sound, as it is called. If you strike a brick wall with 

 hammer it gives a dull sound which is due to its fundamental, but 

 when you hear the cricket through it, I hold the sound is not conveyed 

 by the vibration of the bricks and mortar, but by that of the enclosed 

 ether. A study of the telephone may throw some light on this subject 

 The construction of the telephone involves the principles, (1) that 

 there is a magnetic field about every magnet, (2) that the state of the 

 field is affected every moment by the activity of the currents in the 

 magnet, and (3) that under certain conditions the current may be made 

 to fluctuate in strength. The telephone consists essentially of a trans- 

 mitter and a receiver connected by a wire. In the Bell telephone these 

 two are just alike, and a description of the transmitter answers for the 

 receiver too. (See fig. 400. ) There is a tube of wood or hard rubber, 

 one end of which is closed by a thin iron plate two inches in diameter, 

 and called the diaphragm. This is placed near the mouth when it is 

 talked to or near the ear of one listening. Inside the tube is a straight 

 cylindrical steel permanent magnet, one end coming forward almost, 

 but not quite far enough to touch the diaphragm or disc. Around the 

 forward or disc end of the magnet is a small bobbin wound with a coil 

 of fine insulated wire, the ends of the coil terminating in the wires 



