EAETH TEEMOES. 313 



In addition to these and other contrivances, experi- 

 ments were made with microphones. 



The microphones used were small doubly pointed 

 pencils of carbon about three centimetres long, saturated 

 with mercury, and supported vertically in pivot holes bored 

 in other pieces of carbon, which were the terminals of an 

 electric circuit. These microphones were screwed down on 

 the top of stakes driven deeply into the ground. They 

 were covered with a glass shade thickly greased at its base. 

 The stakes were in the ground at the bottom of a small 

 pit — about two feet square and two feet deep — which was 

 lined with a box. The box was covered with a lid, and 

 earth to the depth of nine inches or one foot. One of- 

 these pits was in the middle of a lawn in the front of my 

 house, and the other was at the foot of a hill at the back 

 of the house. The wires from the microphone passed 

 through the side of the box into a bamboo tube and 

 thence up to my dining-room and bed-room. In one of 

 the circuits there were three Daniell's cells, a telephone, 

 and a small galvanometer. I used the galvanometer 

 because I found that when there was sufficient motion in 

 the microphone to produce a sound in the telephone a 

 motion in the needle of a galvanometer was produced. 

 If in any case motion took place in the magnetic needle 

 during my absence, it was held deflected by a small piece 

 of iron with which it was brought into contact by the 

 movement. 



The sensitiveness of the arrangement may be judged 

 of from the fact that if a person walked on the grass 

 within six feet of the microphone, each step caused a 

 creak in the telephone, and the needle of the galvano- 

 meter was caused to swing and come in contact with the 

 iron. Dogs running on the grass had no effect. A small 

 stone one or two inches in diameter thrown from the 



