5 88 THE POPULAR SCIENCE MONTHLY. 



a small weight attached to a silk fibre, which passes over a delicate 

 ivory pulley, and has a counterpoise attached at the other end. Each 

 pulley has an index and circular scale to mark the angle turned 

 through. The extremity of a wire is fixed at a small distance above 

 the surface of the mercury in each tube. If, then, a horizontal shock 

 occur, the mercury rises in the corresponding tube ; but it rises higher 

 in that one which has its long arm to the north. The pulley is turned 

 through a certain angle, which is measured by the index, and at the 

 same time the mercury in rising comes in contact with the fixed wire, 

 and so completes a galvanic circuit which rings a bell, and stops the 

 clock at the exact half-second when the shock occurred. If the shock 

 comes from some intermediate point, two of the indices will be moved, 

 and the direction and intensity can be measured by observing both of 

 them. We have seen up to this point that the instrument will meas- 

 ure the direction and intensity of a shock, will mark the time at which 

 the shock occurred, and will ring a bell to attract the attention of the 

 observer on duty, who may register succeeding shocks, or, if the 

 earthquake has ceased, may reset the apparatus. But this is not all. 

 The galvanic circuit, which is completed at the moment a shock oc- 

 curs, releases at the same instant the pendulum of a second clock, 

 which has been held out of the vertical by means of a detent. This 

 clock allows a roll of paper to be unwound off a drum, as in any regis- 

 tering telegraph, at the rate of three metres an hour. A pencil rests 

 nearly in contact with the strip of paper. It is connected with one arm 

 of a lever, the other arm of which is slightly distant from an electro- 

 magnet. As often as the current passes, this end of the lever is at- 

 tracted to the magnet, and the pencil in consequence is made to press 

 on the paper, to be released only when the current ceases. By this 

 means, then, a continuous history of the earth's trembling is registered, 

 a pencil-mark corresponding to a time of trembling, and a blank space 

 to a period of cessation. 



This instrument is extremely delicate, and registers motions of the 

 earth which are too slight to be perceptible to the human frame. 

 When we examined it, some one happened accidentally to touch the 

 casing of the instrument. The alarm was immediately given by the 

 bell, and the two clocks were respectively checked and put in motion 

 by the galvanic current. 



In the same room there is apparatus for detecting and measuring 

 atmospheric electricity. A gold-leaf electroscope and a bifilar elec- 

 trometer are observed regularly. These are successively put in connec- 

 tion with the conductor. This consists of a disk of metal above the 

 roof of the house connected with an insulated metallic rod, supported 

 vertically, and capable of being rapidly raised by means of a cord 

 passing over a pulley. When not in use this rod is in connection with 

 the ground. In making an observation, the rod with the disk attached 

 is quickly raised, thereby disconnecting it from the ground. The elec- 



