426 TERRESTRIAL PHYSICS. 



The diameter of the wire is about one millimeter, and that of the spiral 

 twenty or twenty-five millimeters. This spiral is supported at its upper 

 extremity by a thin elastic spring, and can be elevated or lowered by 

 means of a screw. The lower extremity of the spiral is terminated in 

 a cone of copper, pointed with platinum, which is kept at a very minute 

 distance from the mercury contained in an iron cup, which is placed on 

 a column of wood or marble. The distance from the point of platinum 

 to the mercury may be varied at pleasure, but once fixed upon it re- 

 mains invariable, in spite of any changes of temperature, owing to a 

 very simple system of compensation ; the rod which sustains the spiral 

 being of a metal which expands upward, with an increase of tempera- 

 ture, as much as the spiral is lengthened downward. The iron cup and 

 the spiral are in communication with the poles of one of Daniell's gal- 

 vanic batteries of two couples. 



If the surface of the earth is agitated by a vertical shock, even 

 scarcely perceptible, the point of ijlatinum will touch the mercury below 

 it, and will complete the current of the battery. Then two electro-mag- 

 nets, which are in the same circuit, will attract their armatures or 

 keepers, and the first will stoj) the running of a clock, which marks the 

 days of the month, the hours, the minutes, and the half-seconds; and 

 will thus register the exact moment of the commencement of the shock. 

 At the very instant the clock stops it gives a signal of alarm by means 

 of a bell. The second electro-magnet in drawing its armature sets free 

 the pendulum of a second clock, which had been stopped out of the ver- 

 tical line, and this clock in running causes a band of i)aper to move at 

 the rate of three meters an hour. At the same time the armature of 

 the second electro-magnet presses a pencil against the paper as it passes 

 over a little pulley, and causes it to trace a series of dots on the paper, 

 corresponding in length to the duration of the shock; the shock having 

 ceased, the paper will continue its movement unwinding from one cylin- 

 der and winding up on another ; and if another shock occurs, the pencil 

 will register it as before by another series of dots, and thus continue ; 

 so that the intervals remaining unmarked will indicate the hours of 

 repose, and the parts marked the duration of the oscillations. 



To other spirals, Ibrmed with difterent numbers of coils, are suspended 

 small magnets, under which are placed some iron filings, which adhere 

 to the magnets when they oscillate verticall.y ; and thus they preserve 

 the trace of vertical shoclis. One of these spirals causes a light needle 

 to move on a graduated arc, and thus measures the extent of the oscil- 

 lations. 



Four glass tubes, each bent at both extremities at right angles, hav- 

 ing thus two vertical branches, united at the middle of the base of the 

 horizontal portion, serve to indicate the horizontal shocks; one of the 

 two vertical branches has a diameter at least double the other, but is 

 shorter. These four tubes are arranged in the direction of the four 

 cardinal points. As they are all alike and act in the same way, it will 



