286 REPORT— 1851. 



purpose a second brass lever v w, moving on the centre to, is provided, placed 

 parallel to the other lever m g, and as close behind it as possible without 

 touching, and on the same level, both on top. A mercury cup x, behind the 

 lever m g, is provided, to receive the hooked or hanging down outer end of 

 the lever to v, and the lever is kept up, except at the moment of and after de- 

 pression, by the spring z. When therefore the lever w v is pressed down, 

 contact is made between the poles of a second battery through y- conducting 

 wire, the mercury cup y, the fixed wire y x, the lever itself dipping its amal- 

 gamated extremity v into the cup x, while its other end is connected by the 

 mercury cup of brass and binding screw lo, with the conducting wire w'^. 



As it was necessary tliat this contact, when made, should be for some time 

 maintained, the latch z* catches the top edge of the lever wv as soon as it 

 is depressed, and holds it down into the mercury until relieved, when the 

 spring z throws and holds it up again. 



The backs or top edges of both levers, m g and w i', are covered with 

 grooved slips of baked mahogany, to prevent electric contact with the hand. 



Both are pressed down with perfect ease, and in the most absolutely simul- 

 taneous manner, by a sharp stroke or pressure from the flat palm of the hand 

 pronated over them, and the instant the hand is removed, the clock already set 

 in motion is again stopped. 



I now proceed to describe the construction of the Observer's Chronograph, 

 shown in Plate XIII. fig. 1. This chronograph required to be released or 

 set in motion by the depression of the lever lo v just described, and to be 

 stopped by the hand of the observer at the seismoscope on the instant that 

 the passage of the wave or impulse was seen in that instrument. 



a is the clock and dial as before, h the weight in its case. At the back of 

 the clock is a little iron flat lever (fig. 5, c rf), on the same arbor with 

 c e, which, as in the other case, presses on the anchor y of the escapement, 

 and being constantly held upon it by the spring g, prevents any motion 

 until it is withdrawn. 



h (fig. 5 and fig. 1) is a round bar of iron 4tbs of an inch in diameter 

 and 8 inches long, placed horizontally and secured by an iron arm to the 

 clock case, so that one of its ends shall be within about yo^h of an inch of 

 the surface of the little lever c d. 



Round this bar is coiled about 500 feet of insulated copper wire placed 

 between terminal discs of wood ; when a galvanic current is passed through 

 this coil, the bar h becomes a temporary magnet of considerable power, and 

 acting on the lever c d as its armatisre, forcibly pulls its moveable extremity 

 d towards the pole of the magnet, by which the lever c e is withdrawn from 

 the anchor^, and the chronograph is set in motion. 



The wires w^ and y"^ in fig. 2 are supposed in continuation with those 

 similarly lettered in fig. 1 ; and thus when the contact before described is 

 made by pressing down the lever w v, electric communication takes place 

 (through a mile or more of wire) through w"^, I, one end of the copper coil ; 

 k, the other extremity of it ; and y"^, the battery, being supposed somewhere 

 between. 



This completes the arrangement for releasing or starting the Observer's 

 Chronograph. For stopping it, a spring / is placed just above the anchor, 

 in a convenient position to be kept under the point of the index finger of the 

 observer while engaged in looking into the seismoscope ; the instant he sees 

 the disappearance of the cross wires therein, a slight pressure on the spring 

 brings it into contact with the anchor and stops the qhronograph ; a binding 

 screw m was provided above this, for the purpose of fixing the instrument 

 from all chance of motion until the dials were read off and noted: this was 



