OF ARTS AND SCIENCES. 151 



this latter adjustment the wire f is made movable in the direction 

 of its length. 



For releasing the jjendulum and catching it at the end of its spring, 

 the contrivance shown at D is used. The latch h is firmly fastened 

 to the spring t, which serves the double purpose of giving the up- 

 ward force upon the latch-bar to cause it to engage with the catch 

 upon the pendulum boi), and also of lessening the shock produced 

 upon the whole apparatus by the arrest of the pendulum. The spring 

 i is fixed inside the tube h^ which, sliding inside the sleeve /, serves 

 as guide and stop for the motion of the latch. To operate the mech- 

 anism, the tube k is drawn out by the cord attached, and the pen- 

 dulum drawn with it by the latch ; at a certain point, however, the 

 pin ;«, wliich passes thi'ough the latch, is brought against the wedge «, 

 and a slight further motion trips the latch, and the pendulum is 

 released. The point at which the pendulum is released is so adjusted, 

 by sliding the wedge ?«, that the arc through which the bob is lifted 

 is slightly greater than the loss of the arc through friction, etc. in 

 a complete vibration. The latch is drawn up by a cord passing around 

 an axis, L, which is turned by a lever and cord pulled by the observer 

 at the galvanometer. This is to insure that the pendulum is dropped 

 from nearly the same height at each observation, the multiplying 

 arrangement serving to decrease the velocity with which the pendu- 

 lum is raised, so that it rises an exceedingly small distance beyond 

 the point where the latch is tripped. As soon as the pendulum falls, 

 the latch, being at once released by the observer, is drawn back to 

 its lowest position by a spring, not shown in the figure, and the 

 pendulum returning is caught by the latch, and thus, having made 

 a complete swing, is left in its original position ready for the next 

 observation. 



Attached to the frame in which the pendulum swings is a balanced 

 rocking key, p, with two adjustable screws making contact in mercury 

 cups. When the pendulum hangs by the latch, the key rests in 

 contact with the left-hand mercury cup q, through the left-hand screw ; 

 but as the pendulum completes its half- vibration, the pin s, attached 

 to the " fixed " ebonite disk for this purpose, throws the lever over 

 upon the right-hand contact, r, where it remains during the second 

 half-vibration of the pendulum, being replaced in its original position 

 by tlie pin t^ as the pendulum is caught by the latch. 



The cycle of operations performed by the pendulum is explained 

 by the diagram below, which shows the connections as made for the 

 three different kinds of observation : — 



