OF ARTS AND SCIENCE- 151 



I 

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

 of its length. 



For releasing the pendulum and catching it at the end of its spr 

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

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

 ward force upon the latch-bar to cause it to ensa^e with the catch 

 upou the pendulum bob, and also of lessening the shock produced 

 upon the whole apparatus by the arrest oi the pendulum. The sprint 

 i is fixed inside the tube k, 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 m, which passes through the latch, is brought against the wed^e n, 

 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 >t. that the arc through which the bob is lifted 

 - 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 peudu- 

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

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

 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 kev. /'. with two adjustable screws making contact in mercury 

 cups. When the pendulum hang- by the latch, the key r- sts in 

 contact with the left-hand mercury cup q. through the k fi-hand bo 

 but as the pendulum completes its half-vibration, the pin .«. 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 the pin t, as the pendulum is caught by the latch. 



The cycle of operations performed by the pendulum is explained 

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

 three different kinds of observation : — 



