324 HISTORY OF ASTRONOMY. 



a cross, C E F Gr, formed of platinum wire, mounted like 

 a wheel upon the axis D. The arm D G of the cross 

 being slightly the heaviest, rests upon one end of 

 a glass tube, Gr H, bent in the form of a syphon, and 

 containing mercury, while the other end, H, of the tube 

 is without the clock case, so that it can be reached 

 without opening the case. The axis D of the cross 

 communicates by a wire with one pole of the battery, 

 while the .end H of the mercury tube communicates with 

 the other pole. As the pendulum approaches to the 

 extreme left of its arc of vibration, it pulls, by means 

 of the fibre B C, upon the arm C D, and lifts the end Gr 

 out of the mercury, thus breaking the electric circuit ; 

 but during the remaining part of each double vibration, 

 the point Gr rests upon the mercury, and the electric 

 circuit is complete. Accordingly, in Professor Mitchell's 

 register, the clock dots are made at intervals of two 

 seconds. 



In October, 1848, Professor W. C. Bond made the 

 drawings for a clock to break the electric circuit, and 

 his clock was completed in 1850. In this clock the 

 axis of the escapement wheel, and also the axis of the 

 steel pallets are insulated by a ring of shellac. Wires 

 from the two poles of the battery are connected with 

 each axis, so that when either pallet comes in contact 

 with an escapement tooth, the electric circuit is closed ; 

 and when the contact is broken (as it must be at 

 every oscillation of the pendulum) the electric circuit 

 is opened. 



