314 HISTORY OF ASTRONOMY. 



clock, at the end of every hour, to advance or put back 

 the hands of the subordinate clocks, so that all should 

 indicate exactly .the same time. This contrivance was as 

 follows : One of the wheels in each clock carries a flat 

 piece in the form of a spiral, which during the hour, 

 slowly raises a weight acting on a lever. The weight, 

 when raised, is sustained like the trigger of a musket, 

 and the spiral piece has a notch in which the lever may 

 be caught. When the instant arrives for the standard 

 clock to regulate all the others, an electro-magnet (ren- 

 dered magnetic by the instantaneous passage of an electric 

 current) attracts its armature, causing the arm of the lever 

 to fall, and in its fall it catches in the notch of the spiral 

 piece, with which the hands of the clock are connected. 

 If during the preceding hour, the clock has gained or lost 

 time, the fall of the lever carries backward or forward 

 the spiral piece, and with it the hands of the clock, so 

 that on every dial the hands indicate exactly the same 

 time. Thus all the clocks of a large city may be made 

 to strike the hour at the same instant. 



For the ordinary purposes of business, it is not neces- 

 sary that the agreement of the clocks should extend to 

 very minute fractions of time ; if this was however de- 

 sired, the standard clock might act directly upon the 

 pendulums of the subordinate clocks, so as to render their 

 times of vibration perfectly equal. 



In the year 1840, Professor Wheatstone, of London, 

 invented an apparatus, called an electro-magnetic clock, 

 for enabling a single clock to indicate exactly the same 



