3M HISTORY OF ASTRONOMY. 



graver, so that when the circuit was complete, the paper 

 was entirely free, and a dot was made "by the breaking 

 of the circuit. The paper graduated into seconds by 

 this arrangement, exhibited dots with long intervening 

 spaces, as shown on page 325. 



It was still necessary to introduce some nicer ap- 

 paratus for regulating the motion of the surface upon 

 which the dots were to be registered. Professor Mitchell 

 causes his registering disc to revolve with a uniform motion 

 by connecting it with the driving apparatus of his Mu- 

 nich equatorial. Professor Locke prefers, for the moving 

 power, a centrifugal clock. Professor Bond employs a 

 machine of his own invention, called the spring gov- 

 ernor, and described on page 324. Professor Airy, of 

 Greenwich, employs a large conical pendulum, revolving 

 in a circle, the diameter of which is about equal to the 

 arc of vibration of an ordinary second's pendulum* 



Mr. Saxton's arrangement of registering upon a sheet 

 of paper wound round a cylinder, is the most con- 

 venient which has been hitherto employed. 



The electric method of recording transits has been 

 employed at the Washington observatory exclusively 

 since December, 1849 ; it was introduced soon afterward 

 at the Cambridge, Mass., observatory, and it is now used 

 also at the Greenwich observatory. 



