650 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



the iDstrumeuts in the observatory, who wishes to record the exact 

 time at which a heavenly body passes the cross-wires of his instrument, 

 sends a second series of currents through the same pricker, and by a 

 subsequent inspection of the paper, the pricks of these dots compared 

 with the second dots marked by the sidereal clock gives the time of the 

 observation, which can be read to the x^th P ai> t of a second. 



