364 HISTORY OF ASTRONOMY. 



last galvanometer. Again, all the instruments being de- 

 flected, when the battery was cut off, the first galvanom- 

 eter instantly fell to zero ; but the second did not fall 

 until a little while after ; and the third only after a still 

 longer interval a current flowing on to the end of 

 the wire, while there was none flowing in at the be- 

 ginning. 



DIFFERENCES OF DECLINATION RECORDED BY ELECTRO- 

 MAGNETISM. 



Differences of decimation may be recorded by means of 

 electro-magnetism. This is accomplished by inserting in 

 the focus of the meridional telescope two systems of 

 spider lines, one vertical, and the other inclined at an 

 angle of 45. Let A B represent the horizontal wire of 

 the transit instrument, D E the 

 middle vertical wire, and F Gr a 

 wire inclined to the latter at an 

 angle of 45. Let the telescope 

 be pointed upon a star as it ap- 

 proaches the meridian, and let it 

 be bisected by the wire A B, while 

 the time of passing the vertical wire D E is recorded. 

 Let the telescope remain firmly fixed in its position, and 

 suppose a second star enters the field at H and traverses 

 the path H L. Let the instant of passing F Gr at I, and 

 D E at E be recorded. Then if the angle D F is 45, 

 C K (which is the difference of declination of the two 

 stars) will be equal to K I. The line K I is measured by 



