252 HISTORY OF ASTRONOMY. 



Mahler, has an aperture of four and one eighth inches, 

 and a focal length of 65 inches. It has two different 

 modes of illumination; one through the axis as usual,- 

 and the other at the eye-piece, showing bright wires on 

 a dark field. Attached to the eye-piece are two microm- 

 eters for measures both in altitude and in azimuth. 



There is also belonging to the observatory a fine comet- 

 seeker, by Merz and Mahler, having an aperture of four 

 and a quarter inches. The wing on the north side of 

 the tower is designed hereafter to receive a transit in 

 the prime vertical ; but this instrument has not yet been 

 ordered. 



During the summer of 1848, Mr. Bond being engaged 

 with the United States Coast Survey in determining 

 differences of longitude, turned his attention to the electro- 

 magnetic method of recording astronomical observations. 

 The apparatus adopted at this observatory consists of a 

 Grove's battery, a circuit-breaking sidereal clock, and a 

 " spring governor." These are connected by means of 

 copper wires leading to all the principal instruments. 



The spring governor is a machine devised to carry a 

 cylinder with an equable rotary motion, so that it may 

 make one entire revolution in one minute of sidereal 

 time. A sheet of paper is wrapped round the cylinder, 

 and on this paper the commencement of each second is 

 recorded in exact coincidence with the beats of the clock. 

 The observer at each telescope is furnished with a break- 

 circuit key, by which means he is enabled to make a 

 record of his observations on the paper covering the 



