326 



HISTOEY OF ASTRONOMY. 



neously with the event. The beginning of the short line 

 thus printed upon the graduated scale of the register, 

 fixes, by a permanent record, the date of the event. 

 Thus A represents such a record printed upon the gradu- 

 ated paper : 



A 



By tapping upon the key at the instant a star is seen 

 to pass each of the wires of a transit instrument, the ob- 

 servation is instantly and permanently recorded. The 

 usual rate of progress of the fillet under the pen is about 

 one inch per second, and the observations are read off by 

 means of a graduated transparent scale, about an inch 

 square, as represented in the annexed cut, consisting of 

 equidistant and parallel lines, ruled 

 upon a piece of glass, by means of 

 a diamond or etched with fluoric 

 acid. . If the interval between the 

 second dots be greater than the 

 breadth of the scale, the scale is 

 turned obliquely across the fillet, 

 until the first and last divisions exactly comprehend the 

 space between the two second dots. Let the distance 

 from 4s. to 5s. on the above scale, be the distance on the 

 fillet between the fourth and fifth seconds, and let the dot 

 a between them represent the observation. It appears, 

 by inspection, that the observation was recorded between 

 4-7 and 4'8 seconds. The distance of a from the nearest 

 scale division may be estimated to tenths. Thus time is 



