1863.] 259 [Coppee. 



surface of the globule of mercury in a perfect state to receive the 

 next passage of the electricity. 



" So far as known, all other methods are now abandoned, and the 

 mercurial connection is the only one in use. 



*' The Time-scale. — The clock being now prepared to record its 

 beats accurately and uniformly, the next important step was to ob- 

 tain, if possible, a uniformly moving time-scale, which should be ap- 

 plicable to the practical demands of the astronomer. 



" In case the fillet of paper used in the Morse telegraph could 

 have been made to flow at a uniform rate upon its surface, the clock 

 could now record, its beats appearing as dots separated from each 

 other by equal intervals. But it was soon seen that the paper could 

 not be made to flow uniformly; and even had this been possible, a 

 single night's work would demand for its record such a vast amount 

 of paper, that this method was inapplicable to practice. After care- 

 ful deliberation, the ' revolving disk ' was selected as the best possi- 

 ble surface on which the record of time and observation could be 

 made. The preference was given to the disk over the cylinder for 

 the following reasons : The uniform revolution of the disk could be 

 more readily reached. The record on the disk was always under the 

 eye in every part of it at the same time, while on the revolving 

 cylinder, a portion of the work was always invisible. One disk could 

 be substituted for another with greater ease, and in a shorter time, 

 and the measure of the fractions of seconds could be more rapidly 

 and accurately performed on the disk than on the cylinder. 



"After much thought and experiment, it was decided to adopt 'a 

 make circuit ' and ' a dotted scale ' rather than a ' break circuit ' and 

 a 'linear scale;' and I think it will be seen hereafter that in this 

 selection, the choice has been fully justified in practice. These 

 points being settled, the mechanical problems now presented for solu- 

 tion were the following : First, To invent some machinery which 

 could give to a disk of, say twenty inches diameter, mounted on a 

 vertical axis, a motion such that it should revolve uniformly once in 

 each minute of time ; and second, to connect with this disk, the ma- 

 chinery which should enable the clock to record on the disk each 

 alternate second of time, in the shape of a delicate round dot. Third, 

 The apparatus which should enable the observer to record on the 

 same disk the exact moment of the transit of a star across the meri- 

 dian, or the occurrence of any other phenomenon. 



" The first of these problems was by far the most difficult, and 

 indeed, its perfect solution remains yet to be accomplished, though 



