THE ELECTRIC TELEGRAPH. 



telescope. His ear is occupied in noting, and his. mind in 

 counting the successive beats of the pen- 

 Fig- 94- dulum, which in all astronomical clocks is 

 so constructed as to produce a sufficiently 

 loud and distinct sound, marking the close 

 of each successive second. The practised 

 observer is enabled with considerable pre- 

 cision in this way to subdivide a second, 

 and determine the moment of the occur- 

 rence of a phenomenon within a small 

 fraction of that interval. A star, for 

 example, is seen to the left of the wire m m' 



at s, fig. 94, at one beat of the pendulum, and to the right of it at 

 s,' with the next. The observer estimates with great precision 

 the proportion in which the wire divides the distance between the 

 points s and s', and can therefore determine the fraction of a 

 second after being at s, at which it was upon the wire m m'. 



The fixed stars appear in the telescope, no matter how high its 

 magnifying power be, as mere lucid points, having no sensible 

 magnitude. By the diurnal motion of the firmament, the star passes 

 successively over all the wires, a short interval being interposed 

 between its passages. The observer, just before the star approaching 

 the meridian enters the field of view, notes and writes down the 

 hours and minutes indicated by the clock, and he proceeds to count 

 the seconds by his ear. He observes the instant at which the star 

 crosses each of the wires ; and taking a mean of all these times, 

 he obtains, with a great degree of precision, the instant at which 

 the star passed the middle wire, which is the time of the transit. 



By this expedient the result has the advantage of as many inde- 

 pendent observations as there are parallel wires. The errors of 

 observation being distributed, are proportionately diminished. 



When the sun, moon, or a planet, or, in general, any object 

 which has a sensible disk, is observed, the time of the transit is 

 the instant at which the centre of the disk is upon the middle 

 wire. This is obtained by observing the instants which the 

 western and eastern edges of the disk touch each of the wires. 

 The middle of these intervals are the moments at which the centre 

 of the disk is upon the wires respectively. Taking a mean of the 

 contact of the western edges, the contact of the western edge with 

 the middle wire will be obtained ; and, in like manner, a mean of 

 the contacts of the eastern edge will give the contact of that edge 

 with the middle wire, and a mean of these two will give the 

 moment of the transit of the centre of the disk, or a mean of all 

 the contacts of both edges will give the same result. 



By day the wires are visible, as fine black lines intersecting and 

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