TR UE TIME TAKEN BY REG ULAR OBSER VA TIONS. 45. 
the "right ascension" or sidereal time of its meridian passage. The right ascen- 
sion of our 300 stars known as the "standard time stars" have been determined 
with all attainable precision. The greatest variation, if any, cannot exceed the 
tenth part of a second in time ; they are distributed all the way round the circle 
of the heavens at intervals of about five minutes for convenience of observers who 
require accurate time at all hours of the night, the brightest can be observed even 
in daylight, so at all times they are available (if the sky be clear) without much 
delay. A catalogue of them is published every year by the United States naval 
observatory, Washington, for the use of American astronomers and navigators. 
All that is necessary in order to set and regulate a time-piece is a transit in- 
strument adjusted, coUimated, or placed at a right angle with its axis, and set to 
swing north and south, the axis being level. The one I have in use here is a 
two and three-quarter inch aperture achromatic of forty-eight inches focus, mag- 
nifying seventy-five diameters; have it in fine adjustment and set in the meridian, 
stars observed south and north of the zenith agree in time to a fraction of a 
second. This proves it to be in the meridian and the time correct. The passage 
of the star is observed at five equidistant vertical wires, two on each side of the 
meridian wire on line. The instant at which the star is bisected by each wire is 
noted in a new chronograph, a very ingenious machine that works on the follow- 
ing principle : A round disk of paper placed upon a horizontal block of wood 
four inches in diameter, is made by accurately running clock work to turn around 
once in a minute of sidereal time, or nearly so (a slight variation may be corrected.) 
A needle point is arranged a short distance above the moving disc, which simul- 
taneously with the pressing of the spring, makes a hole in the paper and 
re-adjusts itself in position in less than the hundredth part of a second. At each 
round of the disc, the needle point moves in toward the center of the paper the 
sixteenth part of an inch, which prevents confusion in distinguishing the punches. 
To illustrate its use take the transit of Procyon, April 10, 1883. From the cata- 
logue of stars I found that at 7 h., 33m. 12. 17s Procyon would be on the meridian. 
So at about three minutes before that time I arranged and placed the disc of 
paper in position and started the chronograph running, went to the clock I wished 
to time and watched it until it indicated yh., 31m., os. exactly, then touched the 
spring which marks the paper, carried it to the transit, and, simultaneously with 
the star's passing the first vertical wire, I again pressed the spring, and at the 
second, third, and so on until the five were passed, then removed the disc of 
paper and placed it on the reading dial, which showed by the relative position of 
the marks, the interval from the time of the mark made at the clock to those 
made at the transit. In the above observation the time noted was as follows : 
Min. Sec. 
First wire i 46.2 . 
Second wire 2 00.3 
Meridian wire 2 14.4 
Fourth wire 2 28.6 
