S. P. Langley—Allegheny System of Time Signals. 381 
The transit instrument in the western wing, is of four inches 
aperture, and with it and the chronograph, observations for 
time are made on every fair night of the year, except on 
vations of each night, after the other corrections are applied, 
and the results determined from the chronograph and the side- 
real clock. The mean error in the resulting determination of 
é sidereal clock correction, is from three to four hundredths of 
a second, but it cannot be assumed that that of the mean time 
standard is known within these limits, except at the time that 
the observations are freshly made. 
t may be desirable to point out where the system pursued 
here differs from that in which a few signals are sent at stated 
hours, as at Greenwich. In the case of the time ball for 
instance, dropped daily by a clock at Greenwich mean noon, 
it 18 customary to compare the mean time clock which drops it, 
with the sidereal time a few minutes before twelve. If it (the 
Operating clock), be slow, it is caused to gain, and if fast, 
caused to lose, an amount needed to bring it to coincidence 
yy noting coincidence of beats by ear. The resulting errors of 
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