S. P. Langley—Allegheny System of Time Signals. 379 
double wire or “loop,” communicating with the city, is em- 
ployed occasionally for the observatory’s own messages, and 
when (as, for instance, in longitude determinations) it is wished 
to send sidereal time, without interrupting the regular trans- 
mission of signals from the mean-time clock. In the transit 
toom, in the western wing of the observatory, are kept the 
sidereal clock by Frodsham, of London, and the principal mean- 
tme clock by Howard, of Boston. 
On the escape-wheel arbor of this, the standard mean-time 
clock, and turning with it, once a minute, is a wheel cut with 
Sixty sharp radial teeth, of which those corresponding to the 
50th, 51st, 52d, 58d, 54th and 59th seconds of the minute have 
y the minutest lifting of one from the other, and this is 
effected automatically by means of a ruby attached to one of 
them, and placed within reach of the wheel above mentioned. 
As each of these teeth passes, the ruby, and with it the spring, 
is lifted through a minute distance. (Not in practice more than 
One one-hundredth of an inch, and- usually much less.) Once a 
Second, therefore, the circuit is opened during a period of prob- 
ably less than a twentieth ‘of a second, and as the wheel ad- 
vances a tooth with each vibration of the pendulum, the 
armature of the repeater is raised each second of the minute, 
until the 49th is complet 
This action is repeated in every minute of the twenty-four 
hours without variation. The particular second is thus iden- 
tified, but one minute is (so far as the action of the standard 
clock is concerned) not distinguished from another. To do 
