358 
TOPULAR SCIENCE REVIEW. 
to obtain their places for any given time, principally on 
account of small changes of the celestial planes of reference 
which astronomers are compelled to use. Suffice it therefore 
to say, that from the Nautical Almanac may be obtained the 
places of a great number of stars for every day of the year. 
An observation of any suitable star of this list being made with 
the transit-circle, and treated as before described, the result 
compared with the Nautical Almanac right ascension gives 
the error of the sidereal clock, knowing which the error of the 
mean-time clock can be easily found. 
It was always necessary that time should be regularly deter- 
mined at the Royal Observatory for its own special purposes. In 
the year 1833, however, an attempt was successfully made to give 
time by signal to the outer world. A pole, carrying a black 
ball about 5 feet in diameter, was then fixed on one of the 
turrets of the ancient portion of the Observatory. Being raised 
on the pole very shortly before 1 h. (half-way up at 5 m. before 
1 h., and full up at 3 m. before 1 h.), the ball is dropped pre- 
cisely at the instant of 1 h. Greenwich mean time. Its fall 
is at first rapid (this start is the proper instant to note) ; after- 
wards a piston, attached to a rod extending from the ball down- 
wards, entering an open-topped cylinder, the gradual escape of 
the compressed air so checks its fall that it is terminated quite 
gently. The ball was for many years dropped by hand (by 
pressure on a trigger which released the piston), but since the 
year 1852 it has been dropped by automatic galvanic action, 
as will be hereafter described. 
Before proceeding to speak of the wider distribution of 
Greenwich time, we must say a few words in reference to local 
time. This is merely time as determined at any place by 
astronomical observation. Places north or south of a given 
place have the same local time ; places east or west differ. 
Suppose, for instance, two clocks, one at Greenwich and one at 
Bristol, are set right by astronomical observation. Could one 
then be transported to the place of the other, the Greenwich 
clock would be found to be about 1 0 minutes fast of the Bristol 
clock. Or the sun arrives at the Greenwich meridian 10 minutes 
before it arrives at the Bristol meridian. It will be thus under- 
stood how, when railways began to grow up, it became neces- 
sary to employ, for safe regulation of the traffic, one uniform 
time. Greenwich time, now long known as “railway time,” 
came to be adopted, and its use in the country is now universal. 
In course of time the want of some accurate and convenient 
standard of reference seems to have been felt, not alone for the 
service of railways, but also for that of the accompanying tele- 
graphs, which so rapidly sprung up. The Astronomer Royal 
also, viewing the gradual rise of the telegraph system, and 
