364 
POPULAR SCIENCE REVIEW. 
It should be further mentioned, as respects the distribution 
of time in London, that hourly signals are received at the Post 
Office ; and also at the Westminster clock, for facility of regu- 
lating it ; the clock gives also return signals to Greenwich, by 
which its error is there known. As the clock is not controlled 
or in any way acted upon by galvanic current, it may deviate 
slightly from true time ; hut its deviations are small, and seldom 
amount to more than two seconds of time. 
We have now to consider the use made of the hourly signals 
which pass from the Royal Observatory in the line terminating 
at the London Bridge Railway-station. These signals, with the 
exception of that at 1 h. p.m., are placed at the disposal of the 
South-Eastern Kailway Company, who, in return, accord to the 
Koyal Observatory the necessary wire communication at 1 h. 
for an important special purpose. Mr. Walker, to whom the 
establishment of these relations is due, distributes the signals 
received by him on the lines of the South-Eastern Railway, 
principally for regulation of the station clocks, &c. We have 
before us his Table of the Distribution of Time Signals , show- 
ing how extensively and regularly the daily work of distribution 
is done. Many signals are sent by hand, but some are accurate 
transmissions of the Greenwich current itself, and these are 
managed as follows : — At London Bridge a clock (one of those 
controlled by the Greenwich normal clock) acts the part of an 
automatic switch. At each hour it, as it were, turns some rail- 
way or other wire on to the Greenwich wire, so that at different 
hours the signal is transmitted in a different direction. The 
special service accorded to the Observatory at 1 h. consists in 
the giving the necessary wire communication for enabling the 
1 1). current from Greenwich to pass directly to Deal, to dis- 
charge a time-signal ball (similar in principle to that at Green- 
wich) placed on the old semaphore tower. The Greenwich 
current, by relay action at Deal, drops the ball, which, whilst 
falling, makes such momentary changes of wire connections as 
causes a signal (the “ return signal ”) to be received at Green- 
wich, showing there that the ball has really been discharged. 
This ball was erected by authority and at the expense of the 
Admiralty, to give time to shipping in the Downs, and it has 
been in use since the year 1855. 
The Deal ball signal is of special use as giving to masters of 
ships the means of obtaining the errors and approximate “ sea 
rates ” of their chronometers ; the rates of chronometers often 
changing when placed on shipboard. Other such signals might 
be very usefully established at important points of our coasts. 
The Astronomer Koyal has himself urged on the attention of 
the Government the desirability of establishing hourly time- 
signals at the Start Point, and the Ship-Owners’ Association of 
