3G0 
FOrULAR SCIENCE REVIEW. 
Observatory seven such additional dials. One, fixed in the 
boundary wall, near the entrance gate, for public use, is daily 
consulted by great numbers of people, and three in the Chrono- 
meter Room (two showing seconds only) are used for the rating 
of the navy chronometers. Of the remainder it is necessary only 
to say that one (a small one) is placed on the desk of the Super- 
intendent of the Time Department, for a purpose to be yet 
spoken of. In addition to driving these clocks, the normal 
clock further controls (on the excellent principle introduced 
by Mr. R. L. Jones) several other clocks, situated in London. 
In controlling clocks the galvanic force acts not as the motive 
power, but as an auxiliary only, to keep quite right clocks going 
already very nearly right by their own independent power. 
Mr. Jones effects this in the following way : — Taking an ordi- 
nary clock with seconds pendulum, a galvanic coil is substituted 
for the pendulum bob, the hollow of the coil being placed 
horizontal. At each swing the coil encircles permanent bar- 
magnets fixed to the clock-case. Whilst no current is passing 
no effect is produced, and the pendulum keeps its own rate 
undisturbed ; but if a current, received at each second of time 
from a standard clock, is made to circulate through the coil, 
it becomes for the instant magnetic, and such mutual action 
arises between it and the bar-magnets that the clock (within 
certain considerable limits) will, if losing, be accelerated, and, 
if gaining, will be retarded, the pendulum vibrating in com- 
plete sympathy with that of the standard clock. 
The normal clock at Greenwich thus drives certain clocks 
within the Observatory, and controls others in London. But 
the best clock, when set right, will soon begin to deviate. 
Means must therefore be provided for easily correcting small 
deviations. (The way in which the amount of error is found 
will be immediately described : we speak here only of the 
manner of correcting it when known.) Now as the whole 
system of clocks depends on the pendulum of the normal 
clock, acceleration or retardation of its pendulum will simul- 
taneously accelerate or retard the whole of the clocks. The 
only consideration to be regarded is that alteration must not 
be made too rapidly, otherwise the controlled clocks (which 
are, as it were, guided and not driven) might not follow the 
change. The plan adopted for correcting the clocks (one 
devised by the Astronomer Royal) is the following : — A bar- 
magnet, attached in a vertical position to the normal clock 
pendulum, is carried by an arm projecting from the pendulum 
forwards, it therefore swings with the pendulum. Immedi- 
ately below, in a central and vertical position, a hollow galvanic 
coil is supported on a shelf carried by the clock-case. When 
a galvanic current is thrown into the coil it imparts to the 
