504 
POPULAR SCIENCE REVIEW. 
then, of testing the accuracy of such instruments as are confided to his care, 
under every condition of climate in which they may have to be employed, Mr. 
Hartnup has an apartment suitably fitted up in his observatory, and ingeni- 
ously heated with gas, in such a manner as to secure for him, if requisite, 
a considerable variation of temperature ; and in this room there are ranged, 
side by side, from 100 to 200 ships’ chronometers undergoing the strictest 
comparative scrutiny. 
The formidable list in the report to which reference has already been 
made, is simply a record of the variation of sixty-two chronometers which 
were compared and tested during five weeks in April and May, 1861, and 
also in November and December of the same year ; and glancing our eye over 
the first column, we find that for the week ending November 9th, the mean 
daily rate of variation in the least accurate of these instruments was fifteen 
seconds, whilst the most accurate showed no variation whatever ; and the 
average error in the whole sixty-two appears to be about 3i seconds per day. 
This speaks well for the makers of these instruments, upon the accuracy of 
which, as already stated, so many interests are dependent. 
The Astronomer of the Mersey Docks and Harbour Board does not, how- 
ever, limit the employment of his energies to the attainment of precision in 
the working of marine chronometers for insuring the safety of life and pro- 
perty on the high seas, he also influences the movements of his townsmen 
through the time-balls and town-clocks which serve as their guide in daily 
life ; and although perhaps not the most important, this will be to our 
readers the most interesting portion of his labours, and we shall communicate 
the results obtained by him in the words of his report : — 
“ All other observations taken with the Transit Instrument have been used 
for the determination of Greenwich mean time, which has been regularly 
communicated to the port by means of time-balls and clocks. The large 
clock in the Victoria Tower has six dials, each of which is eight feet in 
diameter. This clock lias now been successfully controlled from the Obser- 
vatory for upwards of two years, and the first blow of the hammer at each 
hour of the day indicates Greenwich mean time. On a calm day the striking 
of this clock may be heard at the Observatory, and we frequently compare it 
with the Normal clock. The time which sound takes to pass from the clock 
to the Observatory is two seconds and six-tenths, and so perfectly does the 
small clock at the Observatory control the large one at the Tower, that no 
sensible difference between the calculated and observed time can be detected. 
The striking is not, however, the most convenient test that we have of the 
accuracy of its performance. The large clock is made to send a galvanic cur- 
rent once every minute, and by so doing the needle of a delicate galvanometer 
is deflected at the Observatory, and the deflection of this needle is found to 
be simultaneous with the beat of the Normal clock. I am not aware that 
any clock so large as this was ever before made to perform with such marvel- 
lous accuracy. The mechanical arrangement for making this clock drop the 
time-ball has been found to answer well. For communicating Greenwich 
mean time to ships lying in the river or docks, a time-ball in so conspicuous 
a place as the top of the Victoria Tower is, I believe, the best means that 
could be adopted ; but, for the convenience of those engaged in the manufac- 
ture and regulation of chronometers, and for the public in general, a clock 
