414 
MR. E. H. GRIFFITHS ON THE VALUE OF 
Section VIII.— The Measurement or Time, 
As previously mentioned, an electrical clock (made by Mr, Thomas) was used as 
our standard. The energy required was supplied by two Leclanche cells. The pen¬ 
dulum (a seconds one) was very heavy, weighing about ^ cwt. Its rod was of wood, 
and we were unable to detect that any appreciable change in rate was caused by the 
small changes of temperature to which it was exposed. The working parts of an 
electrical clock of this kind are so simple that, wdren once standardized, it is a very 
reliable instrument. The pendulum was fitted with a rough and a fine adjustment, 
and comparisons were made at regular intervals with a (“rated”) Dent’s chrono¬ 
meter. Until August 21 , 1892, the rate of gain was — 1 in 2424; after that date 
the rate of gain was certainly less than 1 in 25,000, and such variations as were 
observed may have been due to temperature changes, as they were irregular and, in 
any case, so small as to render corrections unnecessary. All observations of time 
made before the above date (August 21) were corrected for clock error, before they 
were used in the calculations. The actual uncorrected observation is, however, the 
one given in all columns headed “time.” 
The chronograph, which was worked by a separate battery, was of a somewhat 
peculiar construction. It was contrived so that on “making” the circuit connecting 
it with the clock, the tape was at once started, and, on breaking that circuit, the tape 
came to rest. Thus the observer could, without moving his position, start or stop the 
chronograph by pressing a key.* 
We rarely, however, used this contrivance, because, as our observations lasted over 
considerable intervals of time, the instrument had to be continuously at work. The 
regulator was so adjusted that the tape passed through about f of an inch per second 
—the fractions of a second being rapidly read off by means of glass scales. The 
labour of numbering the seconds was great, for the instrument was not so contrived 
that it could make a distinguishing mark every tenth second.! 
Immediately before the observer were placed two keys connected wdth the chrono¬ 
graph, one of which made permanent connection with the clock pendulum, the other 
was a spring key for recording the times of passing certain temperatures, or the time 
of each 1000 revolutions of the stirrer. 
Some preliminary observations indicated that the personal error the difference 
between the actual time and those read off from the tape) amounted to about of a 
second. As, however, all times were recorded by the same observer, this constant 
“ lag ” was of no consequence. 
* It was found necessary to place the chronograph at a considerable distance from the galvanometer, 
t The Table XIX., given in Section XII., shows that one experiment alone lasted more than 10,000 
seconds. 
