474 
MR. BAILY ON THE CORRECTION OP 
true time of the coincidence*. This is obviously the most correct mode of 
proceeding : more so than by observing only one disappearance, and one 
reappearance ; and much more so than by observing the disappearance only 
or the reappearance only. It has also this convenience, that it obviates the 
necessity of attending to the minute adjustment of the diaphragm ; and the 
eye is not in such case obliged to wander from one side of the pendulum to the 
other, doubting on which side the disappearance or reappearance will take 
place. I consider this part of the experiment as perfect : and that no appre- 
ciable error can occur when this mode of observing the coincidences is 
adopted j\ In the detail of the experiments, the two moments of disappear- 
ance are written one over the other, with a line between, similar to a fractional 
quantity ; and the same, with the reappearances : the mean of the four is 
annexed in the subsequent collateral column. Much has been said about the 
inutility of observing more than one of these pheenomena ; at which I must 
confess, I have been somewhat surprised^. It is perhaps possible that, if the 
same person always made the observations, always under the same circum- 
stances, always with the same magnitude of the disc, always with the same ex- 
tent of the arc (and that not very small,) and always with precisely the same 
quantity of light, no great difference might be found in the results. But, as these 
are cases never likely to occur in practice, and from the nature of the subject 
must be perpetually varying, it is better to adopt a general and sure guide for 
determining the moment of coincidence : and had I not pursued this plan, I 
should in many instances have been led into considerable error. 
The arc of vibration has always been observed by means of a diagonal scale 
affixed to the clock case ; and the divisions can be easily read off to the hun- 
dredth part of a degree. The scale is 7 inches distant from the pendulum, 
* The experiments with the long cylindrical rod (No. 21) form an exception: as, in this case, 
only one side of the rod could be seen in the vacuum tube. 
t I have also adopted another suggestion of Professors Airy and 'Whewell, by removing the dia- 
phragm from the inside of the telescope, and placing it between the pendulum of experiment and the 
clock pendulum. It is, in fact, attached to the clock case ; and is not only capable of being moved 
in every direction, for the purpose of adjustment, but also of being enlarged or contracted, to suit the 
different pendulums employed. 
: See Philosophical Transactions for 1826, page 4 &c. : and the same volume Part II. page 2 &c., 
containing Lieut. Foster’s experiments on the pendulum. See also ( contra ) Captain Sabine’s Account 
of Experiments, pages 217 — 233. 
