666 ME. MALLET ON THE TEANSIT-VELOCITT OF EAETHQTJAED WAVES. 
the trough next c, where it is produced, to the mid-length where it is observed. This 
involves a correction in the gross transit-time as observed with it. For the methods by 
which the constant for this (seismoscope correction) was determined, I must refer again 
to Report of Brit. Assoc. 1851, pp. 280, 281. It amounts to 0''-065 in time; and as the 
effect of this will in every observation appear to delay the arrival of the earth-wave at 
the instrument, this constant in time^ converted into distance, must be added to the rate 
of wave-transit otherwise obtained. 
The chronograph (originally devised by Wheatstone) is shown in tig. 1*, Plate XXIII. 
It consists, in fact, of a small and finely made clock, deprived of its pendulum, but pro- 
vided with a suitable detent, shown more at large in fig. 4*, by which the action of the 
weight upon it is kept always arrested, but can immediately be permitted to take plaee 
in ghing it motion, upon pressing the hand quickly upon the lever g. 
The running down of the weight causes the anchor and pallets of the escapement [k) 
rapidly to pass the teeth of the escapement-wheel (a), so that the clock “ runs down ” 
by a succession of minute descents, and thus the motion is practically a uniform one. It 
follows that as more weight is added this velocity becomes greater, and by such addition 
the instrument may be made to measure more and more minute fractions of time. 
It registers time upon two dials (fig. 2*), each with an index ; one of these is fixed on 
the axis of the escapement-wheel (a), and its dial is divided into thirty smaller and six 
larger divisions. The pinion on this axis is to the wheel upon the weight-barrel (i) 
as 1:12. This carries the other index, and its dial has twelve divisions, so that one of 
its divisions corresponds to an entire revolution of the former one. The value in actual 
mean time, due to the movement of the instrument as thus recorded, requires to be ascer- 
tained by reference to a clock beating seconds, so that the number of revolutions of the 
index h, and parts of revolutions of that a, during an interval of, say, thirty seconds, 
may be determined by the mean of several experiments. For the methods of perform- 
ing this with the necessary correctness, I again refer to “ Second Report on Earth- 
quakes,” &c.. Report of Brit. Assoc. 1851, pp. 287, 289, &c. 
On the present occasion, as a considerable time elapsed between the successive expe- 
riments, during which the oil on the instrument more or less changed its state, and as 
some were made in summer and others in winter, it became necessary to rate the chro- 
nograph anew for each experiment, or at least to verify the former rating ; for this end 
it was necessary to provide a suitable loud-beating seconds clock, with a divided arc to 
the pendulum, as none such could be procured at Holyhead. The same weight was 
constantly used with the chronograph ; and the extreme differences found, in the rating 
during the several years that these experiments have been in progress, were no more 
than the following : — 
November 1856. Value in mean time of one division of the dial (a) =0-01485 
May 1861. Value of same =0-01806 
Taking for illustration the former value of the smallest division of the dial {a), we see 
