668 ME. MALLET ON THE TEANSIT-VELOCITT OF EAETHQHAKE WAVES. 
It is obvious that a certain loss of time must occur at this contact-maker, in reference 
to our experiments — that in fact the total time registered by the chronograph at D is 
too great, by the minute interval that elapses between the arrival of the galvanic current 
in the coils at a, and the dipping of the poles f,f into the mercury-cups. With the same 
battery power at E and conducting-wires, this delay is practically constant. Its amount, 
however, required to be determined, and the time^ when converted into distance, added 
to the gross transit-rate previously ascertained. 
For this purpose the following little apparatus was employed. Its principle, though 
not the precise details of its construction, is shown in tig. 6, Plate XXIII. Upon a ver- 
tical steel spindle (s) revolving upon an agate step at bottom, and in a polished brass 
collar at top, a cylindric barrel is placed of 1 inch diameter, having an escapement-wheel 
and anchor escapement (v) at its lower end, all the parts being made as light as possible. 
Upon the upper end of the spindle a circular disc of Bristol board (cardboard), y, of 
12|- inches diameter, is secured by a light screw-collar (t) gripping the disc firmly, so 
that it and the spindle must revolve together. Both the upper and under surfaces of the 
card-disc, for an inch or two from the circumference, towards the centre, were slightly 
rubbed with violin-player’s hard rosin, and the whole, resting upon its base B, placed so 
that the disc should rotate horizontally. A fine elastic silk thread is wound a few turns 
round the barrel, and passing over the sheave {r) sustains a weight (W), by the descent of 
which, when required, rotation can be given to the disc, &c., the weight itself being large 
in proportion to the inertia of the rotating parts. By suitable changes in the disposition of 
the parts of the contact-maker (chiefly in getting the cast-iron arm<?, fig. 2, out of the way ), 
it was placed at C with respect to the disc, so that the lower poles of the electro-magnets 
(«, a) were just above the upper surface of the card-disc, and the short end of the lever 
armature {e) just below the same, the card running free in the small space between, and 
the centre of the magnet-poles being exactly at a radius of 6 inches from the centre of 
the disc. Nearly at right angles on the disc to this, the chronograph (D) was placed and 
firmly fixed ; a fixed point (shown in part only in the fig., g), formed of a bit of cylindrical 
mahogany, with its lower end rosined, was so fixed as to be about ^a-th of an inch above 
the upper surface of the disc. The lever (to) of the chronograph, divested of its forked 
pole, and having a small rectangular rod of brass substituted, was so adjusted that its 
sustaining spring beneath should press this brass terminal up against the under surface 
of the disc at directly below the fixed point or stop {g), and, bending the 'card-board 
there, press its upper surface into contact with the lower end of g. 
Thus the weight W being free to descend, this arrangement at ^ acted as a detent to 
keep the disc from moving ; but when the lever (to) was pressed down to start the chro- 
nograph the disc immediately became released, and began to revolve by the action of 
the weight W. At E the contact-making battery, or one of equal power, was placed, 
one of its poles being connected, through the rheostat (B), by conducting-vvares with the 
coil of the electro-magnet [a) and terminating at the £+ pole at the mercmy-cup {n), 
which was in connexion with the other, or s— pole of the battery. 
