ME. MALLET ON THE TEANSIT-YELOCITT OP EAETHQIJAKE WAVES. 669 
The rheostat was adjusted so that the resistance equalled that of the conducting- 
wires along the telegraph poles between C and D, E (fig. 1, Plate XX.). In this state 
of things, when the lever {m) of the chronograph was pressed down, the disc [f) instantly 
commenced rotating, but directly afterwards the electro-magnet (a), whose current was 
estabhshed by the first movement, attracted the lever armature {e) through the disc, and 
the latter was arrested by being gripped between the pole of the magnet and the arma- 
ture. The arc of the circumference of the disc then, at the centre of the magnet-pole 
{i. e. with 6 inches radius), that was intercepted between the marked spot (^) whence 
it started and that at which it was arrested, became a measure of the time lost or elapsed 
between starting the chronograph at the observer’s station and making contact at the 
firing-battery in the actual experiments. The arc thus intercepted was converted into 
time, from the descent of the weight (W), by the common formula ^=-^, s being given 
and equal to ^th the length in feet of the arc described by the circumference of the 
disc before being arrested ; and this was capable of being controlled by measuring by the 
chronograph itself the actual time of a given number of successive revolutions, and parts 
of revolutions, of the disc, the total number of complete revolutions made being taken 
by reckoning the coils wound off the barrel. Upon a mean of ten experiments with this 
apparatus, the delay at the contact-maker appeared to be no more than 0"‘0143, which 
converted into distance, at the greatest transit-rate observed, gives a correction of 17’3 
feet per second, and at that of the least of 12-8 feet per second, both additive. 
It may be remarked that the small error due to inertia, &c. in this apparatus tends 
nearly to correct itself, the extremely small time lost at starting of the disc being very 
nearly equalled by its tendency to be carried a little too far by the velocity impressed ; the 
whole inertia also of the disc-barrel, See. was extremely small in proportion to the moving 
weight W. 
Another con-ection requiring to be attended to in these experiments was the time of 
hang-fire in the charge of the mine, that is to say, the time required for the burning of 
such a portion of the whole charge of powder as should be sufficient to rupture the 
rock around, and so start off from the focus the wave-impulse perceived in the seismo- 
scope — in other w^ords, the time lost between the instant of fii’st ignition of the powder, 
viewed as simultaneous with that of making contact at the firing-battery B, and the 
starting of the wave of impulse to be measured. 
In my former experiments at Killiney Bay, it will be recollected that it was in my 
power to determine this experimentally and rigidly, the moderate charges of powder 
there employed admitting of this, and that I found it amount for 25 lbs. of powder to 
0"-()5051.3, or to about ^th of a second. Such is, in fact, the time that the full charge 
of a 68-pounder takes to bum. But in the present case direct experiment was impossible ; 
and the value for this correction can only be approximately obtained, by observing the 
time that elapsed in some instances between the moment of making contact at B and 
the first great visible movement of rock at the face of the heading. This observation I 
