]ME. MATJ.-RT OX THE TEAXSIT-VELOCITY OE EAETHQUAKE WAVES. 667 
that each di\isioii of the dial (d) is equal one revolution of the index (a), and equal 
0"-01485x30=0"-4455, 
and one revolution of the index (d) equal 
0"-4455xl2=5"-346,— 
an absolute rate of movement of the instrument not widely differing from that employed 
in the experiments of Killiney and Dalkey, with which it is desirable that the present 
results should be comparable. Half a small division of the chronograph can be read ; 
we therefore in these experiments possessed the means of recording time to within 
0"-0074, or to nearly y^pths of a second. 
The additional apparatus of the chronograph consisted merely of such arrangements 
that the releasing lever (^), when pressed down by the hand applied to the wood insulator 
at m, should dip at i into a mercury-cup, and so make contact by the wires (^, V) 
between the poles of the contact-making battery (E). 
It remains to describe the contact-maker (fig 2, Plate XXIII.). c is the base of the 
instrument of mahogany, cariying a vertical and bent arm {d) of cast iron, into the 
upper forked end of which the central iron bars, of about l-ths of an inch in diameter, 
of the electro-magnets a, a (seen in plan in fig. 3) are secured by a cotter. The coils of 
covered wire round these are continuous, the wire {h) from the g + pole passing at its 
further end from the first coil over to the second, and at the extremity of the latter pass- 
ing off to the g — pole by the junctions being effected by mercury-cups in the usual 
way. w is a sliding piece of wood, secured upon the base c, when adjusted in place, by 
the screw at s; this carries a wrought-iron lever armature (c), whose arms are as 8:1, 
the shorter and rather hearier end being adjusted so as to be beneath the poles of the 
electro-magnets, and at such a distance beneath them that, upon passing the current 
through the coils, the magnets shall readily attract the short end of this lever, snatch it 
up into contact with the poles of the magnets, and in doing so depress the other or 
remote end of the lever. The latter extremity of the lever is provided, as seen more at 
large in figs. 4 and 5, with a forked pair of copper poles amalgamated, which, when 
depressed by the action of the electro-magnets, dip into the mercury of the cups/’and/’, 
and in doing so close the holes of the firing-battery, the conducting wires from which 
{h and h) dip respectively into mercury-cups, which by a tube bored through the wood 
are in permanent communication withy and/" (cups) respectively. The lever and forked 
poles, &c., are provided with various screw adjustments as to position, range, &c., and a 
slender spring beneath the lever, ensuring that it shall not be accidentally moved by 
wind, or other cause, until acted on by the powerful grasp of the magnets. 
Ikis instrument was found to answer admirably well. It may be observed, in passing, 
that it gives the means of exploding mines at almost any distance, through telegraphic 
wires and by any moderate contact-making power, and may admit of valuable applica- 
tions hereafter for the explosion, at a determinate instant, of mines for purposes of 
warfare. 
