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these a telescope is mounted, so as to command a full view of 
the space occupied by the mines, which it can sweep from right 
to left or from left to right, by moving it horizontally on a 
vertical pivot. A projecting arm moves with it, and carries on 
its extremity a firing-key, which can be depressed and brought 
in contact with any one of a number of firing points placed on 
the arc of a circle, over which the key moves. The key is con- 
nected with the voltaic battery, and each of the points with one 
of the cables leading to the sunken mines. These are, moreover, 
so arranged, that when the telescope is directed to the position 
of any of the torpedoes, the key moving with it is over the firing 
point corresponding to that mine. Thus all that is necessary is 
to watch the approaching vessel with the telescope, and whenever 
its movement brings the firing-key over one of the points, press 
it down and so close the break in the circuit. In this way 
torpedoes can be fired with perfect accuracy and certainty. 
The second method of firing torpedoes is that in which the 
vessel herself closes the circuit. To accomplish this an appa- 
ratus, called a circuit-closer, is moored above the torpedo, so 
as to float a little below the surface of the water. The electric 
cable passes into the torpedo, and its conducting wire is con- 
nected with one of the wires of the fuze, while the other (in 
the ordinary arrangement the return wire) is joined to the con- 
ductor of a second insulated cable, which is connected with the 
circuit-closer, and through this, when the circuit is closed, the 
voltaic current is carried to earth. As is the case with almost 
every part of the apparatus belonging to the torpedo, there are 
several plans for constructing circuit-closers, and we shall con- 
tent ourselves with describing one which has had its efficiency 
tested by many severe trials, and which will probably be 
adopted whenever torpedoes are used for the defence of British 
harbours. Mathieson’s circuit-closer (^fig. 8) consists of a 
metal cylinder, ab, with an outer covering, c, of light wood 
soaked in tar, which protects and gives buoyancy to the whole. 
This woodwork is held together by iron bands, which are firmly 
attached to the iron foot-plate or base, b. At the top is a ring 
for carrying the apparatus, and there is a similar ring at the 
bottom to which to fasten the mooring line. All this is only 
the case of the apparatus. In the centre of it there is an 
upright flexible metal bar, bd, having at its top a leaden ball, d, 
and about half way up a horizontal brass disk, e, insulated from 
the rod by a piece of ebonite in its centre. To this disk the 
end of the conducting wire of the cable (which is introduced 
through the foot-plate at L by a water-tight and insulated 
joint) is attached. Four metal uprights, kkkk (two only 
appear in the section), support a brass ring, a a, which surrounds 
the pendulum, BD, and sustains four springs, hhhh, the lower 
