TOKPEDOES. 
183 
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circuit, one at the firing point a, the other at b. Now let us 
suppose that a hostile man-of-war attempts to enter the river 
on the course dee. 
When the ship is at n she will be, as seen from a, on the 
bearing line ad corresponding to torpedo No. 1. At a, there- 
fore, the firing-key No. 1 will be pressed down by the man in 
charge there, and one break in the circuit will be closed ; but, 
as seen from b, she will not be on the bearing-line of any of 
the torpedoes ; therefore the second break will still be left open, 
and no current will pass along the wires. As soon as the ship 
has advanced from d towards e, the key will be allowed to rise 
again, and both breaks will be once more open. 
The ship now reaches e. Seen from a she will be on the line 
of none of the torpedoes; therefore none of the keys will be touched. 
But as seen from b she will be on the bearing-line of torpedo No. 3. 
There, therefore, the firing key No. 3 will be depressed, and the 
second break of the circuit will be closed ; but as the first 
(at a) is open, just as before no current will pass. 
The ship still advancing reaches e. As seen from a she is 
now on the bearing-line of torpedo No. 2, and the same is the 
case when she is seen from b. She is in fact directly above the 
torpedo. At each station the key No. 2 is depressed, both 
breaks in the circuit are closed, the voltaic current passes from 
c through A, across the river by the cable to B, and then on by 
another cable to the fuze of torpedo No. 2, which it ignites. 
The charge is fired, and the hostile vessel is either sunk or 
terribly injured. If the ship had succeeded in passing through 
an interval in the first line she would have to run a similar risk 
at the second, and every additional one would increase the 
peril and make the attempt more desperate. 
In some cases it may be possible to occupy, without any 
danger of being cut off, a point about a mile inland on the 
flank of the system of torpedoes. In this way the arrangements 
for the defence can be greatly simplified ; for if the lines of tor- 
pedoes are made to converge towards this advanced point, an 
observer placed there can watch the course of an approaching 
vessel, and when she is passing over the first line, signal the 
fact to a single firing station to the rear of the sunken mines. 
The man in charge there will then observe the bearing of the 
vessel, and if it indicates that she is over a torpedo, he will 
explode it by pressing down the corresponding firing-key. If 
she passes through an interval, the same method will be adopted 
with regard to the second line. In this way the necessity of 
having two firing- stations and a double system of firing-keys 
and electric cables can be avoided. 
To ensure great accuracy in the system of firing torpedoes by 
cross-bearings, telescopic firing-keys have been designed. In 
