182 
POPULAK SCIENCE EEVIEW. 
single torpedo ; let us now see how numbers of them are used 
in combination. We will suppose that the mouth of a river is 
to he defended. The principle on which the torpedoes are to be 
arranged is that they shall be so placed that it will be im- 
possible for a ship to run up the river without passing over, or 
at least coming within close range of, one of them. It would 
seem at first sight that this ought not to be a very diiB&cult 
matter, and that all that would be necessary would be to moor 
the torpedoes in a row across the river, so near each other that 
nothing could pass through the line without coming within 
reach of one of them. But the effect of this arrangement 
would be that the explosion of any one torpedo would probably 
destroy all the rest within range of it, and leave an immense 
gap in the line. It is therefore essential to have more or less 
of an interval between the torpedoes, according to the weight 
of the charges. Between charges of 200 lbs. of gun-cotton, for 
instance, there should be an interval of at least 100 feet. Sup- 
posing, then, there is a channel 500 yards wide to be defended, 
fifteen of theso' torpedoes will be placed in line across it, and, 
as it might easily happen that a ship would run through one of 
the intervals between them, a second line is placed about 
100 yards further back, the torpedoes in which are opposite to 
the intervals of the first ; and then a third and a fourth and 
even more lines may be added, each additional row making it 
more difficult to enter the river in safety. 
There are two ways in which arrangements can be made for 
firing the torpedoes with effect. They may be fired by watch- 
ing the course of the hostile vessel and igniting them with the 
firing-key, or the vessel herself may be made to close the 
circuit. In the first case, before the torpedoes are moored, two 
stations are selected behind the line of defence, and so far apart 
that it will be easy to find the position of each torpedo after it 
is sunk in its place, by taking cross-bearings from those two 
points. To recur to our former example of the river-mouth, 
the firing points, or stations a and b (fig. 12), are selected on 
opposite banks, and probably in two forts, or, if these do not 
exist, in two buildings which command a good view of the 
river, and by position or distance up the banks are secure from 
being destroyed by the fire of ships which have not passed the 
rows of sunken mines. For the sake of clearness, only the 
cables of the first line (the torpedoes of which are numbered 
from 1 to 5) are shown in the diagram. The voltaic battery is 
at c, and is connected, one end with the earth, the other with 
the first series of firing-keys at a. From these, five insulated 
cables cross the river to the second group of firing-keys at b, 
and another series of five cables connects these keys with the 
torpedoes 1, 2, 3, 4, 5. There are thus two breaks in every 
