OUR SUBMARINE DEFENCES. 
367 
very complete and well carried out, and it is hardly necessary 
to add, were very costly. 
The system of mines which it was then proposed to employ 
in defence of harbours generally consisted of lines of ground 
charges of compressed gun-cotton, of 500 lbs. each, the depth 
of water for which such charges were best suited being about 
eight fathoms, and this depth is a very common one. The 
charges were to be in galvanic connection with the look-out 
stations or firing-points, so as to enable them to be fired at the 
will of an operator ; self-acting gear, to explode them on contact 
with an enemy’s ship, being also provided. The object of the 
experiments was to ascertain the distance at which a vessel 
would be destroyed by the explosion of a mine, and also the 
necessary interval between the mines, to secure each one from 
the effects of explosion of those contiguous to it. It is obvious 
that a certain relationship must exist between these distances 
respectively, for while it is imperative that the channel should 
be completely closed against the entrance of an enemy, it is 
most desirable that the first explosion should not extend beyond 
the mine acting on the vessel, because it would open a wide gap 
in the line of defence. With a single line of mines it would be 
necessary that the interval which would be sufficient to prevent 
injury from an adjacent explosion should not, under any cir- 
cumstances, be more than double the distance at which the 
destruction of a vessel might be considered certain. As, how- 
ever, channels are defended by several lines of mines, those 
forming one line covering the intervals in the line next in 
front, the conditions become modified to a certain extent. 
Altogether it was scarcely questioned that an efficient system of 
defence, with large ground charges of 500 lbs. of cotton, might 
be organised ; and if so, this was for many reasons to be pre- 
ferred to the alternative system, consisting of numerous small 
charges floated close to the surface of the water, on which we 
shall have to say more hereafter. 
On August 6 the first experiment took place, at Stokes 
Bay, under the direction of the Special Committee, of which 
Sir W. Jervois is president. We give the details of this trial, 
as obtained from official sources, and published at the time in 
the 64 Engineer,” the editor of which periodical has kindly 
placed the information and cuts herewith at our disposal. 
The Oberon was in the following condition : — The inlet and 
outlet valves of her condensers were left open. The Kingston 
valve of her feed-pipe was closed. The water-line was 2 inches 
higher than the top of her condenser. The original weight of 
her hull, before fitting her with special bottom, had been 
590 tons ; as now fitted, it was 920 tons. Her cables and con- 
densers weighed about 80 tons. Her starboard side had forty- 
