The exterior case is made of well-seasoned elm 1½ in. thick, iron bound,
and screwed together with water-tight packing between the joints, also
cemented with pitch. The interior case is made of stout sheet copper;
the case has two loading holes corresponding in size to two holes in the
deck, or top of the exterior case. These holes are made sufficiently
large to load with gun-cotton discs, if preferred. The loading holes
are fitted with corks, which are inserted before screwing on the brass
cap, to prevent any chance of accident through friction in screwing on
the cap; the bushing has the thread of the screw on the outside for
the same reason. If thought necessary, the cork can be cemented over
before screwing on the cap, which will render the joint doubly secure
from leakage. The centre of the copper case has a stout copper tube,
which is soldered to the top and bottom surfaces of the copper case, the
charge being all round it; into this centre tube is screwed the priming
case. It should be understood that both the exterior and inner cases are
thoroughly water-tight, so that in the event of the outer case being
damaged, still the charge in the interior case is preserved perfectly
dry, the cases being altogether independent of each other.
The capacity of the copper case of the large torpedo is such that it will
contain 77 lbs. of water; the capacity of the small one, 28 lbs.; from
these can be determined the quantity of any explosive agent with which it
may be desired to charge either torpedo.