TOEPEDOES. 
173 
filled with tar to make the torpedo water-tight. The inner case 
(about 4 ft. X 4 ft.) was further protected by being covered with 
plates of zinc, and contained a charge of 4 cwt. of gun-cotton, 
with air-spaces in the centre to make it buoyant. The form 
here adopted — a cylinder of equal height and diameter — seems 
to be the best. In theory the best form would be a globe, the 
charge to be ignited at the centre; but there would be a hundred 
difficulties in the manufacture and use of such a case in practice ; 
and the cylindrical case of these dimensions, which is the nearest 
form to it we can conveniently use, gives us almost all the 
advantages which theoretically belong to the globular form, the 
greatest of which is that the thorough and uniform ignition 
of the charge is secured by its compact form, which brings all 
the outer portions within an equal distance of the point at 
which the ignition commences. A case of English invention 
consists of a cylinder of iron to contain the charge, protected by 
a thick outer covering of fir-wood soaked in tar and painted to 
make it water-tight. Besides wood and iron, a third material 
has been suggested ; this is vulcanised india-rubber made into 
bags to hold the charges. It is found, however, that the bag gives 
way before the whole explosive force is developed, and thus a 
large portion of the charge is lost. To obviate this it has been 
proposed to place in the centre of the torpedo a long perforated 
brass tube containing the fuze and a small priming charge, so as 
to fire the explosive at several points, and thus ignite the whole 
of it before the india-rubber gives way. It is doubtful, however, 
if, even with this improvement, the india-rubber case is suitable 
for torpedoes, at least with charges of gunpowder. Its great 
advantage is the perfect security from water which it afi^ords 
to the charge, but the difficulty of producing a complete 
and effective explosion quite outweighs this. It seems that the 
vulcanised india-rubber bag might be used with advantage in 
constructing wood and iron-cased torpedoes, as any defect in the 
tightness of the case would be neutralised by the charge being 
enclosed in such an inner covering. 
The form and material of the case having been decided 
upon, the next thing is to select the explosive best suited for 
the conditions under which it is to act. A few years ago there 
would have been little to be said on this point, for the only 
available explosive then known was gunpowder ; l 3 ut now there 
are a host of such compounds as gun-cotton, xyloidine, nitro- 
glycerine, dynamite, picrate of potassium, glyoxyline, and more 
of less importance, and chemistry is continually adding to the 
number. The essential qualities required in an explosive for 
this purpose are great power, so that the space within which 
its explosion will be effective may be as wide as possible ; and 
uniformity of action, in order that we may be able to reckon 
