Page:Popular Science Monthly Volume 58.djvu/172

164 is about fifteen pounds to the square inch, or more than a ton to the foot. Solid construction is naturally in order for a submarine boat. But power to resist pressure depends also upon shape. A circular section, because it involves the principle of the arch, is the strongest. With a given thickness of metal, therefore, a spherical boat could safely dive deeper than one of any other form. But the exterior of such a boat is ill-adapted to propulsion, and the interior for the arrangement of machinery.

Since the days of Captain Nemo and the fabulous 'Nautilus' the cigar shape has doubtless been associated with submarine navigation in

the minds of ninety-nine out of every hundred persons who have thought of the matter at all. And it is equally a matter of sober history that this form has been almost universally adopted. Some inventors in the earlier days, with the vision of high speed in mind, have trimmed down the lines to almost needle-like fineness, as in the 'Gustav Zédé.' Now that attempts at high speed have been abandoned, the elongated spheroidal or egg-shape is the favorite, as illustrated both by the 'Holland' and the 'Argonaut.'

But what of power for locomotion under water? Obviously steam power, at least as ordinarily produced elsewhere, will not do. Even supposing the necessary draft to be secured, how shall the smoke be