Page:Popular Science Monthly Volume 68.djvu/274

 270 more than two or three men would form the crew, and on that ground have condemned the construction of these small submarines. They would necessarily be of slow speed and very limited radius of action, while their efficient working would depend upon the nerve and skill of only two or three men working in a very confined space.

Progress in mechanical engineering and in metallurgy has been great since Bushnell constructed and used his first submarine in 1776, during the war between the United States and this country. These advances have made it possible to increase the dimensions, speed and radius of action of submarines; their offensive powers have been enlarged by the use of locomotive torpedoes; and superior optical arrangements have been devised for discovering the position of an enemy while they themselves remain submerged. But it cannot be claimed that any new principle of design has been discovered or applied. From descriptions left on record by Bushnell and still extant, it is certain that he appreciated, and provided for the governing conditions of the design in regard to buoyancy, stability, and control of the depth reached by submarines. Indeed Bushnell showed the way to his successors in nearly all these particulars, and—although alternative methods of fulfilling essential conditions have been introduced and practically tested—in the end Bushnel's plans have in substance been found the best. The laws which govern the flotation of submarines are, of course, identical with those applying to other floating bodies. When they are at rest and in equilibrium they must displace a weight of water equal to their own total weight. At the surface they float at a minimum draught and possess in this 'awash' condition a sufficient freeboard and reserve of buoyancy to fit them for propulsion. When submarines are being prepared for 'diving' water is admitted to special tanks, and the additional weight increases immersion and correspondingly reduces reserve of buoyancy. In some small submarines comparative success has been attained in reaching and maintaining any desired depth below the surface simply by the admission of the amount of water required to secure a perfect balance between the weight of the vessel and all she contains, and the weight of water which would fill the cavity occupied by the submarine when submerged. For all practical purposes and within the depths reached by submarines on service water may be regarded as incompressible; the submarine should, therefore, rest in equilibrium at any depth if her total weight is exactly balanced by the weight of water displaced. If the weight of the vessel exceeds by ever so small an amount the weight of water displaced, that excess constitutes an accelerating force tending to sink the vessel deeper. On the contrary, if the weight of water displaced exceeds by ever so small an amount the total weight of the vessel, a vertical force is produced tending to restore her to the surface. Under these circumstances, it is obvious that if the admission or expulsion of water from internal tanks (or the