Page:Popular Science Monthly Volume 68.djvu/273

 Rh constitutes what is technically called a 'reserve of buoyancy.' In the submersible this reserve of buoyancy and the accompanying freeboard is greater than in the submarine type, and in this respect lies the chief difference between the two types. The submersible has higher freeboard and greater reserve of buoyancy, which secure better seagoing qualities, and greater habitability. The deck or platform is situated higher above water, and to it the crew can find access in ordinary weather when making passages, and obtain exercise and fresh air. Recent exhaustive trials in France are reported to have established the great superiority of the submersible type when the service contemplated may involve sea passages of considerable length. The French policy, as recently announced, contemplates the construction of submersibles of about 400 tons displacement for such extended services, and proposes to restrict the use of submarines to coast and harbor defence for which vessels of about 100 tons displacement are to be employed. All recent British submarines would be ranked as submersibles according to the French classification, and it is satisfactory to know, as the result of French experiments, that our policy of construction proves to have distinct advantages. In addition to these two types of diving or submarine vessels, the French are once more discussing plans which have been repeatedly put forward and practically applied by M. Goubet, namely, the construction of small portable submarine vessels which could be lifted on board large ships and transported to any desired scene of operations. In the Royal Navy for many years past, it has been the practise to similarly lift and carry second-class torpedo or vedette boats about 20 tons in weight. Lifting appliances for dealing with these heavy boats have been designed and fitted in all our large cruisers and in battleships, and a few ships have been built as 'boatcarriers.' The first of these special depot ships in the royal navy was the Vulcan ordered in 1887-8, the design being in essentials that prepared by the writer at Elswick in 1883. The French have also built a special vessel named the Foudre which has been adapted for transporting small submarines to Saigon, and performed the service without difficulty. Whether this development of small portable submarines will take effect or not remains at present an open question, but there will be no mechanical difficulty either in the production of the vessels themselves or in the means for lifting and carrying them. M. Goubet worked out with complete success designs for vessels about 26 feet long and less than 10 tons displacement, with speeds of 5 to 6 knots, the trials of which have been very fully described, but French authorities have not adopted the type, and no decision seems to have been taken to introduce it. In this country no similar action has been taken, and our smallest submarines weighing 120 tons cannot be regarded as 'portable.' Indeed, some leading British authorities on submarines have indicated that experience is adverse to the construction of vessels in which not