The _Narval_ in June, 1901, made the experiment of remaining for twelve hours under water. The Ministry of Marine were represented by Naval Surgeon Gibrat, who wrote a full and detailed report on the condition of the crew after their twelve hours’ submersion. From notices published in the French papers it would appear that “the trial succeeded without incident,” but the impression seems to be general that the crew were in a more or less exhausted condition after their prolonged sojourn beneath the waves, which after all is not to be wondered at. It appears that after six hours under water the inhaling of artificial air became difficult; the long exclusion of natural atmosphere caused a painful irritation of the nerve centres which even the coolest of the officers could not resist, and anæmia set in accompanied by cerebral compression and sick headache. Trouble was also caused by the working of the accumulators, which liberated among the crew salts of lead and sulphur, causing digestive and intestinal complaints. Dr. Gibrat is reported to have expressed to them that, in the present condition of knowledge, 12 hours is the outside limit of efficient work on a submarine under water. The _Narval_ class, besides the eponymous vessel, comprises four other submersibles—the _Sirène_ (launched May 4, 1901), the _Triton_ (launched July 13, 1901), the _Espadon_ (launched August 31, 1901), and the _Silure_ (launched October 29, 1901). These four resemble the _Narval_ in most particulars, though in some respects they are improvements on their prototype. The outer hull of each is made of steel, but the inner hull is made of nickel steel. The choice of the metal is a matter of importance, for the difficulty is to build a hull which possesses sufficient strength to resist the pressure of the water, and yet at the same time is not too expensive. Between the two hulls in the interior of the vessel are seven compartments for water ballast. There are also four water tanks, which are used to regulate the trim of the vessel by introducing water at the last moment.