Such a system (as a French writer has pointed out) is very faulty and dangerous, as may be easily conceived. In the first place, considering the feeble resistance which a canvas can oppose even when mounted on steel springs, it was indispensable to protect it against the pressure of the water during the “dive.” This was believed to have been accomplished by providing the shell with an indiarubber crown, forming a sort of beak, which, when the system was completely folded up and closed, came to rest on another washer of indiarubber placed at the bottom of a groove, where it formed a watertight joint. This would have been very good if there had been certitude that the beak or cap fixed on the shell rested exactly at the bottom of the groove. To make sure of that there had been arranged a small opening, which ought to give water when the closure was not sufficient, but this only gave an almost illusory means of control, the opening being found almost always obstructed by some detritus, _débris_ of sea-wrack, and other foreign bodies encountered by the vessel. It will be seen what permanent danger was created by this apparatus—danger which on two occasions just missed causing the loss of the craft, an unlooked-for chance alone preventing it from foundering. It would further be easy to show that in spite of its danger the movable shell, with canvas folded in accordion pleats, was almost unusable—that it could not serve when there was a slight sea on; that it caused, even during progress on the water-line, infiltrations capable of sinking the boat; and finally that, even in a calm sea and under the best conditions, the length of the manœuvre of raising and lowering made the operations of the “dive” slow and wanting in precision—operations which demand so much rapidity and delicacy, and to which alone it is allowable to sacrifice something on a submarine boat. This movable shell was 0·35 metre (about 13½ to 14 inches) in diameter; it was abandoned after all the Commandants had successively condemned it, and to-day it is never used. For use below the surface the _Gymnote_ carries an optical tube with two inclined mirrors and a periscope, but these have so far given but poor results.