"Lake" boats, fitted with bottom wheels, have, in a competitive test abroad, entered landlocked and fortified harbors without discovery, where the entrance from the sea has been through a tortuous channel. All other vessels, except the one fitted with bottom wheels, were discovered long before reaching the outer fortifications, because it was necessary for them to show their periscopes to sight their way. They struck the sides of the dredged channel, which caused them to broach and be discovered, because they had to maintain a comparatively high speed to be kept under control. In tests carried out in Russia the boat fitted with bottom wheels simply wheeled along in the channel at slow speed and stopped and backed to change course at will. The revolutions of the bottom wheels gave the distance travelled, the manometer gave the depth, and the compass the proper direction; consequently, with a correct chart as to courses and depths, navigation on the bottom in entering harbors is very much easier than on the surface, unless the channels are well buoyed. Most mines, as at present installed, are either of the observation or contact type; the observation mines are fired usually from shore stations when the enemy is seen to be over them, while the contact mine is anchored a few feet beneath the surface and is either exploded by contact with the surface of the vessel's bottom or by the agitation caused by the rush of water due to the swiftly passing vessel. The European belligerents have put out contact mines to protect their capital ships from the submarines. The dread of these mines is holding the submarines outside of the mined areas, and the mines are therefore effective. None of the British vessels are fitted with bottom wheels and diving compartments, and they must be navigated at such speed to keep submerged control that they would explode a contact mine if either the mine or its anchor rope were touched. This also applies to some of my later boats, as the bottom wheels have been omitted to meet the demand for greater speed on the surface and submerged. I am inclined to the belief that this has been more or less of a mistake, because the bottom-wheeled submarine can go to and dig the enemy out of its base in addition to hunting the big surface craft of the enemy on the high seas. With the bottom wheels, navigation can be conducted so carefully over the bottom that inspection of the course can be made, if desired, foot by foot, as progress is made, and all mines can be avoided. [Illustration: MINES PLACED UNDER SHIPS AT ANCHOR Permission Scientific American