_Construction of a Boom._--The general construction of a boom consists of a main cable, buoyed up at intervals by floats. The main cable may be either wire, chain, or rope, the former being very much superior for this purpose to chain or rope. The floats consist of balks of timber built round the main cable and bound together by means of iron hoops &c. A space is left between each float, by which a certain amount of flexibility in the boom is obtained, without which it would be of comparatively little use, as it might be easily overrun. It must be borne in mind, in constructing all such booms, that the smaller the proportion of timber used in forming the floats to the cable, consistent with buoyancy, the stronger will be the structure. A very important feature in connection with such a mode of defence is the manner of mooring it; for if it be moored so as to be unyielding, then its sole power of resisting a vessel charging it is the actual strength of the materials composing the structure, but if it be moored so that it is capable of yielding to a sudden blow, this force will be to some extent absorbed, and resistance of the defence greatly increased. The raft employed to support the main cable should be moored by means of very heavy chains (without anchors) in the direction of the attack, and with ordinary anchors and cables on the other side. As a rule, the booms should be moored obliquely to the direction of the current, where there is any, as the tendency of the current to overrun the boom when so placed will be less, and also a ship ramming it must place herself athwart the current to attack the boom at right angles. _Clearing a Passage through the Torpedo Defences of an Enemy._--The subject of clearing a passage through the torpedo defences of an enemy is one fraught with innumerable difficulties, on account of the varied nature and impracticability of obtaining accurate and _certain_ information of such defences, and thus it is impossible to lay down any fixed rule or plan for carrying out such an operation. In fact, it will be only under the most favourable circumstances that such a service will be successfully accomplished, that is to say, in the case of a harbour or river defended by submarine mines but unsupported by guns, or guard boats, or where the electric light is used. Numerous methods have been devised from time to time to effect the destruction of an enemy's submarine defences, among which are the following:-- 1.--Projecting frames, &c., from the bows of a vessel. 2.--Creeping and sweeping by boats. 3.--Countermining.