lose their power rapidly), and can start or stop almost instantaneously.

As a receiving instrument it is quite as effective as the microphone used in the earlier submarine signalling apparatus.

It is unnecessary to dwell upon the advantages connected with a system like this which makes it possible for a ship to interchange signals with another ship or with a shore station, and this without regard to weather conditions, "zones of silence" or "interference."

Like the microphone tanks of the earlier signalling apparatus, the box containing the oscillator is bolted to the inner skin of the ship well below the water-line, but whereas the tanks containing the microphones of the earlier system were filled with water, the box containing the oscillator is filled with air, and, as it is of the first importance that this air should be dry, it is given a pressure of twenty-five pounds to the square inch before the box is sealed

up.

The oscillator is attached to the skin of the ship in such a way that its diaphragm actually constitutes a part of the ship's plating, a circular hole being cut in the side to admit of this.

The action of the oscillator is in many respects similar to that of a telephone receiver. The diaphragm is vibrated, as in the telephone, by rapid variations in the polarity of an electro-magnet. But as the diaphragm is enormously larger and thicker than that of the telephone, its operation calls for the use of powerful electric currents. The standard type takes a direct current of seven amperes through the main magnetic coil and an alternating current of eleven amperes through the core. The instrument is operated from a switchboard shown on Plate 57. Sending is done by an ordinary telegraph key; receiving, by telephone receivers.

The method of locating the direction of a signal received is exactly the same as with the earlier microphone system which has been fully described.

There is every reason to believe that the oscillator system will before many years be required as a part of the safety outfit of every seagoing vessel.

Boats designed to be carried on shipboard are subject to many conditions which define their characteristics within narrow limits, especially as regards their size and weight. These limits are determined, first of all, by the specific purposes for which the boats are to be used; and next by the facilities available for stowing and handling them.

The boats of a merchant vessel are intended almost exclusively for saving life in circumstances of emergency, when they must be handled by a small number of men and usually under conditions of more than ordinary difficulty. It is therefore as important for them to be light and handy as to be roomy and seaworthy. The boats of men-of-war are used for a great variety of purposes, of which life saving is by no means the first, and in design are necessarily a compromise between conflicting demands.

Merchant Steamers, and especially passenger steamers (including transports), are required by law to carry a number of boats proportioned to the maximum total personnel which they are authorized to carry. Moreover, the law prescribes with much detail the character of the boats and the facilities which shall be provided for their stowage and handling. These facilities are directed almost exclusively toward getting the passengers safely away from the ship in case of necessity and ensuring their safety so long as they may be obliged to remain in the boats. Thus the question of lowering the boats and getting them clear of the ship receives vastly more attention than that of hoisting them. In the matter of stowage, also, questions of convenience must be sacrificed wherever necessary for quick and safe handling in an emergency.

Conditions are entirely different on men-of-war, where everything else gives way to military efficiency. Here the boats are stowed and handled as best they may be after every possible

facility has been provided for pointing and firing the guns. Moreover, since the boats of a man-of-war, instead of being reserved for some special emergency, are designed and used for scores of purposes connected with the everyday life of the ship, the question of hoisting is quite as important as that of lowering.

CONSTRUCTION OF BOATS. (Plates 58 and 59.)

Boats are made of both wood and metal and by a great variety of systems. In wood, three general types of construction are common: Carvel, Clinker and Diagonal. (Plate 58.)

alongside each other Where the build is

In Carvel building (Fig. 1), the planks lie without overlapping, the seams being calked. too light to admit of calking, a narrow batten or riband is run along the seams inside, the calking being in this case limited to the garboard seams and the butting ends of the planks.

Where heavy boats are built on this system, a second layer of planking is sometimes used inside the frames.

In Clinker building (Fig. 2), the planks overlap at their edges like the clapboarding on a house, and are fastened to each other as well as to the frames. As the planks thus support each other, this system has greater strength for a given weight than Carvel building and the frames can be placed farther apart. On the other hand, the planks are liable to split along the line of fastenings, and repairs called for by this or other injuries to the boat are made with some difficulty because of the necessity of removing several planks to repair one. The seams are not calked, the swelling of the planks causing them to bind tightly upon each other. To keep them tight, the boat should be put in the water frequently, or, if that is impracticable, well wetted with a hose from time to time.

In Diagonal building (Fig. 3), the planks run diagonally at an angle of 45° from the keel to the gunwale and two thicknesses of planking are used, at right angles to each other. No frames are needed. This is a strong system of building, but necessarily a heavy one. As a rule it is used only for large boats carrying heavy weights.

The Carvel and Diagonal Systems are sometimes combined, two layers of planking being used, one Carvel laid, the other Diagonal. The parts of a boat are shown on Plate 59.

Metallic Boats. Metallic boats are preferable to wooden ones when they are to be carried in such positions that they are of