THE Societe Centrale de Sauvetage des Naufrages has recently found itself obliged to provide a pontoon boat-house for one of its motor boats, owing to the difficulty of obtaining a satisfactory site for a boat-house and slipway in the crowded harbour of Marseilles.

As the Institution is liable to meet with a similar difficulty, owing to the increased shipping at certain harbours where a Life-boat is stationed, we give a translation of the description of the French floating dock, which we believe will be interesting to our readers.

The boat - house with movable platform comprises two pontoons of rectangular section, each 44 ft. in length and 3 ft. in breadth and 9 ft. 10 ins.

in height. They are kept separated and parallel to one another by means of iron ties and cross bars, leaving a space of 16 ft. 4 ins. between the two.

The rear ends of the two pontoons are joined to one another by a vertical metallic partition, suitably strengthened, and, at the dock entrance the pontoons are joined by two strong bottom gussets shaped to take the boat. Each float is fitted with an elm keel as a protection in case of stranding and a side fender of pitch pine as a protection against collision.

The structure is covered with a corrugated-iron roof with glazed skylights to afford light. This roof is carried by four corner posts, and elsewhere by uprights and iron trusses.

The front of the dock is closed by a wide door in two portions, of openwork and of sliding pattern, actuated from the inside of the dock by means of tackle.

The remainder of the boat-house in front, at the rear and at the sides, between the pontoons and the roof, is closed by timber boards resting on iron cills, glazed windows being provided at the sides.

The usual access to the house is at the rear by means of an iron ladder and an ordinary door, which can be locked.

A metal platform, placed in the space between the pontoon, is provided for carrying the Life-boat and for affording means of lowering her into the water and raising her clear of it.

The platform is covered with movable and interchangeable iron gratings for effecting repairs to the boat. The boat's keels rest on three timber longitudinals fixed to the platform. The latter, with suitable guides at the four corners, is slung on four flexible steel cables fitted with rigging screws and coiled round the drum of four windlasses with worm gear.

By means of two longitudinal shafts, connecting the windlasses in pairs and by means of bevelled pinions, the windlasses are set in motion simultaneously by a transverse shaft with cranked handles placed at the rear of the bouse, and worked by six men. At each angle the ratio between the pinions is 1 to 2.

The cog-wheels of the windlasses have fifty teeth and are driven by worm gear. The thread of the worm on the port side is necessarily the opposite hand to that of the worm on the starboard side.

Under these conditions, the time required to lower the boat into the water is from seven to eight minutes,and it requires about twenty minutes to hoist the boat into position again.

In order to fix the platform and not leave it slung on the drums, four chains are provided with hooks and straining screws secured to the windlass frames.

After the platform has been hoisted to the required height the hooks are passed into the eye-bolts on the platform, and the latter is allowed to drop until the chains are taut.

A platform, consisting of an iron grating, fixed at the rear, and two j movable timber gangways at the front | of the house, complete the installation.

Ballast consisting of four tons of cement is needed to ensure stability and the draught required by the structure, so that it may offer sufficient resistance to wind-pressure on its sides.

The installation, in addition, includes four mooring rings, fixed at the ends of the pontoons, and a portable pump for emptying them.

The total weight is 39 tons..