or a displacement of about 700 cubic feet, corresponding to a maximum
draught of 3.7 feet. The mean speed is 4 knots, or 4½ miles per hour, a
great velocity being unnecessary, owing to the small distance to cross
in a port often obstructed by the general movement of vessels taking
place therein.
The engine is from 16 to 18 horse-power. Its frame is inclined
perpendicularly to the direction of the screw-shaft, the extremity of
which is supported near the screw by a strengthened cross-stay serving
as a pillow-block. The cylinder is 8 inches in diameter, and the piston
has a stroke of 6 inches, causing the screw (which is 3¼ feet diameter)
to make 200 revolutions per minute. The screw, although it has a wide
surface of thrust, gives, nevertheless, a recoil of about 30 per cent.,
because of its location between the hulls and its oblique action on the
shaft.
The steam is furnished by a tubular boiler having an internal fireplace
and a heating surface of sixteen square meters, the draught being
effected by the exhaust of the engine. This boiler, which is tested
up to 14 pounds, is fed by a steam pump, or by a pump actuated by the
engine. The feed pumps take water successively from one or the other of
the reservoirs in the hulls. The reservoirs are filled in the morning,
and their level is ascertained by two small and ingenious Decondun
indicators, the dials of which are placed against the walls of the
engine-room.
Taken altogether, these little boats are well arranged and quite
handsome; and, since they were put into service in June, 1880, they have
proved a great convenience to the hard-working and active population for
which they were built.
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