tio of the cubes
of the velocity. Suppose that we wish to gain these two knots advance
on eight. It is evident that, if the boilers have to generate, and the
engines to use twice the power, and exert twice the force, they must
have also twice the strength. The boiler must be twice as strong and
heavy; the various working parts of the engine must be twice as
strong: the shafts, the cranks, the piston and other rods, the beams,
the cylinders, the frame work, whether of wood or iron, and even the
iron wheels themselves, with every thing in any way employed to use
the power, overcome the resistance, and gain the speed. There is no
working arrangement in any way connected with the propulsion of the
ship that does not partake of this increase; every pump, every valve,
every bolt connected directly or indirectly with the engine economy of
the ship.

2. In the second place, seeing that much greater strength of parts is
required to overcome the increased resistance, it is equally evident
that this high speed and power thus require a larger outlay in every
point of the prime construction of the vessel and engines by which the
speed is to be attained. The hull's heavier timbers cost a higher
price according to size than the direct proportion of size indicates.
Large and choice timbers are difficult to get, and costly. The hull
must also be strengthened to a large extra extent by heavy iron
strapping and bracing, which, unlike the rest, cost in the ratio of
the material used. So with the engines. The shaft, which weighs twice
as much, does not cost only twice as much, but frequently three or
four or five times as much. This arises not from the weight of the
metal, as is evident; but from the difficulty of forging pieces that
are so large. The persons engaged in the forging and finishing of the
immense shafts, cranks, pistons, etc., used in our first class
steamers, frequently consider that the last and largest piece is the
_chef d'oeuvre_ of the art, and that it will never be transcended,
even if equalled again. They have expended all of their skill and
ingenuity in the task, and have not succeeded sometimes until they
have forged two or three new pieces. When a great work of this kind is
done, it may be discovered in the turning, polishing, and fitting up,
that it has at last a flaw, and that it will not do for the service
intended. As a matter of course, it must be thrown aside and a new
piece forged. This was but recently the c