being driven by the thread of a horizontal screw, to the head
of which is attached a crank. By a machine of this construction,
properly proportioned, one man may raise about twenty tons weight.
Vertical screws, turned by levers, have been frequently used for the
purpose of raising vessels to repair. But in these cases a large
portion of the power applied is lost in the friction of the screw, and
the process is laborious and tedious. This is probably the most
awkward and injudicious method that has been applied to that purpose.
Another method which has been applied to the purpose of elevating
vessels, is decidedly ridiculous, although less laborious than the
former. It is called the "hydraulic power," and consists in forcing
water into large cylinders, by forcing pumps which are operated by
steam power; while the water thus forced into the cylinder moves a
piston and piston-rod, to which is connected several stout chains,
which passing over corresponding pulleys, descend to a platform, on
which rests the vessel to be raised. An expensive apparatus, called
the "Marine Railway," constructed on the principle of the _inclined
plane_, with a huge and complicated carriage to travel thereon, has
been extensively used for taking vessels out of the water to repair.
This plan is objectionable, however, on several accounts. It requires
the application of a great quantity of power to overcome the friction
of its many axles and machinery, in addition to what is requisite to
overcome the gravity of the vessel. It is, moreover, injurious to the
vessels which are taken up thereby, on account of its elevating the
forward part, before the centre and stern become seated on the
carriage. The most judicious mode in present use, for raising vessels
to repair, and which must be preferred to all others, where there is a
supply of water from an elevated reservoir, is on the principle of
locks; the vessel being floated into one apartment, is elevated by the
induction of water from above, till it can be floated over an elevated
platform, where it is left at rest, while the water is allowed to pass
off below. The sides of this upper box or apartment, are moveable,
being attached to the bottom or platform by hinge joints, so that they
may be let down to a horizontal position, thus giving the workmen the
advantage of light and convenience. The "dry dock" in the Navy Yard at
Charlestown, Mass., is constructed awkwardly enough; but as the
vessels at th