ame, which can be raised or lowered in grooves like a window
and the screw shaft within the ship can be protruded or withdrawn by
appropriate mechanism, so as to engage or leave free this short shaft as
may be required. When the screw has to be lifted, the screw shaft is drawn
into the vessel, leaving the short shaft free to be raised up by the
sliding frame, and the frame is raised by long screws turned round by a
winch purchase on deck. A chain or rope, however, is better for the purpose
of raising this frame, than long screws; but the frame should in such case
be provided with pall catches like those of a windlass, which, if the rope
should break, will prevent the screw from falling.

DETAILS OF THE PADDLES AND PADDLE SHAFT.

483. _Q._--What are the most important details of the construction of
paddle wheels?

_A._--The structure of the feathering wheel will be hereafter described in
connection with an account of the oscillating engine; and it will be
expedient now to restrict any account of the details to the common radial
paddle, as applied to ocean steamers. The best plan of making the paddle
centres is with square eyes, and each centre should be secured in its place
by means of eight thick keys. The shaft should be burred up against the
head of these keys with a chisel, so as to prevent the keys from coming
back of their own accord. If the keys are wanted to be driven back, this
burr must be cut off, and if made thick, and of the right taper, they may
then be started without difficulty. The shaft must of course be forged with
square projections on it, so as to be suitable for the application of
centres with square eyes. Messrs. Maudslay & Co. bore out their paddle
centres, and turn a seat for them on the shaft, afterward fixing them on
the shaft with a single key. This plan is objectionable for the two
reasons, that it is insecure when new, and when old is irremovable. The
general practice among the London engineers is to fix the paddle arms at
the centre to a plate by means of bolts, a projection being placed upon the
plates on each side of the arm, to prevent lateral motion; but this method
is inferior in durability to that adopted in the Clyde, in which each arm
is fitted into a socket by means of a cutter--a small hole being left
opposite to the end of each arm, whereby the arm may be forced back by a
drift.

484. _Q._--How are the arms attached to the outside rings?

_A._--Some engineers join the pad