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 paddle arms to the outer ring by means of bolts; but unless very carefully fitted, those bolts after a time become slack sideways, and a constant working of the parts of the wheel goes on in consequence.  Sometimes the part of the other ring opposite the arm is formed into a mortise, and the arms are wedged tight in these holes by wedges driven in on each side; but the plan is an expensive one, and not satisfactory, as the wedges work loose even though riveted over at the point.  The best mode of making a secure attachment of the arms to the ring, consists in making the arms with long T heads, and riveting the cross piece to the outer ring with a number of rivets, not of the largest size, which would weaken the outer ring too much.  The best way of securing the inner rings to the arms is by means of lugs welded on the arms, and to which the rings are riveted.

485. Q.—­What are the scantlings of the paddle floats?

A.—­The paddle floats are usually made either of elm or pine; if of the former, the common thickness for large sea-going vessels is about 2-1/2 inches; if of the latter, 3 inches.  The floats should have plates on both sides, else the paddle arms will be very liable to cut into the wood, and the iron of the arms will be very rapidly wasted.  When the floats have been fresh put on they must be