ing disc by six bolts 2 inches diameter,
countersunk in the back of the disc, and tapped into the malleable iron
flange. Besides this attachment, each end of the pin, reduced to 4-1/2
inches diameter, passes through a hole in its corresponding disc, and the
ends of the pin are then riveted over. The crank pin is perforated through
the centre by a small hole about 3/4 of an inch in diameter, and three
perforations proceed from this central hole to the surface of the pin. Each
crank shaft bearing is similarly perforated, and pipes are cast in the
discs connecting these perforations together. The result of this
arrangement is, that a large part of the oil or water fed into the bearings
of the shaft is driven by the centrifugal action of the discs to the
surface of the crank pin, and in this way the crank pin may be oiled or
cooled with water in a very effectual manner. To intercept the water or oil
which the discs thus drive out by their centrifugal action, a light paddle
box or splash board of thin sheet brass is made to cover the upper part of
each of the discs, and an oil cup with depending wick is supported by the
tops of these paddle boxes, which wick is touched at each revolution of the
crank by a bridge standing in the middle of an oil cup attached to the
crank pin. The oil is wiped from the wick by the projecting bridge at each
revolution, and subsides into the cup from whence it proceeds to lubricate
the crank pin bearing. This is the expedient commonly employed to oil the
crank pins of direct acting engines; but in the engine now described, there
are over and above this expedient, the communicating passages from the
shaft bearings to the surface of the pin, by which means any amount of
cooling or lubrication can be administered to the crank pin bearing,
without the necessity of stopping or slowing the engine.

[Illustration: Fig. 54. DOUBLE DISC CRANK. Messrs. Bourne & Co.]

651. _Q._--What is the diameter of the screw shaft?

_A._--The screw shaft is 7-1/2 inches diameter, but the bearings on each
side of the disc are 8-1/2 inches diameter, and 16 inches long. Between the
side of the disc and the side of the contiguous bearings there is a short
neck extending 4-3/4 inches in the length of the shaft, and hollowed out
somewhat to permit the passage of the piston rod; for one piston rod passes
immediately above the shaft on the one side of the discs, and the other
piston rod passes immediately below the shaft on th