to the highest point it can attain, whereby it is placed in mid stroke, and cannot rest with the piston rods and connecting rod in a horizontal line.  The crank pin is 8-1/2 inches diameter, and the length of the bearing or rubbing part of it is 16 inches.  It is secured at the ends to the discs by flanges 18 inches diameter, and 2 inches thick.  These flanges are indented into thickened parts of the discs, and are each attached to its corresponding 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 the other side of the discs.  A short piece of one piston rod is shown in fig. 54.