round holes in the centre of the guide blocks.  Those blocks are of cast iron 6 inches deep, 11 inches wide, and 14 inches long, and they are formed with flanges 1 inch thick on the inner sides of the blocks.  The projection of the air pump lever from the centre of the cross head is 1 foot 9 inches, and it is bent 5-3/4 inches to one side to enable it to engage the air pump rod.  The eye of this arm is 6 inches broad and about 2 inches thick.  At the part where one of the piston rods passes through it, the arm is 8 inches deep and 6 inches wide; but the width thereafter narrows to 3 inches, and finally to 2 inches; and the depth of the web of the arm reduces from 8 inches at the piston rod, to 4 inches at the eye, which receives the end of the air pump rod.  The feed pump arm is only 3 inches thick, and has 9 inches of projection from the centre of the cross head; but the eye attached to it on the opposite side of the cross head for the reception of the other piston rod is of the same length as that part of the air pump arm which one of the piston rods passes through.  The piston rods have strong nuts on each side of each of these arms to attach them to the arms, and also to enable the length of the piston rods to be suitably adjusted, to leave equal clearance between the piston and each end of the cylinder at the termination of the stroke.

[Illustration:  Fig. 53.

CROSS HEAD AND PUMP ARMS. Messrs. Bourne & Co.]

649. Q.—­Will you recapitulate the main particulars of the air pump?

A.—­The air pump is made of brass 12-1/2 inches diameter and 42 inches stroke, and the metal of the barrel is 9/16ths of an inch thick.  The air pump bucket is a solid piston of brass, 6-1/2 inches deep at the edge, and 7 inches deep at the eye; and in the edge three grooves are turned to hold water which answers the purpose of packing.  The inlet and outlet valves of the air pump consist of brass plates 1/2 inch with strong feathers across them, and in each plate there are six grated perforations covered by india rubber discs 7 inches in diameter.  These six perforations afford collectively an area for the passage of the water equal to the area of the pump.  The air pump rod is of brass, 2-1/2 inches diameter.

650. Q.—­What are the constructive peculiarities of the discs and crank pin?

A.—­The discs, which are 64 inches diameter, are formed of cast iron, and are 2-1/2 inches thick in the body, and 5 inches broad at the rim.  The crank shaft is 8-1/2 inches diameter, and the central boss of the disc which receives the shaft measures 10 inches through the eye, and the metal of the eye is 3 inches thick.  In the part of the disc opposite to the crank pin, the web is thickened to 10 inches for nearly the whole semicircle, with the view of making that side of the disc heavier than the other side; and when the engine is stopped, the gravitation of this heavy side raises the crank pin