is formed generally of malleable iron, but in some engines the side frames consist of oak with iron plates riveted on each side.  The guard plates are in these cases generally of equal length, the frames being curved upward to pass over the driving axle.  Hard cast iron blocks are riveted between the guard plates to serve as guides for the axle bushes.  The side frames are connected across the ends, and cross stays are introduced beneath the boiler to stiffen the frame sideways, and prevent the ends of the connecting or eccentric rods from falling down if they should be broken.

504. Q.—­What is the nature and arrangement of the springs of locomotives?

A.—­The springs are of the ordinary carriage kind, with plates connected at the centre, and allowed to slide on each other at their ends.  The upper plate terminates in two eyes, through each of which passes a pin, which also passes through the jaws of the bridle, connected by a double threaded screw to another bridle, which is jointed to the framing; the centre of the spring rests upon the axle box.  Sometimes the springs are placed between the guard plates, and below the framing which rests upon their extremities.  One species of springs which has gained a considerable introduction, consists of a number of flat steel plates with a piece of metal or other substance interposed between them at the centre, leaving the ends standing apart.  It would be preferable, perhaps, to make the plates of a common spring with different curves, so that the leaves, though in contact at the centre, would not be in contact with the ends with light loads, but would be brought into contact gradually, as the strain conies on:  a spring would thus be obtained that was suitable for all loads.

505. Q.—­What is the difference between inside and outside cylinder engines?

A.—­Outside cylinders are so designated when placed upon the outside of the framing, with their connecting rods operating upon pins in the driving wheels; while the inside cylinders are situated within the framing, and the connecting rods attach themselves to cranks in the driving axle.

506. Q.—­Whether are inside or outside cylinder engines to be preferred?

A.—­A diversity of opinion obtains as to the relative merits of outside and inside cylinders.  The chief objection to outside cylinders is, that they occasion a sinuous motion in the engine which is apt to send the train off the rails; but this action may be made less perceptible or be remedied altogether, by placing a weight upon one side of the wheels, the momentum of which will just balance the momentum of the piston and its connections.  The sinuous or rocking motion of locomotives is traceable to the arrested momentum of the piston and its attachments at every stroke of the engine, and the effect of the pressure thus created will be more operative in inducing oscillation the farther it is exerted from the central line of