A.For large engines, pistons with a metallic packing, consisting of a single ring, with the ends morticed into one another, and a piece of metal let in flush over the joint and riveted to one end of the ring, appears to be the best species of piston; and if the cylinder be oscillating, it will be expedient to chamfer off the upper edge of the ring on the inner side, and to pack it at the back with hemp.

They are generally square-morticed for the rails, which are frequently what we should call split hoop- holes, but in the best kind are slats of hard wood, about two and a half inches wide and one in thickness.

The internal breadth of the mortice in which the valve finger moves is 5/16 inch, and its external depth is 1-3/4 inch, which leaves three eighths of an inch as the thickness of metal round the hole; and the breadth, measuring in the direction of the hole, is 1-1/2 inch.