figure. The bottom side of the piston was formed with a flange, and the upper side with a recess to receive the "junk ring." Between this flange and junk ring, J R, a packing or expansion ring, E R, of cast iron was fitted. This ring was pressed out against the sides of the cylinder by small springs, as shown in the plan. The junk ring was fixed down by pins, P, which screwed into brass

OLD FORM OF LARGE CYLINDER PISTON.

nuts, recessed into the body of the piston at suitable intervals round its circumference. The defect of this old form of piston lay in the springs, which could not be properly adjusted to give a uniform pressure on all parts of the ring. Besides which, the springs often broke, and were not free to move laterally, or to follow the bore of a cylinder when it became worn.

The St. Rognvald's Low Pressure Cylinder Piston.-It will be seen that the Buckley spring with which this piston is fitted consists of a straight spiral spring, bent into a circle just like that of the spiral spring in the Lancaster piston. It forces the two piston rings out against the walls of the cylinder, and at the same time presses them firmly between the junk ring and the under side of the piston. The pressure between the piston rings and the cylinder, results from the natural elasticity of the spiral spring trying to regain its natural shape. There is, therefore, not much danger of excessive friction being set up between the rings and the sides of the cylinder, if the whole has been properly proportioned for and fitted into the cylinder.

The packing rings in most pistons are made of hard springy cast-iron, turned to a slightly larger diameter than the bore of the cylinder, and afterwards cut across to permit of compressing and of inserting them into the cylinder along with the piston.

Specification for the s.s. "St. Rognvald's" Pistons.-To be of cast iron, 1 inch thick, turned to size of cylinders, and made hollow with strong feathers uniting top and bottom metal, fitted with Buckley's patent packing rings, and springs. The pistons to be turned tapered for inner edge of junk ring so that it can be easily removed. The junk ring to be secured with square-headed screw pins, screwed into brass nuts recessed into the body of the piston; the heads of the screw pins to be secured with solid guard rings. No core holes to be made in top or bottom metal of pistons.

Piston Rods and Crossheads.-Piston rods are for the purpose of joining the piston to the connecting rod or direct to the crank shaft in the case of oscillating engines. They are usually made of forged mild steel, turned round, and parallel with tapered or conical ends, the one end being carefully fitted and bolted to the piston, as in the following illustration, and the other keyed to the crosshead.

In the following figure the part which connects the end of the piston rod to the connecting rod is termed the crosshead. In direct-acting engines, the crosshead requires to be supported by guides, in order to bear the side pressure thrown on it by the oblique action of the connecting rod. If the crosshead were not supported, and the piston rod not guided in any manner, then this side pressure (or pressure at right angles to the direction of motion of the piston) would cause injurious bending of the piston rod, and very excessive wear of the glands and packing in the stuffing boxes. In the case of a beam engine, the intervention of the parallel motion obviates the necessity for guides, since the pistonrod end moves in a straight line, and the side pressure is supported by the parallel motion bars. If the wear at the joint of the crosshead and connecting rod is intended to take place in the connecting rod, the pin which passes through both is fixed in the crosshead or forms part of it; and the connecting rod rotates through a certain angle about the pin, and is fitted with bushes in order to provide for the wear. If, however, as is more generally the case, the wear is imposed upon the crosshead, the pin is then fixed in the connecting-rod end, and the crosshead is provided with a bush where the pin passes through it. The pin therefore oscillates with the connecting rod, as shown in our figure. A square hole is cut out of the solid steel to receive the bush, which is formed in halves. The connecting-rod end is forked, and passes over the ends of the bush. The pin is prevented from turning in the connecting rod, and all the wear takes place in the crosshead bush. The halves of this bush are held together by a wrought-iron cover with bolts passing through it. These bolts must be of sufficient section at the bottom of the thread to withstand a tension equal to the initial effective pressure of the steam multiplied by the area of the cylinder. The crosshead is carried out at the sides to receive two flat cast-iron slide plates or slippers, which bear against the planed columns, and resist the side pressure of the connecting rod. These plates fit round the crosshead with three flanges only, so that they may be pushed into position, or taken out without disconnecting any of the larger parts; and they are prevented from slipping out at the one side by small screwed pins which are fitted in all round.

S.S."St. Rognvald's" Piston Rods. To be forged of mild steel, well fitted to the pistons, and secured to same by a nut (not recessed into piston). Rods to be 6 inches diameter below pistons, and 4 inches diameter above pistons. The lower ends (or crossheads) to be forged solid, slotted, and fitted with strong hard brasses (flat top and bottom) secured with malleable-iron covers and two steel bolts, 3 inches diameter, with guard rings on the nuts. Steel pinching pins, also split pins, to be put through the points of these bolts.