matter that might, under the ordinary conditions of working, be deposited on the valve faces.

In the next figure a more modern form of double-beat valve is illustrated. This is an improvement on the previous design, because for a given area the valve is lighter. In all valves of this type it is important that both valve and seat should be of the same metal, so as to avoid difference between the expansion of the two parts, otherwise there will be a continual leakage of steam.

Fig. 128.

By combining two double-beat valves an extremely small lift for a given area is obtained, a point of some importance in large engines where trip gear is used. Such a valve is shown in the annexed figure. With this type the necessity of attention to equal expansion is emphasised, there being four faces liable to leakage; and first-class workmanship is indispensable to satisfactory working. A common arrangement of Cornish valve motion is shown in outline in Fig. 125. The position of the cams depends upon the desired point of cut-off. The valves, of course, can have no lap, but by placing the steam cams so that they do not lift the valve until the valve-rod is past its central position the same effect is produced as by a lap on a slide valve. If the levers and cams were set so that when the valve-rod was in the centre of its travel all the valves were just on the point of being lifted or released, the

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resulting distribution would be similar to that given by a slide valve having neither steam nor exhaust lap.

In designing the cams it is important to see that the first point of contact is near the fulcrum of the cam. The advantage of this

Fig. 124.

is twofold. It throws less strain on the valve gear, because at the instant of contact the valve is not in equilibrium. In the second place the speed of the striking part of the cam is slow, and the action, therefore, quiet. When the valve has been lifted it is in equilibrium, and as the point of contact is continually receding from the fulcrum of the cam, the opening is rapid; also, on closing,

the valve is let down to its seat without a jar. This action will be better understood from Fig. 126, where the cam and lifting lever are shown in two positions, the first in which the valve is on the point of being lifted from its seat, the second in which the valve has attained its extreme lift.

The Cornish valve, when adapted to a tripping arrangement,

forms a useful gear for large winding engines. The essential feature of this class of engine is that it must start rapidly and maintain its speed to the latest possible point in the wind. The gear about to be described accomplishes the desired action in a very effective manner. It was designed by the author in 1898 for a large winding engine in the Lancashire coalfield. In Fig. 127 the valve-rod A and the rocking-rod B receive motion from the eccentrics through the medium of a reversing motion of the straight link type. Until the maximum speed is attained the steam is carried on for nearly the whole length of the stroke, but when the maximum is reached the governor moves the small cams C into a position that brings Fig. 126.

them in contact with the rising trip hook.

The valve, spindle, stirrup, and hook are then left to fall to the closed position by reason of their weight, and the air cushion, which can be regulated by a small thumb screw, prevents the valve falling violently on its face. In cases where the valve is light and the pressure high, a spring would be necessary to close it promptly, because of the unbalanced area of the valve spindle and the friction of the stuffing box. The maximum speed being once attained, a very small amount of steam is sufficient to maintain it, because the load becomes lighter every instant, owing to the diminishing weight of rope on the ascending cage and the increasing weight on the descending rope. Tripping, of course, must take place before the trip hook begins to descend—that is, somewhat before half-stroke -as in the single eccentric Corliss gear. The indicator diagram from a complete wind will illustrate the action clearly.

From the figure it appears that after about five revolutions the speed at which tripping occurs is obtained. For several strokes the speed increases, as shown by the early tripping, until, towards the end of the wind, a mere breath is admitted. The stop valve is then closed, the brakes applied, and the engine rapidly brought to rest.

The action of the trip may be clearly shown by means of a Zeuner diagram. In the figure the clearance between the lifting Fig. 128.

Atmospheric

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lever and the trip hook is drawn as a lap circle, and the lead being fixed upon the angle of the eccentric is known. Draw the valve circle, and from the centre A, with a radius equal to the distance

between the pallett of the catch and the cam, + clearance between lifting lever and trip hook, + overlap of tripping edges (D in Fig. 127), describe an arc intersecting the valve circle.* A line drawn

*This construction applies where the distance from the fulcrum of the catch to the point at which the cam strikes is equal to the distance from the fulcrum to the working edge of the catch. Should these be unequal the radius of the arc will be increased or diminished proportionally. In Fig. 127, should the distance E exceed the distance F, the radius of the arc will be greater by the difference of these distances referred to the distance D.