(2) The low-pressure cylinder at the top. (3) The low-pressure cylinder encasing the high-pressure cylinder.

But in each of these cases, as also in compound horizontal engines, it is usual to confine the expansion to one pair of cylinders.

THE USE OF AN INTERMEDIATE RECEIVER.

In another class of compound engines there are two cranks at right angles, but only one cylinder connected with each crank.

Here each cylinder forms, as it were, an engine complete in itself; the cylinders (called A and B, as before) are placed side by side, and are of equal length, and the point to be noticed is, that the pistons in A and B no longer move together, but that one leads the other by half a stroke.

It is clear that the mode of exhausting at once from A into B is no longer applicable, and that some special method of distributing the steam, different from anything that we have yet seen, must be arranged.

The difficulty arises from the fact that the directions of motion of the pistons cross each other, whereby, for example, when the piston in A is at the end of its stroke and about to ascend, that in B is in its middle position and is desending.

In order to get over this obstacle, Mr. Cowper

ΙΟ

has proposed to place an intermediate receiver between the cylinders A and B, which shall act as an exhaust reservoir for the steam coming from A, and as a boiler for the steam going into B. It appears that engines with a receiver have worked well in practice, but it seems difficult to justify the use of this arrangement by a strict reference to the principle of the theory of heat.

A general idea of the arrangement of the engine proposed by Mr. Cowper may be gathered from the Fig. 18, where the cylinders A and B

are placed side by side, and the high-pressure cylinder A is enveloped in a steam receiver or reservoir, marked C, the content of which is perhaps three times that of A.

In a working engine on this plan, steam (say at 70 lbs pressure) would enter A and be cut off at half-stroke; it would thus expand and finally exhaust itself into the receiver, where the pressure would vary from, say, 10 lbs. to 14 lbs.

The receiver would supply steam for the lowpressure cylinder B, just as if it were the boiler of an ordinary engine, and the pressure of the steam in C would fall to 10 lbs. when the

demand upon it was made, but would rise to 14 lbs. when fresh steam entered it from A. The temperature of the steam in the jacket surrounding A is therefore much below that of the entering steam, which is so far a departure from the old practice.

CHAPTER VI.

SLIDE VALVES.

THIS is a branch of the subject deserving the special attention of the engineer.

Its importance in regard to the economical working of the steam engine cannot be overestimated.

The slide valve ordinarily used in high-pressure engines, and the manner of its operation, are well known to nearly every practical mechanic and engineer. It will be remembered that the operations of admitting the fresh steam, and releasing, the waste steam, are alternately performed by the same valve and the same motion.

The valve being made to slide backwards and forwards upon the face of the ports, opens and closes the several passages in their turn.

The two extreme ones, called the steam ports, communicate with each end of the cylinder.

The middle one is called the exhaust port, and its corresponding passage terminates in a pipe open to the atmosphere.

Steam is admitted freely into the steam chest from the boiler, and the valve is made of sufficient length to cover all the ports, when it is placed in the centre of the stroke.

When it is in this position no steam can enter the cylinder, but as the valve moves on one of the ports opens, the arrangement of the valve gearing being such that when the piston is ready to begin its stroke, the steam port begins to open.

During the stroke of the piston, the valve not only travels to the end of its stroke, but also returns to the point from whence it set out, and its continued motion in the same direction finally closes the port and prevents any further admission of steam. The steam has now done its work, and must be removed. In the middle of the valve a hollow chamber is formed of sufficient length to open between the ports.

As soon as the edge of this chamber passes the edge of the steam port, the pent up steam finds vent and rushes through the exhaust port and escapes through the exhaust pipe into the atmosphere.

Now looking at Fig. 19 you notice that

the exhaust port opens when the steam port closes, and that both happen just at the end of the stroke.