Steam, Steel and Electricity

by James W. Steele

We are now getting the air-pressure through the pipe in both ends of the tube alike, and do not move the pellet either way. To make it move we must do something more, and open one end of the tube, and close that fork of the air-pipe, and thus get all the pressure on one side of the pellet. Remove one finger from the end of the tube, and pinch the fork of the air-tube that is on that side. The pellet will now move toward that end of the tube which is open. Reverse the process, and it can be pushed back again with air-pressure to the other end, and so on indefinitely.

Let us improve the process. We will close each end of the tube permanently, and insert four cocks in the tube and forked pipe.

We have here two tubes inserted at each end of the large tube, and in each of these is a cock. We have each cock connected by a rod to the lever set on a pin in the middle of the tube. We must have these cocks so arranged that when the lever is moved (say) to the right, A. is opened and B. is closed, and D. is opened and C. is closed. Now if the air-pressure is constant through the forked air-tube, and the cock E. is open, if the top of the lever is moved to the right, the pellet will be pushed to the left in the large tube. If the lever is moved to the left, and the two cocks that were open are closed, and the two that were closed are opened again, the pellet will be sent back to the other end of the tube. This movement of the pellet in the tube will occur as often as the lever is moved and there is any air-pressure in the forked tube. There is a supply-cock, opened and an escape-cock closed, and an escape-cock opened and a supply-cock closed, at each end of the tube, every time the lever is moved.

We are using air instead of steam, and the movement of these four cocks all at the same time, and the result of moving them, is precisely that of the slide-valve of a steam-engine. The diagrams of this slide-valve would be difficult to understand. The action of the cocks can be more readily understood, and the result, and even much of the action, is precisely the same.

But to make the arrangement entirely efficient we must go a little further into the construction of a steam-engine. The pellet in the tube has no connection with the outside, and we can get nothing from it. So we give it a stem, thus: and when we do so we change it into a piston and its rod. Where it passes through the stopper at the end of the tube it must pass air- (or steam-) tight. Then as we push the piston back and forth we have a movement that we can attach to machinery at the end of the rod, and get a result from. We also move the cocks, or valves, automatically by the movement of the rod.

Turning now to Fig. 3 again let us imagine a connection made between the rod and the end of the lever in Fig. 2. Now put on the air (or steam) pressure, and when the piston has reached the right-hand end of the tube it automatically, by its connections, closes B. and opens A., and opens D. and closes C. The pellet will be pushed back in the tube and go to the other end of it, through the pressure coming against the piston through the part of the air tube where the cock D. is open. It reaches the left-hand end of the tube, and we must imagine that when it gets there it, in the same manner and by the proper connections, closes D., opens C., closes A. and opens B. If these mechanical movements are completed it must be plain that so long as the air (or steam) pressure is continued in the forked pipe the piston will automatically cut off its supply and open its escape at each alternate end, and move back and forth. Any boy can see how a backward and forward movement may be made to give motion to a crank. All other details in an engine are questions of convenience in construction, and not questions of principle or manner of action.

Of older readers, I might request the supposition that, in Fig. 2, only the valves A. and B. were automatically and invariably opened and closed by the action of the piston-rod of Fig. 3, and that C. and D. were controlled solely by the governor, before mentioned, which we will suppose to be located at E. Then the escape of the steam ahead of the piston must always come at the same time with reference to the stroke, but the supply will depend upon the requirements of each individual stroke, and the work it has to do, and afford to the piston a greater or less push, as the emergencies of that particular instant may require. This arrangement would be one of regularity of movement and of economy in the use of steam. That which is needed is supplied, and no more. This is the principle and the object of the Corliss cut-off, and of all others similar to it in purpose. Their principle is that only the escape is automatically controlled by the movements of the piston-rod, occurring always at the same time with reference to the stroke, while the supply is under control of the movement of the governor, and regulated according to the emergencies of the movement. The governor, in any of its forms, as ordinarily applied, performs only half of this function. It regulates the general supply of steam to the cylinder, but the supply-valve continues to be opened, always to full width, and always at the same moment with reference to the stroke. With the two separate sets of automatic machinery required by engines of the Corliss type, the piston does not always receive its steam at the beginning of the stroke, and the supply may be cut off partially or entirely at any point in its passage along the cylinder, as the work to be done requires. The economic value of such an arrangement is manifest. No attempt is made here to explain by means of elaborate diagrams. It is believed that if the reason of things, and the principle of action, is clear, the particulars may be easily studied by any reader who is disposed to master mechanical details.