WATT’S ENGINE.
I have already told you that Watt suggested the use of steam alternately on each side of the piston; and carried it out by closing the top of the cylinder, and allowing the rod of the piston to pass through a stuffing-box or gland. I now have to explain to you how this alternate admission of the steam may be effected.
You evidently require first an opening at the top and bottom of the cylinder, communicating with the boiler, one only being open at a time; but in this case, where is the steam to escape that was on one side of the piston when the opposite side was being acted upon? It must go somewhere, but evidently must not return to the boiler. Hence, some method has to be contrived by which, when one end of the cylinder is open to the boiler, the other may be open to the air or to the condenser (in which the steam is cooled under Watt’s plan). Fig. 61 will, I think, render clear one or two of these arrangements.
Fig. 61.
The first is the four-way cock, a very simple contrivance, easily and frequently used in models. You must first understand how a common water or beer tap is made. Fig. 61, A, represents one in section, turned so as to open the passage along the pipe to which it is attached; C is the pipe in which is the tap, a conical tube of brass set upright, and with a hole right and left made through it, fixed into a short horizontal tube (generally cast with it in one piece). Into this fits very exactly the conical plug B, also with a hole through it sideways. When this is put into place, no water or other liquid can pass, unless the hole in the plug is in the same direction with the hollow tube forming an open passage. If a key is put on the square part of the plug, and it is turned half round, the passage through the pipe will be closed. A steam tap would be made in a similar manner, if its only office were to open and close a passage in a tube. But we now want two passages closed and two opened, and then the alternate pair closed and opened. This is cleverly effected by a four-way cock.
At D is shown a section of the steam cylinder and piston, with the stuffing-box and all complete. A pipe enters this at the top and bottom, and another crosses it in the middle, making four passages. Shaded black is the four-way cock, the white places showing the open channels through the plug. When this plug stands as at D, steam can pass from the boiler to the top of the cylinder only, above the piston, which it drives downward; the steam below the piston escapes through the other open-curved channel into the air, or to the condenser. Just as the piston reaches the bottom of the cylinder, the tap is turned, and the passage stands as seen at E. Steam now passes to the bottom below the piston, driving it upward, and the steam above it, which has done its work, passes outward through the other open channel of the tap.