246 EIGHTEENTH CENTURY. FT. in. 



at one end of the beam, and the weight of the lump of 

 iron, e, at the other end. 



The lever-beam of this engine is balanced in such a 

 way that when it is not at work the weight e pulls it down 

 on the side away from the engine, and the piston, /, /, is 

 drawn up to the top of the cylinder, as in the figure. To 

 set the engine going a fire is lighted under the boiler, and 

 the tap or stopcock, a, is opened, so that the steam rises 

 into the cylinder, driving out the air through the air-vent, c. 

 As soon as the cylinder is full of steam, a is turned off, and 

 the stopcock, b, turned on. Immediately a small jet of 

 cold water from the tank T rushes through b into the cylin- 

 der, turning the steam back into a few drops of water, 

 which flow out with the cold water down the pipe d. Now 

 notice, the cylinder is quite empty; for the steam drove out 

 the air, and the cold water carried the steam away with it, 

 while no air can come in at c or d, because the little valves 

 in them are kept shut by the weight of the atmosphere out- 

 side. So there is nothing to hold up the piston, which is 

 being heavily pressed down by the air above it. The con- 

 sequence is, down it comes to the bottom of the cylinder, 

 dragging with it the end of the lever-beam. Directly it 

 reaches the bottom the stopcock b has to be shut, and a 

 opened again. Up rises the steam directly from the boiler, 

 driving up the piston, and the whole thing begins again. In 

 this way the lever-beam is kept moving up and down by 

 simply turning the two stopcocks one after the other. These 

 were at first opened and shut by boys ; but one day an in- 

 genious lad named Humphrey Potter, who wanted to save 

 himself the trouble of turning the cocks, found that by 

 tying strings from the handles to the different ends of the 

 beam he could make the engine open its own cocks as the 

 beam went up and down. This rough arrangement was 

 scon improved, and the machine worked by itself. 



