STEAM AND GASOLINE ENGINES 181 



and the steam inlet. The steam pressure used was slight, and it 

 did not push the disk up. This was raised by the weight on the 

 end of the lever, which weight also pushed the pump rod down. 

 When the air was all expelled from the cylinder, and it was full of 

 steam, the valve on the air vent was closed as also was the one 

 on the steam inlet. Then the valve on the water pipe was opened 

 and cold water let into the cylinder. This condensed the steam 

 to water, which occupied only one two-thousandth of the space of 

 the steam. Then air pressure forced the disk down, which brought 

 down the arm of the lever to which it was attached and raised 

 the other end with the attached weight and pump rod. The 

 valves in the air pipe and in the water vent were now opened, the 

 water let out of the cylinder, and the process was started over 

 again. In spite of the fact that this engine was very crude and 

 that the valves were operated by hand it was used to pump 

 water out of the British mines, for it was an improvement on 

 hand- or horse-power. 



It remained for a resourceful Scotch lad, Humphrey Potter, 

 who tended the valves on such a pumping engine at a mine, to 

 rig ropes from -the valve handles to moving parts of the engine 

 so that they were opened and closed at the proper times. The 

 engine thus became automatic. This arrangement was called a 

 "scroggin" a Scotch word, meaning "lazy." 



A model of Newcomen's engine in the museum of the Uni- 

 versity of Glasgow was turned over for repair in the year 1763 

 to James Watt, an instrument maker connected with the univer- 

 sity. This led him to think of various means of improving this 

 crude device and to the invention of a real steam engine, one in 

 which steam alone furnished the propulsive power. Watt called 

 his engine a "fire engine " (Fig. 71, p. 182). He saw that the 

 expansive power of the steam itself could be used to force the 

 piston head first one way and then the other in the cylinder. 

 He built the cylinder of his engine closed at both ends with the 

 piston rod coming out at one end through a steam-tight packing 

 of greased tow. He arranged the valves in a way to let steam in 



