CH. xxvni. WATT'S ENGINE. 251 



piston down. But now, the piston having pulled down the 

 beam, a and c will be closed and the other two cocks, b and 

 /, will be opened. So the steam above the piston will rush 

 out at b into the condenser, while the steam from the boiler 

 will pass directly from e down to d y and coming in below the 

 piston, will drive it up again. In this way, although the cylin- 

 der is never cooled, the piston moves steadily up and down ; 

 because the steam is driven off into the condenser standing 

 in B, where it is turned into water, and is drawn up by the 

 two pumps D and E, and sent along the pipe, s, s, back to the 

 boiler. 



This was the principle of Watt's double-acting steam- 

 engine, and if you understand the difference between Figs. 

 41 and 43 you will see that, though Watt was not the first to 

 make engines move by steam, he was 'the first to make a 

 pure steam-engine, where the piston moves up and down 

 without any help from the outside air, or of the counter- 

 balancing weight e, Fig. 41, and without the enormous waste 

 of heat and fuel which made all the earlier engines com- 

 paratively useless. 



I have only told you here of the way in which he applied 

 steam to his engines ; all the numberless other improvements 

 which he made you must read about in books on engineering. 

 For twenty long years he went on improving and inventing 

 without reaping any reward for his labour. Other men tried 

 to steal his ideas and to make a profit out of his genius, and 

 he had to fight against prejudice and injustice, and against 

 constant depression caused by his own ill-health. Yet he 

 found many kind friends upon his road, and amongst the 

 most famous of these was Boulton, the Birmingham manu- 

 facturer, who became his partner in 1769, and stood by him 

 manfully in all his difficulties and troubles. It was from 



