248 EIGHTEENTH CENTURY. yv. hi. 



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 soon improved, and 

 the machine worked by itself. 



Watt's Separate Condenser — Such was the engine as 

 Watt found it. Wlien he began to examine it he saw at 

 once what an immense quantity of heat was wasted. Every 

 time the piston came down, the cyhnder, as well as the 

 steam in it, had to be cooled down ; every time the piston 

 rose, the cylinder had to be heated again ; and the thing 

 which puzzled him most about it was, that it took six pounds 

 of cold water to condense only one pound of steam. 



It was in this difficulty that he came to Dr. Black, and 

 learnt from him the theory of latent heat, which showed that 

 there is an immense store of heat hidden in steam, which 

 has to be drawn out before it can become water. This was 

 an entirely new light to Watt, and it led him to make many 

 experiments still more exact than those of Dr. Black, which 

 convinced him that no engine would ever work well or 

 economically, while so much power was wasted in cooling 

 and re-heating the cylinder at every stroke. But how was 

 he to cool down the steam without cooling the cylinder 

 which held it ? 



For months he pondered over this without finding any 

 answer. At last, one Sunday afternoon, when he was walk- 

 ing on the Green of Glasgow, the way to do it flashed upon 

 his mind. If he could draw the steam off into a separate vessel 

 and condense it there, the cylinder might still be kept hot, and 

 the thing would be done. Fig. 42 will help you to understand 

 how this could be effected. Here the two flasks, a and 

 B, are first quite emptied of air, and b is half filled with 

 water. Under b is placed a lamp, d ; under a, a basin of 



