JAMES WATT 131 



rare at that time, and later that the University possessed a 

 model of such an engine, which however was at the time in 

 London for repairs. The model was sent for at Watt's re- 

 quest, to enable him to repair it: but it did not arrive until 

 1763. In the meantime, Watt had already made a number of 

 experiments with a Papin digester as boiler, and with the 

 piston and cylinder of a syringe, and had also learned all 

 he could from books, whereby Black also helped him in con- 

 versation. He saw that the pressure of the steam, if the 

 boiler were sufficiently heated, would be amply sufficient to 

 drive an engine, without the use of the air pressure (which is 

 the case to-day with all high pressure engines, for example 

 locomotives); but at that time it was not to be thought of 

 that boilers capable of standing the necessary high pressure 

 in safety could be built sufficiently large; this was no doubt 

 also the reason why Papin had already preferred the low 

 pressure machine, in which the steam was only used to form 

 the vacuum. Watt then soon set the model going, and he 

 also saw the grave defects which had prevented these 

 machines being popular; namely, the excessive consumption 

 of steam. At every stroke of the piston, the cylinder, which 

 had just been cooled, had again to be heated to the tempera- 

 ture of the boiler by the inflowing steam, only to be cooled 

 again, which fact was quite sufficient explanation of the ex- 

 cessive consumption of fuel. While walking one day on a 

 Sunday in spring 1765, Watt conceived the idea of the separ- 

 ate condenser. He perceived that it would suffice to connect 

 the cylinder, which could then remain always hot, with a 

 space always kept cool, after it had been cut off from the 

 boiler, in order to obtain the required vacuum, since the 

 steam would of itself stream into the cold space and there 

 condense to water. He suddenly saw clearly the fact, now a 

 commonplace, that in every space filled by saturated steam, 

 the final pressure will always correspond to the coldest part 

 of the space. 



