262 DISCOVERY OH. 



piston would thus be brought to the top of the cylinder 

 again, ready for another stroke. 



Watt's constructive work on the steam-engine began 

 after 1757, when he was given a workshop as a mathe- 

 matical instrument maker within the walls of the 

 University of Glasgow. He first tried some experiments 

 on a Papin's digester, which is merely a small sealed 

 iron vessel having a safety-valve, so that water can be 

 boiled in it under a pressure much greater than that of 

 the atmosphere. Watt made a form of steam-engine 

 by fixing upon the digester a syringe one-third of an inch 

 in diameter with a solid piston, and furnished also with 

 a tap to admit the steam from the digester or shut it off 

 at pleasure, as well as to open a communication from 

 the inside of the syringe to the open air, by which the 

 steam contained in the syringe might escape. The 

 steam from the digester forced the piston up, and was 

 allowed to escape ; the pressure of the atmosphere then 

 pushed back the piston to its original position, and 

 the action could be repeated. This single-acting high- 

 pressure syringe engine was made and experimented 

 upon by Watt about 1761 ; and, with certain additions, 

 still constitutes one of the best forms for road motors. 



In 1763-64, Watt had to repair a model of Newcomen's 

 engine belonging to the Natural Philosophy Department 

 of the University of Glasgow. He saw that the alternate 

 heating of the cylinder by steam and cooling it by water 

 involved great loss of heat ; and he set to work to 

 devise a means of remedying this and other defects of 

 the engine. The solution of the chief difficulty occurred 

 to him while taking a walk on a fine Sunday afternoon 

 in 1765. He tells us : 



I was thinking upon the engine at the time, and had gone as 

 far as the herd's house, when the idea came into my mind that, 





