T1IK WATT AND THE TREVITHICK ENGINES. 121 



The term " single " refers to its open-top cylinder 

 as originally erected by Newcomen, when it was called 

 the Carloose engine, and so it remained after its re- 

 erection in 1775, under the name Dolcoath new engine, 

 alias Bullan Garden ; but after the last re-erection in 

 1799 it had a cylinder-cover, and was called the Sham- 

 mal 45-inch engine; "working 20 Ibs. to the inch" 

 meant the force on each inch of the piston, including 

 vacuum on the one side of 14 Ibs. and steam on the 

 other side of 6 Ibs. to the inch. 



Watt, on his first visit to Cornwall, in 1777, spoke 

 disparagingly of the Newcomen atmospheric engines 

 " burdened to 6 or 7 Ibs. net to the inch." Fifty years 

 later Stuart described Watt's engine as " using steam 

 of a somewhat higher temperature than 212 degrees, so 

 as to produce a pressure between 17 and 18 Ibs. on each 

 square inch of the piston ; yet in practice, from imper- 

 fect vacuum and friction, it cannot raise more water 

 per inch than would weigh about 8^ Ibs.," 1 or an 

 increase of net force when compared with the New- 

 comen atmospheric of only a pound or two on 

 the inch in the lapse of years embracing the active 

 lifetime of Watt. The cause of this slight increase of 

 power is so simple that it has been passed by unnoticed 

 by very many. The steam pressure in the Newcomen 

 atmospheric was continued unaltered in the Watt va- 

 cuum engine. Trevithick constructed the first boiler 

 and engine capable of safely and economically using 

 the power of high-pressure steam. Nelson was obliged 

 to come to close quarters, that his shot, propelled by 

 weak cannon and low-pressure powder, might penetrate 

 wooden ships. We now manufacture and control high- 



fc>tuart' ' History of the Steam-Engine,' published 1824. 



