300 OPEN-TOP CYLINDER. 



It was something like the Newcomen open-topped 

 cylinder of a hundred years before, but with a heavy 

 piston, on the top of which a guide-wheel equal in 

 diameter to the cylinder turned on a pin, to which the 

 main connecting rod was jointed. The guide-wheel 

 prevented any tendency to twist the piston from the 

 angular positions of the connecting rod, and allowed 

 the crank-shaft to be brought comparatively near to the 

 cylinder top. The boiler was cylindrical, of wrought 

 iron, with internal fire-tube and external brick flues; 

 and gave steam of about 40 Ibs. on the inch above 

 the atmosphere, which, acting under the piston, caused 

 the up-stroke, an expansive valve reducing the average 

 pressure in the cylinder by one-half. The down-stroke 

 was made by the atmospheric pressure of 14 Ibs. on the 

 inch, on the piston, its lower side being in vacuum, 

 together with the weight of the thick piston and con- 

 necting rod, and the momentum of the revolving parts. 



My readers must not suppose that this was an 

 attempt to revive the discarded Newcomen engine ; the 

 likeness was only apparent ; its power was mainly from 

 the use of strong expansive steam, giving motion in the 

 up-stroke through a rigid connecting rod, with con- 

 trolling and equalizing crank and fly-wheel. It was 

 not, as the Newcomen, 1 dependent for its power on the 

 atmospheric pressure; and having no cylinder cover, 

 or parallel motion, or beam, was not a Watt engine, 

 though it had the Watt air-pump and condenser. 



The Dolcoath engines continued to work with open- 

 topped cylinders a quarter of a century after the Watt 

 patent ; and when they had passed away, many of 

 Trevithick's high-pressure steam-engines retained the 

 same form of outline, but had neither cylinder covers, 



1 Sec vol. i., p. 5. 



