Steam Engine, 219 
to the action of cold, in the manner as a great surface of 
air is admitted to the wick of Argand’s lamp. 
3dly. The accurate fitting of the metal piston to the 
cylinder by means of springs, which saves much trouble 
and expence in packing. 
4thly. As all the steam is brought back into the boiler, 
it enables us to use ardent spirits, if necessary. 
An account and drawing of Cartwright’s engine may 
be seen in 1 Tilloch’s Phil. Mag. 1. 
I do not find, however, that Cartwright’s engines are 
much in use. 
Mr. Hornblower’s improvements do not seem to con- 
sist in the adaption of any new principle, for reasons 
stated by professor Robison, in the article steam engine, 
in the Edinburgh Encyclopedia, p. 771, but in the me- 
thods used to make his collars air tight, in the structure of 
his condensing vessel, and the framing of his beam. A 
plate of Hornblower’s engine is given by professor Robi- 
son in that article, and by Dr. Gregory in the 2nd vol. of 
his mechanics. 
The improvements introduced by Mr. Woolf consist 
ill 
1st. His ascertaining the law of the expansion of steam, 
by a volume for each pound per square inch on the safe» 
ty- valve : hence 
2dly. In his using steam at much higher temperature 
than in the common engine : and 
3dly. In his using it twice over in separate cylinders ; 
though this idea seems to have occurred also to Mr. 
Hornblower. 
4thly. In applying heat to the hot steam in the second 
cylinder : and using as the metalline substances as the 
mpan of communicating and preserving heat to the steam. 
5thly. In the form of his boilers, of which I shall give 
