) 
; 
Steam-Engine—Physics, §c- 159 
Mr. Watt soon added the airpump to the condenser, to ex- 
tract the air extricated from the water in boiling, together with 
the water injected. 
The next step was to close the upper end of the cylinder, 
the piston-rod working through a tight packing to exclude the 
air, letting the steam in above, as well as below the piston, by 
an alternate communication, and then condensing it in both 
cases alternately, thus producing a double stroke: at the same 
time deriving some aid from the expansive force of the steam 
on the side of the piston opposite to the vacuum. This is 
essentially the form of all the engines in use at the present 
day. The minor parts devised by Mr. Watt, as the working 
of the valves, &c. were such as would readily occur toa scien- 
tific mechanician. : 
While he was bringing the engine to its present perfection, 
and furnishing it for the numerous mines, manufactories, and 
breweries in Great Britain, variations were devised by Cart- 
Wright, by Hornblower, Woolf, and others in England, and 
more recently by Evans and by Ogden in America, evincing 
much ingenuity, but (with the exception of Evans’s, which is 
a simple engine of high pressure) making the machine more 
complex. 
Watt and Bolton’s engines are more generally used, being 
Properly an atmospheric engine, or working with steam so low 
a8 merely to produce a vacuum in the cylinder, became of enor- 
Mous dimensions, when the power required was that of an 
hundred horses : a scale of estimate adapted to the comprehen- 
Sion of those who had before used the labour of that animal, and. 
Preferred to substitute the steam-engine. 
It had not, however, escaped the notice of Mr. Watt, that 
there existed in steam another source of power besides that of 
atmospheric pressure. The experiments of his learned friend, 
Dr. Black, of Glasgow, as well as those of the French chemists, 
and of Papin, in the instance of his digester, had ascertained 
the laws of its expansive force, and amongst other interesting 
lacts, those subservient to our present purpose; viz. That 
a" “r water has reached the boiling point, 212° of Fahrenheit, 
* caloric which enters it no longer becomes latent, but sen~ 
