Steam-Engine — Physics^ ^c. 159 



Mr. Watt soon added the airpump to the condenser, to ex- 

 tract the air extricated from the water in boihng, 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 jsame 

 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 to a 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 engine, as most generally used, being 

 properly an atmospheric engine, or working with steam so low 

 as merely to produce a vacuum in the cylinder, became of 

 enormous dimensions, when the power required was that of 

 an hundred horses : a scale of estimate adapted to the com- 

 prehension 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 

 facts, those subservient to our present purpose ; viz. That 

 after water has reached the boiling point, 212*^ of Fahrenheit, 

 the caloric which enters it no longer becomes latent, but «.pn- 



