Steam Engines which work by a Pijon. 22$^ 



tlafticity, and by the indireft confequence of its condfenfation, which affords a vacuum. 

 This laft is the only principle difplayed in Newcomen's engine. In order to produce the 

 vacuum at pleafure, it becomes requifite that various apertures fhould be formed at the bot- 

 tom of the cylinder or fyringe we have been fpeaicing of: — one to commimicate fteam froni 

 a boiler ; another to admit a jet of cold water, to condenfe that fteam during the interval in 

 which the communication from the boiler is cut off; a third provided with a pipe called the 

 educlion-pipe, to carry off the condenfed fteam and injeftion water; and laftly, a fmall 

 lateral aperture or valve through which the permanently elaftic fluid, which cannot de- 

 fcend through the edudlion-pipe, may be driven out. This laft is called the fnifting' 

 clack. By thefe provifions the operation is made to take place a'- follows: The pifton 

 being up, the fteam-cock is opened, and fteam iffues from the boiler, which being lefs 

 than half the weight of common air, rifes to the top and expels the air through the 

 edu£lion-pipc, of which the lower extremity is covered with a flap valve in a trough 

 of water. When the noife of its efcape is heard the fteam-cock i$ (hut, and the injeftionf 

 pipe being opened throws a ftream of cold water againft the bottom of the pifton. 

 The fteam becomes immediately condenfed, and the preffure of the atmofphere fortes the 

 pifton down into the vacuum. Upon its progrefs downwards the injedlion pipe is clofedj 

 and when it has arrived nearly to the bottom of the cylinder, the fteam-cock is a^ain open- 

 ed. The elaftic fteam then not only fills the fmall fpace between the cylinder and the bot- 

 tom, but its preffure affifts the eduftion water to pafs off through its pipe, and drives the- 

 difengagcd elaftic fluid through the fnift. In this ftate therefore the fteam is foijiewhat 

 ftronger than the atmofphere, and counterpoifes its a£lion on the upper furface of the 

 pifton; whence the pifton itfelf rifes by the action of the counter-weight, and regains Mi. 

 original pofition at the top of the cylinder. A fecond repetition of the procefs, namely, of 

 fliutting off the fteam and injefting cold water, caufes it again to defcend, and in this^ 

 manner the alternations may be continued without limit. 



As the preffure requifite to wrork an engine of this kind with fpeed, and to overcome the- 

 friftion of its parts, may be eftimated at about yf pounds for every round inch, the effe£t 

 •fa pifton three feet in diameter, with ten ftrokes of fix teet each per moment, will be to. 

 raife a weight of near half a ton at the rate of one foot per iecond. Such a power will 

 therefore be very confiderable. 



It is to be underftood, that the opening and ftiutting of the fteam and injedtion cocks 

 are performed by apparatus fixed to the working beain^in iuch a manner as to ftrike the 

 levers of thofe cocks at the precife inftants of time when their effcfls are required to be pro- 

 duced. The attendant has no other office to perform than keeping up the fire. 



This curious and moft eminently ufeful engine became an objeft of the refearches of 

 James W;it', then of Glafgow, but fince alFociated with Matthew Boulton of Birmingham. 

 It would be fuperfluous for me to enter into any general account of the fcience and intelli» 

 gence of either of tJiefe engineers, or of the great advantages which fociety muft ever derive- 

 when the fpirit of enterprife is joined to the exertions of inenrjl ability. Neither can I here 

 attempt to detail the variety of new refuks which are exhibited in the fpecificaiions lodged- 

 by Watt in the Chancery offices concerning tnis objtft. My prefent limits and the atten- 

 tion which the world has already paid to thefe eminent men^ and the produi^s of their en- 

 lightened 



