ON STEAM-ENGINES. 295 



expofed to the atmofphere : for example, that mafles or quan- Mr. WoolPs 

 titles of fleam of the expand ve force of 20, 30, Or 50 pounds ^^7^° 

 the fquare inch of a common fafely-valve, will expand to 20, 

 30, or 50 times its volume, and flill be refpeftively equal to 

 the atmofphere, or capable of producing a fufficient a6lioh 

 againft the pifton of a fleam-engine to caufe the fame to rife 

 in the old engine (with a counterpoife) of Newcomen, or to 

 be carried into the vacuous part of the cylinder in the im- 

 proved engines firft* brought into effect by Meflrs. Boulton and 

 Watt. 



In confequence of this difcovery Mr. Woolf was enabled 

 to ufe his fleam twice (if he chofe), and with complete ef- 

 fect; nothing more being necefTary than to admit high fleam, 

 fuppofe of 40 pounds the fquare inch, into one cylinder, to 

 work there by its expanfiye force, and then to allow the fame 

 fleam to pafs into, and expand itfelf in, another cylinder of 

 forty times the fize of the firft, there to work by condenfation 

 in the common way. Or with only one cylinder, by admit- 

 ting a proportionally fmall quantity of high fleam into it from 

 the boiler, Mr. Woolf, found that he could effect a confider- 

 »ble faving in fuel. 



In this firft improvement of Mr. Woolf, though the faving 

 might be carried a confiderable length, it was ftill necefiarily 

 limited by the ftrength of materials; for in the employment 

 of high fleam there muft always be fome danger of an explo- 

 jion. Mr. Woolf, however, by a happy thought, has com- 

 pletely obviated every danger of this kind, and can now take 

 the full advantage of the expanfive principle without the leafl 

 danger whatever. This he effects by throwing into common 

 ileam the additional temperature necefTary for its high expan- 

 fion, after thefieam is admitted into the working cylinder, which 

 is healed by means adequate to the end intended to be gained ; 

 and the advantage which he thus gains he effectually fecures by 

 a moft ingenious improvement in the pifton. It may be eafily 

 conceived that fleam of fuch high rarity as Mr. Woolf employs, 

 could not be made fully effective with the pifton in common 

 ufe; for in proportion to its rarity fo muft be the facility w,ith 

 whtch a portion of it would efcape, and pafs by the fide of \he 

 pifton to the vacuous part of the cylinder : but Mr. WoolPs 

 contrivance feems perfectly adapted to prevent the lofs of even 

 the fmalleft portion of the fleam, * ■ 



* Befide* 



