JAMES WATT. 25 



proved, could not have operated with much effect. But, 

 imperfect as it was, it exemplified two new principles of the 

 highest importance, neither of which appears to have been 

 thought of, in the application of the power of steam, before his 

 time. The first is the communication of the moving force of 

 that agent to bodies upon which it cannot conveniently act 

 directly, by means of the piston and its rod. The second is 

 the deriving of the moving force desired, not from the expansion 

 of steam, but from its other equally valuable property of con- 

 densibility by mere exposure to cold. Papin, however, it is 

 curious enough, afterwards abandoned his piston and method 

 of condensation, and reverted to the old plan of making the 

 steam act directly by its expansive force upon the water to be 

 raised. It is doubtful, however, whether he ever actually erected 

 any working engine upon either of these constructions. Indeed, 

 the improvement of the steam-engine could scarcely be said to 

 have been the principal object of those experiments of his 

 which, nevertheless, contributed so greatly to that result It 

 was, in fact, as we have seen, with the view of perfecting a 

 machine contrived originally without any reference to the 

 application of steam, that he was first induced to have recourse 

 to the powers of that agent. The moving force with which he 

 set out was the pressure of the atmosphere ; and he employed 

 steam merely as a means of enabling that other power to act. 

 Even by such a seemingly subordinate application, however, of 

 the new element, he happily discovered and bequeathed to his 

 successors the secret of some of its most valuable capabilities. 



We may here conveniently notice another ingenious con- 

 trivance, of essential service in the steam-engine, for which we 

 are also indebted to Papin — we mean the safety-valve. This is 

 merely a lid or stopper, closing an aperture in the boiler, and 



