THE STEAM-ENGINE. 



441 



atmosphere. If the principle of expansion, as above explained, be attentively 

 considered, it will be evident that the extent of its application will mainly 

 depend on the density and pressure of the steam admitted from the boiler. If 

 the density and pressure be not considerable when the steam is cut off, the 

 extent of its subsequent expansion will be proportionally limited. It was in 

 consequence of this, that this principle from which considerable economy of 

 power has been derived, was applied with much less advantage by Mr. Watt 

 than it has since been by others, who have adopted the use of steam of much 

 higher pressure. In the engines of Boulton and Watt, where the expansive 

 principle was applied, the steam was cut off after the piston had performed 

 from one half to two thirds of the stroke, according to the circumstances under 

 which the engine was worked. The decreasing pressure produced by expan- 

 sion was, in this case, especially with the larger class of engines, little more 

 than would be necessary to allow the momentum of the mass moved to spend 

 itself, before the arrival of the piston at the end of the stroke. 



Subsequently, however, boilers producing steam of much higher pressure 

 were applied, and the steam was cut off when the piston had performed a 

 much smaller part of the whole stroke. The great theatre of these experiments 

 and improvements has been the mining districts in Cornwall, where, instead 

 of working with steam of a pressure not much exceeding that of the atmosphere, 

 it has been found advantageous to use steam whose pressure is at least four 

 times as great as that of the atmosphere ; and instead of limiting its expansion 

 to the last half or fourth of the stroke, it is cut off after the piston has performed 

 one fourth part of the stroke or less, all the remainder of the stroke being ac- 

 complished by the expansive power of the steam, and by momentum. 



For several years after the extension of Watt's first patent had been obtained 

 from parliament, he was altogether engrossed by the labor of bringing to per- 

 fection the application of the steam-engine to the drainage of mines, and in 

 surmounting the numerous difficulties which presented themselves to its general 

 adoption, even after its manifold advantages were established and admitted. 

 When, however, these obstacles had been overcome, and the works for the 

 manufacture of engines for pumping water, at Soho, had been organized and 

 brought into active operation, he was relieved from the pressure of these 

 anxieties, and was enabled to turn his attention to the far more extensive and 

 important uses of which he had long been impressed with the conviction that 

 the engine was capable. His sagacious mind enabled him to perceive that 

 the machine he had created was an infant force, which by the fostering in- 

 fluence of his own genius would one day extend its vast power over the arts 

 and manufactures, the commerce and the civilization of the world. Filled with 

 such aspirations, he addressed his attention about the year 1779, to the adapta- 

 tion of the steam-engine to move machinery, and thereby to supersede animal 

 power, and the natural agents, wind and water. 



The idea that steam was capable of being applied extensively as a prime 

 mover, had prevailed from a very early period ; and now that we have seen 

 its powers so extensively brought to bear, it will not be uninteresting to revert 

 to the faint traces by which its agency was sketched in the crude speculations 

 of the early mechanical inventors. 



Papin, to whom the credit of discovering the method of producing a vacuum 

 by the condensation of steam is due, was the earliest and most remarkable of 

 those projectors. With very limited powers of practical application, he was, 

 nevertheless, peculiarly happy in his mechanical conceptions ; and had his 

 experience and opportunities been proportionate to the clearsighted character 

 of his mind, he would doubtless have anticipated some of the most memorable 

 of his successors in the progressive improvement of the steam-engine. 



