HISTORY OF SCIENCE. 



regularly made to work in the class lecture-room before the students. 

 But Watt was not content with his success, for his mind had been 

 directed to many points concerning the theory of the machine, and he 

 had ascertained many facts relating to that theory. He made experi- 

 ments to discover the proportion in which water is expanded by con- 

 version into steam ; the quantity of water that is vaporized by a given 

 weight of coal ; the quantity of steam expended at each oscillation of 

 Newcomen's engine ; the quantity of cold water that must be thrown 

 into the cylinder to cause a certain effective force to operate on the 

 descending piston ; and, lastly, he determined the elastic force of steam 

 at various temperatures. It appears to have been about the year 1763 

 or 1764 when Watt's attention was first directed to the improvement 

 of the steam-engine, but it was not until 1774 that he succeeded in 

 making a model to work satisfactorily. To describe all the steps by 

 which Watt brought the steam-engine to perfection, and the beautiful 

 mechanical combinations he devised, would require a separate treatise, 

 while but a short notice of the main improvements is all that can here 

 be given. The several machines previously projected or constructed, 

 in which steam appeared as the agent, do not in general require any 

 special description here, as with- two exceptions they did not prove suc- 

 cessful in practice. We may pass over Hero's machine and the contro- 

 versies of Solomon de Caus, of the Marquis of Worcester, and of Sir 

 Samuel Morland. DENIS PAPIN, a Frenchman, who however was living 

 in England at the time he was engaged in his experiments, conceived 

 the idea of creating a vacuum by means of steam beneath a piston 

 moving in a cylinder. His plan was to place a small quantity of water 

 in the bottom of his cylinder, which, when heated, converted some of 

 the water into steam, and this, by its expansive force, raised the piston 

 against the pressure of the atmosphere. This done, the fire was re- 

 moved and the cylinder allowed to cool, when the steam which filled 

 it condensed again into water, and in this state, taking up only a small 

 portion of the space of the steam, left a vacuum which, opposing 

 nothing to the weight of the atmosphere, allowed the piston to be 

 pressed down with a force which would be nearer to about 15 Ibs. on 

 the square inch in proportion to the perfection of the vacuum within 

 the cylinder. Not only was the notion of using a piston new in the ap- 

 plication of steam, but the still more remarkable novelty was the con- 

 densation of the steam by cold as the means of obtaining the effective 

 force from the pressure of the atmosphere. 



There is another contrivance, for which we are indebted to Papin. 

 As already stated (p. 290), when heat is applied to an open vessel 

 of water the temperature will rise to 212 R, at which point it will 

 remain stationary until all the water has been converted into steam 

 at 212, the steam carrying off in the form of latent heat, the heat 

 passing into the vessel. If, however, the escape of the steam be 

 prevented by completely closing the vessel, the temperature of the 



