THE STEAM-ENGINE. 



411 



There is also another circumstance which increases the consumption of 

 fuel. The water must be forced through B, not only against the atmospheric 

 pressure, but also against a column of sixty-eight feet of water. Steam is 

 therefore required of a pressure of forty-five pounds on the square inch. Con- 

 sequently the water in the boiler must be boiled under this pressure. That 

 this should take place, it is necessary that the water should be raised to a 

 temperature considerably above 212°, even so high as 275° ; and thus an in- 

 creased heat must be given to the boiler. Independently of the other defects, 

 this intense heat weakened and gradually destroyed the apparatus. 



Savery was the first who suggested the method of expressing the power of 

 an engine with reference to that of horses. In this comparison, however, he 

 supposed each horse to work but eight hours a day, while the engine works 

 for twenty-four hours. This method of expressing the power of steam-engines 

 will be explained hereafter. 



The failure of the engines proposed by Captain Savery in the work of 

 drainage, from the causes which have been just mentioned, and the increasing 

 necessity for effecting this object, arising from the large property in mines 

 whiqh became every year unproductive by being flooded, stimulated the inge- 

 nuity of mechanics to contrive some means of rendering those powers of steam 

 exhibited in Savery's engine available. 



Thomas Newcomen, the reputed inventor of the atmospheric engine, was an 

 ironmonger, or, according to some, a blacksmith, in the town of Dartmouth, in 

 Devonshire. From his personal acquaintance and intercourse with Dr. Hooke, 

 the celebrated natural philosopher, it is probable that he was a person of some 

 education, and therefore likely to be above the position of a blacksmith. Be- 

 ing in the habit of visiting the tin mines in Cornwall, Newcomen became ac- 

 quainted with the engine invented by Savery, and with the causes which led 

 to its inefficiency for the purposes of drainage. 



John Cawley, who was the associate of Newcomen in his experiments and 

 inquiries, was a plumber and glazier of the same town. Newcomen and Caw- 

 ley obtained a patent for the atmospheric engine, in 1705, in which Savery 

 was associated, he having previously obtained a patent for the method of pro- 

 ducing a vacuum by the condensation of steam, which was essential to New- 

 comen's contrivance. It was not, however, until about the year 171 1, that any 

 engine had been constructed under this patent. * 



Newcomen resumed the old method of raising the water from the mines by 

 ordinary pumps, but conceived the idea of working these pumps by some 

 moving power less expensive than that of horses. The means whereby he 

 proposed effecting this, was by connecting the end of the pump-rod D (fig. 4) 

 by a chain with the arch-head A of a working-beam A B, playing on an axis 

 C. The other arch-head B of this beam was connected by a chain with the 

 rod E of a solid piston P, which moved air-tight in a cylinder F. If a vacuum 

 be created beneath the piston P, the atmospheric pressure acting upon it will 

 press it down with a force of fifteen pounds per square inch ; and the end A 

 of the beam being thus raised, the pump-rod D will be drawn up. If a pres- 

 sure equivalent to the atmosphere be then introduced below the piston, so as 

 to neutralize the downward pressure, the piston will be in a state of indiffer- 

 ence as to the rising or falling ; and if in this case the rod D be made heavier 

 than the piston and its rod, so as to overcome the friction, it will descend, and 

 elevate the piston again to the top of the cylinder. The vacuum being again 

 produced, another descent of the piston, and consequent elevation of the pump- 

 rod, will take place ; and so the process may be continued. 



Such was Newcomen's first conception of the atmospheric engine ; and the 

 contrivance had much, even at the first view, to recommend it. The power of 



