36 THE HISTORY OF [SECT. i. 



the piston, which by springs are forced outwardly against the surface of the cylinder, 

 so that the piston may adapt itself to any inequality in its form. The piston rod is 

 also made steam-tight by a metallic box, constructed in the same manner as shown 

 at N. O is the fly wheel for regulating the motion of the engine. 



The metallic piston is the only part of the engine which was really new in 

 principle, and for its invention we are undoubtedly indebted to Cartwright ; and 

 though we cannot say any other part is new except in arrangement, we admire the 

 appearance of simplicity and originality which distinguish his design, even knowing 

 that both theory and practice forbid us attempting to use the methods he proposed. 



Cartwright included in his patent a rotary engine, which is simple in appear- 

 ance, but in reality involves many difficulties in construction, besides the loss of 

 effect which must necessarily follow from steam acting on a rotary piston. 



1797. JOHN CURR. 



I 



44. A work containing the proportions of the parts of atmospheric engines, as 

 they were executed in 1797, with brief technical directions for constructing them, 

 illustrated by plates showing the parts on a large scale, was published at Sheffield, 

 by John Curr. 1 It contains no general description of the engine, and he assigns no 

 reasons for any of the proportions he has given, except when speaking of the 

 pressure on the piston, he says, that when the pressure was increased from seven to 

 eight pounds and a half per square inch, the engine did less, and also when reduced 

 to 6'1 Ibs. it did somewhat less ; and he does not recommend a greater load than 

 six and a quarter or six and a half pounds. 4 The engine had a sixty-one inch 

 cylinder, and made twelve strokes, of eight feet and a half each, per minute. The 

 consumption of coals was ten hundred weight of small coal, or sleek, per hour. 

 The power of the engine would be nearly equal to fifty-four horses' power ; and as 

 the ratio of coal to sleek is about as three is to four, it is equivalent to about 

 840 Ibs. of coal per hour : and at this ratio, one pound of sleek raises 97,600 Ibs. 

 of water one foot high, and one pound of coals 130,000 Ibs. one foot high. 



45. In 1797 an engine on Savery's principle was described by William 

 Nicholson in his ' Philosophical Journal,' which Mr. Kier had erected in 1793. 

 It acted wholly by condensation ; the steam vessel being raised somewhat above 

 the height to which the water was to be raised. It had a provision for letting in 

 a small portion of air between the steam and the water, and the construction was 

 extremely simple and judicious. The boiler was seven feet long, five feet deep, 

 and five feet wide ; and it consumed six bushels (522 Ibs.) of good coal in twelve 



1 The Coal Viewer and Engine Builder's Practical Companion. * Idem, p. 40. 



