APPENDIX B. 253 



stroke is entirely filled with steam, formed under 

 the pressure of 35 atmospheres. The steam pro- 

 duces no effect by the expansion of its volume,, for 

 no space is provided in which the expansion can 

 take place. It is condensed as soon as it leaves 

 the small cylinder. It works therefore only under 

 a pressure of 35 atmospheres, and not, as its use- 

 ful employment would require, under progressively 

 decreasing pressures. The machine of Mr. Per- 

 kins seems not to realize the hopes which it at 

 first awakened. It has been asserted that the 

 economy of coal in this engine was j\ above the 

 best engines of Watt, and that it possessed still 

 other advantages (see Annales de Chimie et de 

 Physique, April, 1823, p. 429). These assertions 

 have not been verified. The engine of Mr. Per- 

 kins is nevertheless a valuable invention, in that 

 it has proved the possibility of making use of 

 steam under much higher pressure than previously, 

 and because, being easily modified, it may lead to 

 very useful results. 



Watt, to whom we owe almost all the great im- 

 provements in steam-engines, and who brought 

 these engines to a state of perfection difficult 

 even now to surpass, was also the first who em- 

 ployed steam under progressively decreasing pres- 

 sures. In many cases he suppressed the introduc- 

 tion of the steam into the cylinder at a half, a 



