152 OF THE POWER OF STEAM, [SECT. n. 



the power which would have resulted from condensation will be lost, and the loss 

 is equal to the pressure of the atmosphere acting through the height to which the 

 piston was raised by the steam. This is the nature of the common high 'pressure 

 steam engine. It is obvious, that the greater the elastic force of the steam, the 

 less is the loss by neglecting to condense it under these circumstances ; but it may 

 be remarked, that unless the valve aperture be equal to the diameter of the 

 cylinder, the steam cannot escape at the necessary rate without part of the load 

 acting to expel it ; and so much more of the effective force will of course be lost. 

 The effective power is as the space the steam occupies, multiplied by the excess 

 of elastic force above the atmospheric pressure. 



297. By Expansion. Retaining the same loaded piston, let it be raised by 

 the conversion of a given quantity of water into steam, to the height which cor- 

 responds to the load and temperature ; then if the load on the piston be wholly 

 removed at that height, the steam will raise the piston by expanding till it becomes 

 nearly of the same elastic force as the atmosphere, and its condensation will pro- 

 duce the same effect as if the steam had been generated of atmospheric elastic 

 force at first ; consequently, the effect in raising the load on the piston is wholly 

 additional, and the joint effect of a high pressure and condensing engine is pro- 

 duced by the same steam. The effective power of steam applied in this manner 

 is equal to the space it occupied, as high pressure steam, multiplied by the excess 

 of its elastic force above the atmospheric pressure, added to the amount arising 

 from multiplying the space it occupies when of atmospheric elastic force by the 

 atmospheric pressure. Hence, by this combination of effect the power of steam 

 of high elastic force will be nearly doubled. 



298. This is not, however, the mode by which steam can be applied with the 

 greatest advantage ; for, instead of removing the load on the piston wholly at the 

 height to which it was raised by the generation of the high pressure steam, a part 

 of it may be removed, and then the steam would expand to a height depending 

 on the portion of the load removed ; at that height remove a second portion, and 

 so on, successively, till the steam becomes of atmospheric elastic force. In this 

 case, as far as the load was raised in parts by the expansion of the steam, the effect 

 is greater than in the preceding combination : the mode of calculating it will be 

 afterwards shown, but the principle is that of the expansion steam engines. 



299. The preceding is not the only mode of deriving advantage from expan- 

 sion ; indeed it is only a late discovery 7 , and most probably belongs to Woolf as 

 far as he was capable of appreciating it. The methods of Hornblower and Watt 

 only apply to the case now to be considered. Let the piston be raised unloaded, 

 as in the first case, by the conversion of a certain quantity of water into steam of 

 atmospheric elastic force. When the piston is at that height, add a weight equal 



