Prime Movers. 1 7 



The steam and ether engine was intended to economize the 

 heat which is wasted in the condenser, when the steam is changed 

 into water. Few attempts to improve the steam-engine have 

 excited such sanguine expectations; and the invention was actually 

 purchased by the French Government, on the recommendation 

 of eminent French engineers, who estimated the saving it was 

 supposed to effect at seventy-four per cent. ; even Rennie, who 

 seems to have examined it carefully, considered it to save 

 seventy per cent. — Ether was evaporated, by the heat given out 

 in condensing the steam of an ordinary low-pressure engine ; 

 and the resulting ether vapour was employed to move a piston. 

 It was assumed, that all the work done by the ether was so 

 much gained ; but, among other inconveniences, the evapora- 

 tion of the ether was found incompatible with condensation of 

 the steam, at a sufficiently low temperature; and the effect 

 derived from the steam was, therefore, less than it should be. 

 Without great care, also, there was danger of ignition or explo- 

 sion, with the ether vapour ; and ether was unavoidably wasted. 



It was supposed, from the very low temperature at which 

 certain fluids boil, that they might be used, with the steam- 

 engine, more advantageously than water. Thus, while the 

 latter, under ordinary pressure, boils at 212", alcohol boils at 

 175°, and ether at 112 s . And, as the elastic force of the 

 vapour produced is, in each case, the same, it was considered 

 that the vapour of alcohol, ether, and other fluids which 

 boil at comparatively low temperatures, might be used in 

 producing motive power, more economically than that obtained 

 from water. Their boiling points being lower, they can be 

 evaporated with less expenditure of fuel ; and from this it was 

 assumed that, at a given cost, they would do much more 

 work. This reasoning was extremely plausible ; nevertheless 

 it was found that the vapour of water, though produced at a 

 higher temperature, is more economical than that of any other 

 fluid. The reason is a very simple one : the mechanical effect of 

 a vapour depends, not only on its pressure, but also on the 

 distance through which that pressure is exerted. Now, as a 

 cubic inch of water, at 212°, produces one thousand six hundred 

 and ninety- six cubic inches of vapour : while a cubic inch of 

 alcohol produces only six hundred and sixty, and a cubic inch 

 of ether, only four hundred and forty -three : it follows that, to 

 fill as much space as is occupied by the vapour obtained from 

 one cubic inch of water, nearly three cubic inches of alcohol, or 

 four of ether, must be evaporated. And hence, the motion of 

 a piston through a given distance is produced far more cheaply 

 by the evaporation of water, than by the evaporation of alcohol, 

 or ether. The same is true with regard to every other liquid 

 that has been tried — and, we may reasonably suppose, with 

 vol. i. — no. i. c 



