SfK WILLIAM SIEMENS, F.R.S. 7 1 



would be less than the impact. Hence the motion of the particles 

 would be diminished, in proportion as the piston was urged for- 

 ward, and the result would be a reduction of temperature. Ex- 

 pansion might be carried to a point, where the elastic force was 

 entirely exhausted, at which point the elastic fluid must have 

 turned the liquid, or solid state. Commencing with steam of the 

 density of water, and expanding it down, until a perfect vacuum 

 was obtained, would form, in point of principle, a perfect engine, 

 \vlii.-h nothing could supersede. But the practical difficulty of 

 carrying expansion to its utmost limits, was the great strength and 

 size of the cylinders required to withstand the enormous pressure 

 of steam of that density, and to be at the same time of sufficient 

 dimensions to allow of the expansion of the steam down to that 

 extraordinary degree. All the various air and other engines, 

 designed to supersede the steam-engine, might be said to aim at 

 one point, that of carrying the expansive action to a farther 

 degree than could be done with the steam-engine, without having 

 to resort to cylinders of great size and strength. In the combined 

 ether-engine, the analogy was evident. Instead of expanding 

 the steam down to a minimum pressure, the expansive action in 

 the steam cylinder was stopped at a certain point, and the steam 

 was made to impart its remaining heat to another liquid, which 

 evaporated at a lower temperature, and the vapour of which occu- 

 pied less room than steam at a given temperature, and could 

 consequently continue the expansive action in a cylinder of smaller 

 dimensions than would otherwise be required. This was all the 

 advantage that could be claimed in favour of the combined ether 

 engine. It was a matter of calculation, whether that advantage 

 was sufficient to balance the disadvantages of a necessarily im- 

 perfect vacuum, in both the steam and the ether cylinders, of the 

 complication of parts, and of other drawbacks which had been 

 mentioned in the paper. There was an omission of one fact in 

 the paper, which perhaps the author could supply, that was, the 

 quantity of ether evaporated, for a given amount of steam con- 

 densed. No mention had been made of this ; nor was he 

 acquainted with any experiments on the subject. It was a well- 

 ascertained fact, that the vapour of ether was about four times 

 heavier than steam at the same pressure, and the question was, 

 how much of that ether vapour would be produced for a given 



