7<D THE SCIENTIFIC PAPERS OF 



had adverted to the combined-vapour engine. The results then 

 arrived at, from calculation, were very similar to those now 

 recorded by the author. The comparison of the performances of 

 the combined engine and of the ordinary expansive steam-engine, 

 as shown by the diagrams, was very remarkable. There was a 

 difference in the consumption of fuel, per indicated H. P. per hour, 

 as between 3'1 Ibs. in the combined engine, and 7'4 Ibs. in a direct 

 acting engine, of nearly the same power, working steam expan- 

 sively. That would naturally seem to indicate the vast superiority 

 of the ether engine. But before accepting such a fact, which 

 was the result of experiment, as a proof of the superiority of the 

 principle involved, he thought the conditions under which the 

 experiments were made should be carefully inquired into, and 

 an estimate be formed of the causes, either in construction or in 

 working, which might have influenced the result. For instance, 

 it was well known, that an unprotected cylinder condensed a 

 large quantity of steam, if worked expansively. He had known 

 cases, where the result was modified to the extent of 50 per cent., 

 by that cause alone. On the other hand, the ether-engine seemed 

 to have been under some disadvantage. There was a bad 

 vacuum, of 5 Ibs. only, in the ether cylinder, which was, no 

 doubt, in a great measure, to be attributed to the difficulty of 

 condensing ether, the condensing point being very low ; but he 

 thought it might also be due, partly, to the exhaust ports being 

 of insufficient area, for ether steam being five times heavier 

 than water steam, the ports for its discharge should be more 

 than twice the usual area. To judge of the merits of the inven- 

 tion now brought forward, it would be necessary to revert to 

 broader principles, instead of limiting the inquiry simply to the 

 facts stated. Perhaps he might be allowed, shortly, to develop 

 the relative merits, in a theoretical point of view, of the combined 

 ether-engine, and of a well arranged expansive steam-engine. He 

 believed he was the first to advocate, in the Institution, the 

 dynamic theory of heat. According to that theory, heat and 

 power were identical. The particles of a heated body were in a 

 more agitated condition than in a cooler body, the motion of the 

 particles being greatest in elastic fluids, and their pressure upon 

 the sides of the vessel such, that if one side yielded to the impact, 

 or pressure, as was the case with a working piston, the reaction 



