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THE SCIENTIFIC PAPERS OF 



one fourth or one fifth of the theoretical maximum of mechanical 

 effect due to the heat put into the air. Another drawback was 

 that in most air engines, and particularly in Stirling's, owing to 

 the low conducting power of air and insufficient amount of heating 

 suri'ace, the cylinder or vessel in which the air was heated by the 

 fire was found to get fully red-hot, so that the products of com- 

 bustion reached the chimney at that elevated temperature. This 

 source of loss was obviated in the engine described in the present 

 paper, by causing the products of combustion to pass through the 

 working cylinder, the air being heated by direct contact with 

 them ; and if the expansion in the cylinder could be carried far 

 enough, no doubt the whole of the heat in the products of 

 combustion might in this case be utilised ; but it was clearly 

 impossible to carry expansive action very far in this engine, owing 

 to its low working pressure, and moreover the working of the air- 

 pump constituted a very heavy loss of useful effect. The loss of 

 sensible heat escaping at the exhaust might be remedied by the 

 application of a regenerator ; but this could not be done except at 

 a sacrifice of the simplicity of construction which appeared to him 

 to constitute the chief recommendation of the engine. It was 

 impracticable he believed to carry out the principle of the hot-air 

 engine on a scale sufficient to give any large amount of power ; 

 but a question of much practical importance was to produce a 

 safe engine of small power, which could be put up anywhere, in 

 any room, because requiring no boiler, and therefore necessitating 

 no increased rate of insurance against fire. This object had been 

 already accomplished by various constructions of gas engines, and 

 was also effected by the hot-air engine now described, which he 

 hoped would be so far perfected in its details as to give an 

 effective power of as much as 4 or 5 horse power ; and even 

 though the consumption of coal were not reduced below 8 Ibs. 

 per horse -power per hour, there were no doubt many cases in which 

 such a source of motive power could be advantageously employed. 

 He moved a vote of thanks, which was passed, to Mr. Cooke for 

 his paper, and also to Mr. Wenham for the additional information 

 he had kindly given. 



