FIRST LITERARY LABOURS. 47 



engine, which he had seen at work in Dundee. From 

 a rather brief account it appeared that this engine 

 was not driven by steam but by heated air. This 

 idea interested me exceedingly, since it appeared to 

 afford a foundation for an advantageous transformation 

 of the whole engine-constructing art. In a paper 

 entitled "On the use of heated air as mechanical 

 power", contributed in 1845 to Dingier' s Polytechnic 

 Journal, I described the theory of such air-eno-ines, 



/ ' C? 



and gave also a sketch of the construction of such 

 a one as I conceived to be practicable. 



My theory was based entirely on the principle of 

 the conservation of energy, which had been advanced 

 by Mayer arid mathematically worked out by Helmholtz 

 in his celebrated memoir "On the Conservation of 

 Energy"; originally read before the Physical Society. 

 Later on my brothers William and Frederick occupied 

 themselves a good deal with these engines, and con- 

 structed them in various forms. They too however 

 unfortunately had to undergo the common experience 

 of finding, that engineering had by no means advanced 

 far enough to allow of the discovery being utilised with 

 advantage. Only small engines could be constructed 

 on the basis of the above principle so as to work well 

 for a length of time; for large ones the right material 

 for the heating apparatus was and is still wanting. 



In the same year I printed in Dingier* s Journal 

 ii description of the already mentioned differential 

 governor, to which in collaboration with my brother 

 \\ illiam I had tried to give the most varied forms. 



