APPENDIX B. 257 



the second of ample dimensions, with moderate 

 strength. 



Double-cylinder engines, although founded on 

 correct principles, often fail to secure the advan- 

 tages expected from them. This is due principally 

 to the fact that the dimensions of the different 

 parts of these engines are difficult to adjust, and 

 that they are rarely found to be in correct propor- 

 tion. Good models for the construction of double- 

 cylinder engines are wanting, while excellent de- 

 signs exist for the construction of engines on the 

 plan of Watt. From this arises the diversity that 

 we see in the results of the former, and the great 

 uniformity that we have observed in the results of 

 the latter. 



NOTE I. Among the attempts made to develop \ 

 the motive power of heat by means of atmospheric 

 air, we should mention those of MM. Niepce, which 

 were made in France several years ago, by means 

 of an apparatus called by the inventors a pyre- 

 olophore. The apparatus was made thus: There 

 was a cylinder furnished with a piston, into which 

 the atmospheric air was introduced at ordinary 

 density. A very combustible material, reduced to 

 a condition of extreme tenuity, was thrown into it, 

 remained a moment in suspension in the air, and 

 then flame was applied. The inflammation pro- 



