APPENDIX R 255 



On which he remarked,, that the mean pressure is 

 more than half the original pressure; also that in 

 employing a quantity of steam equal to a quarter, 

 it would produce an effect more than half, 



Watt here supposed that steam follows in its ex- 

 pansion the law of Mariotte, which should not be 

 considered exact, because, in the first place, the 

 elastic fluid in dilating falls in temperature, and 

 in the second plac3 there is nothing to prove that 

 a part of this fluid is not condensed by its expan- 

 sion. Watt should also have taken into considera- 

 tion the force necessary to expel the steam which 

 remains after condensation, and which is found in 

 quantity as much greater as the expansion of the 

 volume has been carried further. Dr. Robinson 

 has supplemented the work of Watt by a simple 

 formula to calculate the effect of the expansion of 

 steam, but this formula is found to have the same 

 faults that we have just noticed. It has neverthe- 

 less been useful to constructors by furnishing them 

 approximate data practically quite satisfactory. 

 We have considered it useful to recall these facts 

 because they are little known, especially in 

 France. These engines have been built after the 

 models of the inventors, but the ideas by which 

 the inventors were originally influenced have been 

 but little understood. Ignorance of these ideas 



