214 Mr. Meikle on the QZconomy of the Steam-Engine. 



claiming for our countrymen the invention of whatever is va- 

 luable in the steam-engine, is no less industrious in bringing 

 forward the important discovery that steam is preferable to 

 other vapours in an ceconomical point of view. By turning 

 to the Philosophical Magazine for July 1826, page 41, it will 

 be seen that if this idea be still new to Mr. Ainger, who is un- 

 commonly well versed in every minutia connected with the hi- 

 story of the steam-engine, it is by no means new to the public ; 

 particularly, so far as regards the vapour of alcohol, which 

 next after steam had been so much oftener proposed than any 

 other, that the consideration of its case was quite sufficient in 

 starting the subject : the same mode of comparison being so 

 obviously applicable to other vapours. 



I am, however, very much at a loss to see the need of such 

 a complicated calculation as Mr. Ainger has employed. The 

 method which I followed is incomparably more simple. It is 

 evident that if we know what rises can be produced in the 

 temperature of a certain mass of cold water, by the condensa- 

 tions of known quantities of different vapours having equal 

 tensions, we have the main datum for making the comparison, 

 so far as regards the expense of heat. The rest is too ob- 

 vious to require any remark. 



It will be found that some of the numbers on page 41, 

 line 27, have not been correctly copied from Dr. Ure; as I 

 had put down 42° and 49 0, 5 in place of 42°*5 and 49°. For- 

 tunately this oversight has scarcely any influence on the re- 

 sult. 



MM. Dulong and Petit, in stating their results, place in 

 one column a series of temperatures ; and opposite to these, in 

 another column, the dilatation. This undefined dilatation is 

 understood by some to signify the expansion for one degree 

 at the temperature opposite ; by others, the mean expansion 

 for one degree of the interval between the temperature oppo- 

 site and the temperature next before it ; both of which senses 

 are generally far from what the authors meant. Whoever takes 

 the trouble to examine thoroughly the memoir of Dulong and 

 Petit, will find that they reckon all their intervals from the 

 freezing point up to the temperature opposite the dilatation ; 

 and that such dilatation is the mean for one degree of the 

 whole interval. The quotations from Dulong and Petit in 

 most of our English authors are tainted with these mistakes ; 

 but as it is some time since I attended to this, I can only men- 

 tion at present the quotations from Dulong and Petit in those 

 very useful works, Dr. Ure's Dictionary of Chemistry, and the 

 Library of Useful Knowledge; many of which will be found 

 very incorrect, particularly where they state the expansions 



of 



