416 HENRY A. KOWLAND 



The experiments of Fairbairn and Tate 34 are probably the best direct 

 experiments on the density of saturated vapor, but even those do not 

 pretend to a greater accuracy than about 1 in 100. With Eegnault's 

 values of the other quantities, they give about Joule's value for the 

 equivalent, namely 425. Him, Herwig, and others have also made the 

 determination, but the results do not agree very well. Herwig even 

 used a Geissler standard thermometer, which I have shown to depart 

 very much from the air thermometer. 



Indeed, the experiments on this subject are so uncertain, that physi- 

 cists have about concluded to use this method rather for the deter- 

 mination of the volume of saturated vapors than for the mechanical 

 equivalent of heat. 



From the Steam-Engine and Expansion of Metals 



The experiments of Hirn on the steam-engine and of Edlund on the 

 expansion and contraction of metals, are very excellent as illustrating 

 the theory of the subject, but cannot have any weight as accurate deter- 

 minations of the equivalent. 



From Friction Experiments 



Experiments of this nature, that is, irreversible processes for con- 

 verting mechanical energy into heat, give by far the best methods for 

 the determination of the equivalent. 



Rumford's experiment of 1798 is only valuable from an historical 

 point of view. Joule's results since 1843 undoubtedly give the best 

 data we yet have for the determination of the equivalent. The mean of 

 all his friction experiments of 1847 and 1850 which are given in the 

 table is 425-8, though he prefers the smallest number, 423-9, of 1850. 

 This last number is at present accepted throughout the civilized world, 

 though there is at present a tendency to consider the number too small. 

 But this value and his recent result of 1878 have undoubtedly as much 

 weight as all other results put together. 



As sources of error in these determinations I would suggest, first, 

 the use of the mercurial instead of the air thermometer. Joule com- 

 pared his thermometers with one made by Fastre. In the Appendix 

 to Thermometry I give the comparison of two thermometers made by 

 Fastre in 1850, with the air thermometer, as well as of a large number 

 of others. From this it seems that all thermometers as far as measured 



3* Phil. Mag., ser. 4, xxi, 230. 





