30 Prof. A. Schuster and Mr. W. Gannon. [Nov. 22, 



means of a comparison made by Joule himself with one of Rowland's 

 thermometers. Joule's original thermometers have been temporarily 

 placed by Mr. B. A. Joule in the hands of Professor Schuster, 

 in order that an accurate comparison may be instituted between it 

 and modern thermometers. A full description of the comparisons 

 made will be given on another occasion. The result arrived at shows 

 that the correction is less than that assumed by Rowland, and would 

 bring his value up only to 775 at the temperature indicated. Griffiths 

 compares his result with that deduced by Rowland from Joule's 

 observations. Rowland combined the different values obtained by 

 Joule in his various investigations, attaching weights according to 

 his judgment as to their relative merits. He finds in this way that 

 the difference between him and Joule amounts to one part in 350, but 

 if equal weights are attached to all Joule's results, the difference is 

 reduced to one part in 4281. Little value can be attached, however, 

 to a combination of Joule's results which gives equal weights to that 

 obtained in 1847 and that deduced from his latest and most careful 

 work. There is, moreover, in Rowland's table a misprint or error in 

 the reduction of Joule's 1847 result from foot-pounds to kilogram- 

 metres, which lowers the value as given by Griffiths from 779'2 to 

 about 778. It does not seem to us advisable to go beyond Joule's 

 1878 results, and the value assigned by him in this latest research 

 should be taken as giving his final judgment on the matter. Reducing 

 to the nitrogen thermometer of the Bureau International, Joule's 

 result is 775 foot-pounds at Greenwich, at a temperature of L6'5 C. 

 At the same temperature Griffiths' number is 779'8. 



Great weight must be attached to Rowland's determination, which 

 at the temperature to which Joule's number applies is 777*6, and at 

 19*1, 776*1, corresponding to our 778'5. Rowland's value is there- 

 fore halfway between our and Joule's result. But it must be taken 

 into consideration that if the comparison between Rowland's and 

 Joule's thermometers as made by the latter is to be trusted, Rowland's 

 value referred to the " Paris " nitrogen thermometer would be slightly 

 reduced. At any rate it seems probable that if his value is in error, it 

 is rather in the direction of being too high. We have therefore a 

 difference of three parts in a thousand to account for between our 

 result and that of Rowland, and of nearly four parts in a thousand 

 between Griffiths ami Rowland at a temperature of 19*1. These 

 results are summarised in the following table : 



Table III. Equivalent in foot-pounds at Greenwich at 19'l 

 referred to the " Paris " Nitrogen Thermometer. 



Schuster and 

 Joule. Rowland. Griffiths. Gannon 



774 776-1 779-1 778*5 



