452 Royal Society : — Prof. J. Thomson on the 



lished principles from existing experimental data. I may say, how- 

 ever, that the method devised by myself was also a true method, and 

 that I have since worked it out to its numerical results, and have 

 found that these are quite in accordance with those brought out 

 by my brother. The two indeed may be regarded as being essenti- 

 ally of the same nature ; and I think it unnecessary to occupy 

 space by giving any details of the method I planned and have 

 carried out. Its general character may be sufficiently gathered 

 from the concluding passages of the British-Association 1872 paper, 

 as printed in the Transactions of the Sections, Brighton Meeting. 



In order to discover whether the feature now developed by theo- 

 retical considerations is to be found showing itself in any degree 

 in the experimental results of Eegnault on the pressures of steam 

 at different temperatures*, I have made careful examinations of 

 his engraved curve (plate viii. of his memoir), and of his empi- 

 rical formulae adapted to fit very closely to the results exhibited 

 in that curve, and of his final Tables of results at the close of 

 his memoir ; and by every mode of scrutiny which I have brought 

 to bear on the subject (in fact by each of some seven or eight 

 varied modes) I have met with clear indication of the existence of 

 the expected feature; and by some of them I have found that 

 it can readily be brought prominently into notice. The engraved 

 curve drawn on the copper plate by Eegnault himself is offered by 

 him as the definitive expression of his experiments, as being an ex- 

 pression which satisfies as well as possible the aggregate of his ob- 

 servations — subject, however, to a very slight alteration, which he 

 has pointed out as a requisite amendment in the part of the curve 

 immediately below the freezing-point, a part with which the in- 

 vestigations in the present paper are specially concerned. 



After telling (page 581) of the great care with which he had 

 marked the curve on the copper plate and got it engraved, he 

 says : — " Je n'ai pas pu eviter cependant quelques petites irregu- 

 larites dans les courbes ; mais une seule de ces irregularites me 

 parait assez importante pour devoir etre signalee. Elle se presente 

 pour les basses temperatures comprises entre 0°et — 16°; la courbe 

 creuse trop vers l'axe des temperatures, elle laisse, notablement au- 

 dessus d'elle, toutes les determinations experimentales qui ont ete 

 faites entre 0° et —10°. Ainsi les valeurs, que cette petite portion 

 de la courbe donne pour les forces elastiques, sont un peu trop 

 faibles, et j'ai eu soin de les augmenter, de la quantite convenable, 

 dans les nombres que je donnerai plus loin." Whether we are 

 now to think that this bend downwards f of the curve towards the 

 axis of temperatures, involving what Eegnault regarded as a small 

 faulty departure of his drawn curve from his actual experiments, 

 was introduced merely by a casual want of accuracy in drawing, 



* Eegnault, "Des Forces Elastiques de la Yapeur d'Eau aux differentes 

 Temperatures," Memoires de 1'Academie des Sciences, 1847. 



f In M. Kegnault's curve the temperatures are measured horizontally across 

 the sheet, and pressures are measured upwards. 



