12 Mr. J. H. Alexander on the Tension of Vapour of Water. 



Of the earlier observations in the preceding table I need 

 not say much. Those of Mr. Watt, which he suffered to lie 

 by him for forty years, and, in the caustic phrase of Tredgold, 

 only produced when they had become unnecessary, he was 

 himself dissatisfied with, but, as appears upon comparison, 

 with more modesty than reason. I have specially calculated 

 but two or three of his temperatures ; and of the whole sixty- 

 two experiments, have inserted but twenty-two; among which, 

 however, both the limits are to be found. Of his friend 

 Robison's, I have had to calculate none specially; but all 

 happened to find a place in the table. Of M. Betancourt's 

 numerous observations, which were reported originally in de- 

 grees of Reaumur and French inches, I have inserted only 

 those which have been reduced to English scales by Sir 1). 

 Brewster for the Edinburgh Encyclopaedia. 



The experiments of Mr. Southern are, in fact, the supple- 

 ment of those of Mr. Watt; having been made and reported 

 at the desire of the latter. The numbers will be found to 

 differ somewhat from those generally found in professed 

 treatises on the steam-engine ; they are in fact the mean of 

 the actual observations; while those usually given have been 

 selected now from one and now from the other set, and re- 

 duced (by himself) to what they might have been, had the 

 pressure at 212° been thirty inches. For the present purpose 

 it seemed to me proper to state the real, not the possible 

 result. 



Mr. Dalton's experiments were distinct, and are therefore 

 given in distinct columns. The numbers in the earlier column, 

 marked with an asterisk, were not from actual experiment, 

 but by interpolation, according to the method he has him- 

 self explained. I have inserted them opposite to experimental 

 numbers in the adjoining -column, for the sake of comparison 

 and the benefit of the inference which may flow from the va- 

 riations. The numbers in the later column were not in 

 every case given by his own experiments ; but they were ac- 

 cepted by him as authentic, and the most reliable he knew. 

 It is more complimentary' to his reputation than to their own 

 research, that compilers of chemical manuals, even down to 

 the present time, retain among their tables his ancient results 

 whose inaccuracy he himself has recognised. All of his expe- 

 riments, of Southern's, and of Dr. Ure's, are in the table. 

 To the originals of M. Arzberger I have not had access; but 

 I have found these quoted in so many authorities and so uni- 

 formly accordant that I have not hesitated in recording them. 

 Of the extensive table of Mr, Philip Taylor, whose remark- 

 able accord with the results from the formula I may be allowed 



