394- Royal Irish Academy. 



When the corresponding numbers in the three columns are com- 

 pared, it will be at once observed, that the values of f, investigated 

 by the method just explained, are somewhat less than those extracted 

 from the table I have been hitherto in the habit of using ; but that 

 they are considerably greater than the values of Kamtz, the differ- 

 ences being generally better than twice as great in the latter in- 

 stance as in the former. This will be more manifest by taking a 

 mean of the different results in column 2, and comparing it with the 

 force of vapour corresponding to the same temperature as given in 

 the two other tables. Now, the mean of the temperatures is 61°'63, 

 the quotient got by dividing their sum by twenty. But the corre- 

 sponding mean value of /, in column 2, must be differently calcu- 

 lated, seeing that the temperature and the corresponding tensions of 

 the vapour augment at a very different rate. For temperatures, 

 in fact, in arithmetic progression, the corresponding tensions are in 

 geometric progression, and, although this is well known to be but an 

 approximate law, it may be considered as rigorously true for the limit- 

 ed range of temperature within which my experiments have been made. 

 To calculate, therefore, the mean force of vapour, as deducible from 

 the numbers in column 2, and which must correspond to the tempe- 

 rature 61 0, 63, it is only necessary to add together the logarithms of 

 the numbers in this column, and divide their sum by twenty, and 

 the quotient will be the logarithm of the mean. When this process 

 is gone through, the mean logarithm is found to be "73699, and the 

 corresponding number *54575. The following, therefore, are the 

 tensions of aqueous vapour at 61 0- 63, as deduced from my experi- 

 ments, and as extracted from the tables of Dal ton and Kamtz. 

 My experiments. Dalton. Kamtz. 



61°'63 -5457 '5523 '5349 



Difference between Dalton's number and mine = + "0066 



Difference between Dalton's number and that of Kamtz = + '0174. 



It thus appears, that the result at which I have arrived is some- 

 what less than the Daltonian number, but considerably greater than 

 that given by Kamtz ; and that, therefore, my experiments, as far 

 as they have been discussed, give at least a. prima facie countenance 

 to the opinion, that the values of the elastic force of aqueous vapour, 

 as given by the latter philosopher, are, at and about 61 0- 63, below 

 the truth. 



Before, however, this conclusion can be considered as fully esta- 

 blished, and before we can judge correctly of the amount of the 

 errors by which his table is affected, it will be necessary to inquire 

 whether the thermometer I have employed be a true one. This 

 essential inquiry I have been enabled to institute by my friend Pro- 

 fessor Lloyd, who has put into my possession, for the purpose, a 

 thermometer given him by Professor John Phillips, together with a 

 table of differences between it and the standard thermometer belong- 

 ing to the Royal Society. Upon a comparison of the two instru- 

 ments, I find, that at and about 60°, the thermometer I have em- 

 ployed stands *6 of a degree higher than that lent me by Professor 

 Lloyd, while the latter stands *3 of a degree higher than the standard 

 in possession of the Royal Society ; so that the indications of my 



