14 Mr. J. H. Alexander on the Tensioji of Vapour of Water. 



The temperatures of the Franklin Institute, which were 

 taken for the composition of the table, come from the second 

 series of their reported experiments. Pressures have been 

 also taken from both the other series, when their temperatures 

 were already in the table ; and, adopting this method as a 

 uniform system, I did not allow myself to exclude the ano- 

 maly which shows itself between the different series at the 

 temperature of 300°. 



Of the experiments of M. Regnault, I have already spoken 

 sufficiently. 



It is apparent, upon a slight examination of the table, that 

 the calculated pressures do not differ more from the average 

 of the experimental ones, than these experimental ones do 

 among themselves ; which is about as much as could be de- 

 sired to show the validity of the formula, and the reasonable- 

 ness of its application, instead of others which are in general 

 merely means of interpolating a particular experimental series. 

 But in order to establish this more clearly, it will be necessary 

 to ascertain more distinctly what the difference is between the 

 results of the formula and those of observation. This differ- 

 ence is, of course, best expressed in the arithmetical scale of 

 temperatures ; as I have tabulated it here, upon the maximum 

 deviation in each instance. 



The mean of the sum of these differences is +1°'09 Fahr- 

 enheit ; which is the maximum error of the formula, com- 

 pared with these six series. 



It will be observed that I have left out of this comparison 

 the last observation of the Academy ; because it was the very 

 utmost point which the apparatus could carry, and because it 

 might therefore be expected to be affected by the untrustwor- 

 thiness which forbade the series from being extended further. 

 I have also neglected the last observation of Arzberger, which, 

 compared with the Academy's, is in error more than 10°, — 

 a deviation sufficient to discredit it entirely. Ure's experi- 

 ments I have not compared at all ; because, if we admit the 

 series just now tabulated, his results are altogether too high. 

 He may, however, be compared with himself, in the two 



