BY THE BOILING POINT OF WATER. 241 



Upon carefully projecting Dr Hooker's results in Plate III., fig. 2, in the same 

 manner as my own, that is, by exhibiting the logarithms of the pressure (or the 

 heights) in terms of the temperatures of boiling water, I found, in the first place, 

 large deviations among the results, increasing also at great elevations. The 

 breadth of the space is so considerable over which the individual observations 

 are distributed, that it seems impossible, from the observations only, to assign 

 any one curve as particularly indicated by them ; and for the most part they are 

 as well represented by a straight line as by any curve not absolutely sinuous. I 

 must, however, note that below the temperature of 187°, or at heights above 13,600 

 feet, something like a dislocation occurs in the continuity of the observations. Dr 

 Hooker was aware of this circumstance, and ascribes it to "the metal of the 

 kettle, and consequently of the thermometer, getting heated above the tempera- 

 ture of the boiling water." Whatever may have been the cause, this part of the 

 series, the most important for testing a formula, can hardly be relied on for that 

 purpose. 



There is no doubt that M. Regnault's numbers represent, as well as any num- 

 bers can be expected to do, the main features of Dr Hooker's observations.* But 

 the differences between M. Regnault's numbers and my approximate formula are 

 trifling, compared to the latitude of error which the projection of the observations 

 themselves discloses. Balancing the errors as nearly as possible, the observations 

 between 212° and 190° are well represented by a line which gives 538 feet of ascent 

 for a fall of one degree in the boiling point, which it will be seen differs only by 

 jgoth part from the corrected result of my Alpine observations.! I De » to observe, 

 that this coincidence is the more striking, because, from the method of projection 

 used, it was impossible to guess at the numerical result until the interpolating 

 line had been fixed upon. 



In criticising Dr Hooker's results, I do so with every feeling of courtesy and 

 respect, in the same spirit, in short, in which I am sure he found it necessary to 

 state his objection to my formula. The whole of his barometrical observations 

 appear to have been made with the greatest care and fidelity, and, judging by the 

 results, with great success. From not knowing his thermometric apparatus, I am 

 unable to determine why these observations are of less value. I should attribute 

 it rather to the boiler, or to the mode of using it, than to the thermometers ; for 

 Dr Hooker speaks of a coincidence in the readings of different thermometers so 

 exact as to be unusual. Dr Hooker states that he finds the errors by actual cal- 

 culation considerably less, if reduced by M. Regnault's numbers than by mine ; 



* A few points marked by the letter r, calculated from Regnault's formula, are inserted in the 

 figure for the sake of comparison. 



f The entire series of Dr Hooker's observations is best represented by 548 feet for 1°, when 

 we include the (somewhat doubtful) highest observations. This agrees almost exactly with my 

 earlier determination. 



