Platinum Thermomctry at Kew Observatory. 41 



I first took the data in Table XII and added to the individual values 

 for pt s in KI, K 3 , and KT the mean of the differences between the values 

 of pt s found with these respective thermometers and the corresponding 

 values in K 4 . As the corresponding values of pt s in these four thermo- 

 meters answered to nearly the same barometric pressure, one got in this 

 way a mean pt g answering very fairly to the mean barometric pressure. 

 Applying formula (14) to the figures so obtained, I obtained by least 

 squares 



x =+0-0063, y= +0-00018 (15). 



Next I took all the observations made with K 3 and K 4 excluding 

 one when the asbestos cone fell off and combining the observations 

 made on the same days got twenty -four values of pt s as given by the 

 formula (10). Combining these two and two in the order of the corre- 

 sponding barometric pressures, I found from the resulting twelve data 

 by least squares 



x= +0-0087, y = +0-00021 (16). 



For ready comparison I show side by side the formulae resulting from 

 (15) and (16), as well as those based on the Callendar -Griffiths value. 

 In these formulae, t g and pt g are, as before, the temperatures on the air 

 and platinum scales of sulphur vapour boiling under the standard pressure 

 760 mm., while t and pt are the corresponding temperatures when the 

 pressure is not 760, but p mm. It is supposed that S = 1-5 in the 

 formulas for pt. 



Callendar and Griffiths, {pt = pt s + 0'072 (p - 760), 

 from Regnault. l* = t s + Q-082(p - 760). 



From (15), i.e., by ex-^ 



periments on KI, K 3 , | pt = pt s + 0-078 (p - 760) + 0-00018 (p - 760) 2 , 



K 4 , and K?, between } 



Sept. 1897 and Jan. t = t, + 0'Q88(p -760) + 0-00020 (^-760) 2 . 



1899. 







K 4 between Oct. 1895 [ f = , + o-092(p -760) + 0-00024 (p-760) 2 . 

 and Sept. 1899. 



In restricting the second calculation to K 3 and K 4 I was guided by 

 the obvious change in KI and by the comparative fewness of the experi- 

 ments at the sulphur point with K>, K 5 , and K^. According, however, 

 to Table XIII the variation of pt s , per 1 mm. pressure, deduced from 

 the experiments on K 3 , is exceptionally high. Further, the agreement 

 between observed and calculated values in the second calculation was 

 not so good as in the first. I am thus disposed to attach somewhat 



