Platiniuii Tkermometry at Kew Observatory. 43 



fundamental intervals, and were compared in the great majority of 

 instances on the same or on successive days, the differential results are 

 but little exposed to the chief sources of uncertainty. 



Before discussing the differential results I shall glance briefly at the 

 absolute data obtained when the leads were interchanged. A summary 

 of these is given in Table XIV, along with results of observations with- 

 out interchanged leads on October 4 and December 8 and 9, 1897 : 



Table XIV. Values of R . 



The leads had been ad j listed to approximate equality between 

 September 21 and October 4, 1897: we may thus reasonably suppose 

 that the changes experienced prior to December 9, 1897, were small. 

 Also, judging by what was observed on October, 1898, such change as 

 might have occurred would tend to diminish R . We may thus regard 

 the results obtained on October 4, and December 8 and 9, 1897, as 

 proving that the observations On September 21 did not suffer from any 

 notable source of error peculiar to the day. 



Comparing the results obtained on September 21, 1897, with the 

 mean results for 1899, we obtain a surprisingly close agreement in the 

 case of every thermometer except KI. 



This certainly is pretty strong evidence of constancy in at least 

 three of the thermometers during the last two years. It is of course 

 possible that a common change might have occurred, and that it was 

 masked by a corresponding equal change in the box coils ; but as these 

 coils are of a different material, platinum silver, and have been exposed 

 to wholly different conditions, such a contingency is improbable. 



41. For the purpose of intercomparison, I have selected as standards 

 of reference K 3 and K 4 . These two thermometers are almost identical 

 in pattern, and have been exposed to almost exactly the same treat- 

 ment, usually being compared in immediate succession to one another. 

 Taking the mean RQ, RI, and R g given by these thermometers on each 

 occasion, I have found the difference between these means and the R , 

 RI, R s of each of the other thermometers. I preferred taking the mean 

 for the two thermometers to taking either alone as a standard, because 



