Platinum Thcrmonutry at Kew Observatory. 35 



case of an average platinum thermometer. It has been customary at 

 Kew to allow a fully longer immersion than ten minutes in ice before 

 taking the first reading, and I think that in the case neither of the ice 

 nor the steam point observations has there been any sensible error 

 through excessive hurry. 



At the sulphur point the attainment of the steady state is a tedious 

 process. Readings taken with the short-stem thermometers KS and K 4 

 within thirty or forty minutes of their first exposure to sulphur vapour 

 had nearly always to be rejected, being conspicuously low. One had 

 in fact to allow about an hour with these thermometers. Even with 

 the long-stem thermometers, whether glass or porcelain, at least forty 

 minutes' exposure to sulphur vapour was found desirable. A good deal 

 depends on the gas supply and on the quality of the Bunsen burner 

 used. 



In some of the sulphur point experiments at Kew it is open to doubt 

 whether the stationary temperature had been absolutely attained, but 

 I do not think any serious uncertainty was introduced in this way. 



Impurity in the Ice used in yetting KO, or Variability in the Method of 



Treating it. 



32. The ice employed at Kew Observatory is supplied in large 

 blocks, which are washed prior to use. As the testing of thermometers 

 in ice is a frequent occurrence, the assistants have no lack of experience, 

 and since a planing machine was introduced some years ago the ice as 

 prepared has been very uniform and finely divided. The ice employed 

 in the platinum thermometry has been taken from the supply used in 

 the ordinary test work, and as the purity of this is frequently checked 

 by observations with Kew standard thermometers no serious impurity 

 need be feared. The check would, I allow, be hardly adequate to 

 settle a question of two or three thousandths of a degree, and no doubt 

 for work of the highest accuracy a more stringent test would be 

 desirable. Strong confirmatory evidence of the uniform purity of the 

 ice is afforded, as will be seen presently, by the small variability in the 

 observed values of the fundamental intervals of the thermometers. 



The method of preparing and moistening the ice seems to exert a 

 small but sensible influence on the depressed zero readings of mercury 

 thermometers, possibly because the amount of the depression must be 

 influenced by the rate of cooling. There seems, however, to be very 

 little if any effect on the zero reading of a platinum thermometer, so 

 long as the immersion is ample and the temperature of the room is 

 moderate. 



Error in the Barometer. 



33. Ordinary errors of reading, lag, or improper temperature 

 correction would merely introduce irregularities into the calculated 



D 2 



