Platinum Tlicrmmndnj at Kew Observatory. 



oo 



between the leads did not vary much, so that the application of a cor- 

 rection for this difference does not much improve the agreement between 

 the results of different days. 



The value of EO is conspicuously high on May 10 in all the ther- 

 mometers. This was clearly not the fault of the ice, because, as 

 Table XXII shows, the fundamental intervals found on May 10 were 

 not below the average, but in fact rather above. Again, the cause can 

 hardly have been error in the correction for temperature. For on 

 May 10 the temperature was only very slightly above 20 C., while on 

 the next occasion, June 27 when the values of RO were smaller it 

 was nearly 22 C. ; and if we may trust Dr. Marker's experiments, 

 the temperature coefficient applied in the reductions is certainly not 

 too low. 



Phenomena similar to those occurring on May 10 were by no means 

 uncommon. 



51. Additional information as to the degree of accuracy attained 

 in determining the fundamental interval is afforded by Table XXIII. 

 This gives the mean difference, irrespective of sign, between the indi- 

 vidual results and their arithmetic mean for two epochs, viz., the year 

 1896, and the period from September, 1897, to January, 1899. In the 

 former epoch there were six observations with each thermometer, in the 

 latter epoch from twelve to fourteen. 



Table XXIII. Mean Departures of Fundamental Intervals from their 

 Arithmetic Means. 



During the former epoch the original box with brass plug holes was 

 in use, while during the latter epoch the plug holes were of fusible metal 

 in Doulton ware. During the second epoch bridge centres were taken 

 every day, and readings were always taken with the current both ways ; 

 in the first epoch neither of these precautions was taken. Yet we see 

 that the mean departures are almost all greater during the second epoch 

 than during the first. This certainly does not point to any superiority 

 in fusible metal as compared to brass, but quite the reverse. 



The size of the mean departures in KI is partly due to change in the 

 fundamental interval, especially during the second epoch. 



The fundamental interval in KO, being 2 '5 times as large as in the 



