132 SCIENCE PROGRESS. 



whose indications he compared with a mercury thermo- 

 meter previously standardised by the Bureau International 

 des Poids et Mesures. Admirable as were his arrange- 

 ments I consider that the comparison of electro-motive 

 forces is better adapted for the detection, than for the 

 accurate measurement of small differences of temperature. 



This is hardly the place in which to enter into a dis- 

 cussion of the question, and I only here give my opinion 

 for what it is worth, though I have reason to believe that 

 it is shared by others. 



Again, he gives no precise details in his published 

 paper of the circumstances under which the readings of 

 the mercury thermometer were taken — such as the con- 

 stancy of the temperature, the rate of rise (if any), the 

 observed changes of zero, etc., nor does he give any 

 record of the results of the differences between repeated 

 comparisons at the same temperature; thus it is impossible 

 to estimate his probable limit of error. 



There is little doubt that measurements of resistance 

 can, under favourable circumstances, be made with greater 

 accuracy than comparisons of the electro-motive force due 

 to a thermo-electric couple. 



The investigations by Professor Callendar and myself 

 have, I think, placed beyond question the relation between 

 the platinum and the air thermometer over a range of o° to 

 6oo Dl C, and the almost exact correspondence subsequently 

 discovered (a difference of '005° C. in elevation, and of 

 •ooi° in range between 14 and 25° C.)' 2 between the mer- 

 cury thermometers used in my investigation (which were 

 standardised by means of platinum thermometers) and the 



Callendar, Phil. Trans., A, 1887. 



„ Phil. Mag., July, 1891, and January, 1892. 



„ Journal Iron and Steel Institute, No. 1, 1892. 



Callendar and Griffiths, Phil. Trans., A, 1891. 

 Griffiths, Brit. Assoc. Report, 1890. 

 ,, Phil. Trans., A, 1891. 



„ 1893, pp. 420-433. 

 „ Phil. Mag., 1 89 1. 



'Phil. Trans., 1893, p. 430. 



