6 



Messrs. C. W. Waidner and F. Mallory on 



density is that of 0° 0., sea-level latitude 45°, 100° C. For 

 the third temperature the boiling-point of some substance, 

 which is accurately known from a direct measurement with 

 an air-thermometer, is used. The one most frequently used 

 is the boiling-point of sulphur, 2 = 444 0, 53, on account of the 

 accuracy with which it is known. Callendar and Griffiths 

 have shown that if platinum thermometers be standardized 

 by these three temperatures (melting ice, steam sulphur 

 vapour), the above formula will give temperatures on the air- 

 scale to within 0°*01 C. over the range 0° C. to 100° O. This 

 conclusion is further confirmed by a direct comparison made 

 by Dr. W. S. Day and the authors between a platinum 

 thermometer and a Tonnelot thermometer standardized at the 

 Bureau International, whose scale of temperature is based on 

 measurements made with a constant- volume gas thermometer. 

 The comparison referred to above, made by Callendar and 

 Griffiths, is based on a constant-pressure air thermometer. 



Description of Apparatus. 



Mercury Thermometers. — The thermometers used by Row- 

 land in his experiments on the mechanical equivalent were con- 

 structed by Baudin in 1876-77, and are numbered 6163, 

 6165, and 6166. Of these thermometers, Baudin 6163 is 

 perhaps the most important, as it was used in eight of the 

 fourteen determinations.; Baudin 6166 was used in four 

 determinations, and has besides an historical interest, as Joule 

 compared his thermometers with it; Baudin 6165 was used 

 in only one determination of the mechanical equivalent. 

 Besides these, Kew 104, constructed by Welsh in 1853, was 

 used in one determination. This thermometer was not avail- 

 able for this comparison, but inasmuch as it was only cali- 

 brated to o, 5 F., and was given but little weight in the final 

 results, its effect on the values of the mechanical equivalent 

 is entirely negligible. 



The stems of the thermometers are graduated in millimetres. 



Thermometer. 



Range. 



1° C. occupies about 



Baudin 6163 , ,, 

 6165 , 

 6166 



-6°C. to40°C. 

 -3°C. to33°C. 

 -2° C. to 31° C. 



9-0 mm. 

 11-7 „ 

 12-9 „ 



Platinum Thermometer. — The platinum thermometer used 

 in these experiments, and shown in fig. 1, was constructed by 



