_'l Mi. E. H. Griffiths. Appendix to a Communication [Jan. 18, 



and two air thermometers constructed by Hicks under Mr. Calendar's 

 direction.* 



The indications of these air thermometers are independent of ex- 

 ternal pressure, and no difficulty was experienced in obtaining the 

 temperature of the bulb to 1/1000 C. The results were, however, 

 not entirely satisfactory. It was found impossible to maintain the 

 rjrfrrior portion of the tank (where the comparisons had to be made) 

 at a temperature constant to 1/1000 C, especially at higher tempera- 

 tures. A reference to pp. 374 378 will show that the regulating 

 apparatus was designed to maintain at a constant temperature the 

 interior of the steel chamber there described, and this purpose it 

 fulfilled admirably. Fluctuations, however, amounting to as much 

 as 1/100 at 50 C. occurred in the surrounding water, and an element 

 of uncertainty was thus introduced into our comparisons. I am now 

 so modifying the apparatus as to eliminate these fluctuations in the 

 exterior temperature, and thus render the tank more suitable for 

 such observations. 



I believe Mr. Callendar proposes to make a communication to this 

 Society in which the details of this comparison will be given, and he 

 has, with this object in view, taken with him to Canada the records 

 of our experiments. I therefore propose on this occasion to confine 

 myself to results. I may, however, mention that extreme care was 

 taken with the cleaning and drying of the air thermometers ; obser- 

 vations were made with, the thermometers filled with air, hydrogen, 

 and nitrogen, and all the precautions observed which Mr. Callendar's 

 considerable experience of air thermometers could suggest. 



The conclusions, as far as they affect my previous determinations 

 of temperature, are that over the range through which the experi- 

 ments were conducted (14 to 26 C.) the limit of error does not 

 exceed 0'003 C. of the nitrogen thermometer. An error of such a 

 magnitude would affect my final value of J by about 1 in 4000. 



Another possible cause of error, mentioned on p. 424, is the differ- 

 ence caused by unequal lag of the rising mercury thermometer at the 

 beginning and end of the temperature range, and I have pointed 

 out on p. 424 that a possible error of 0'008 C. might be due to this 

 cause. 



I have recently performed the experiments by which I hoped to throw 

 some light on this point using as a thermometer a naked platinum 

 wire immersed in pentane. The experiments are difficult to conduct, 

 and I do not regard the results as entirely satisfactory. They agree with 

 my former experiments in indicating that the lag is greater at the 

 beginning than at the end of the range by a quantity between 0'002 

 and 0-009 C. The mean result of my observations gives 0'004 C. 



* For a description of these air thermometers ww ' Hoy. Soc. Proc.,' January, 

 1891. 



