the late Dr. Joule's Thermometers. 495 



The result of this calculation, therefore, would be that the 

 scale of the Joule thermometer is about one part in a thou- 

 sand smaller than that of the Tonnelot, the difference being 

 due either to a difference in the glass or to faulty cali- 

 bration. 



In order to compare the observed differences in the read- 

 ings of the two thermometers with the values calculated from 

 the most probable scale -value of the Joule, I have added 

 columns VIII. and IX., the former giving the calculated 

 value of Tt — T 7 -, and the latter the difference 8 which is either 

 due to errors of observation or to irregular errors of gradua- 

 tion of one or other of the thermometers. Although the 

 obvious fault in this respect shown by the Joule prepares us 

 for occasional differences of about o, 01,I was not, for several 

 reasons, satisfied with the results of this series of comparisons. 

 The apparatus had not reached its final form during these 

 experiments, the stirring was not as good, and the thermo- 

 meter had not yet been protected against the inflow of cold 

 water through the opening in the roof of the inner box. A 

 great difficulty was als® found in comparing together directly 

 the Joule thermometer, which was rather sluggish in its 

 motion, with the Tonnelot, which answered very quickly the 

 smallest change of temperature. Unless care was taken, there- 

 fore, to make the rise exceedingly uniform errors were easily 

 made. Additional uncertainty was introduced by the frequent 

 redeterminations of the zero of the Tonnelot. The probable 

 error of the calculated coefficient was too great to allow me 

 to be satisfied with its value. 



A second series of experiments was therefore decided upon, 

 and as in a joint research in the equivalent of heat I had 

 occasion, together with Mr. Gannon, to determine with con- 

 siderable accuracy the scale-value of a Baudin thermometer 

 graduated directly to a 50th of a degree, I made use of the 

 latter in the second series. 



The experiments were made exactly in the same way as 

 before, the Joule being directly compared with the Baudin, 

 and zero readings being dispensed with. The results are 

 embodied in Table YI. The first column gives the tempera- 

 tures according to the Joule thermometer, the coefficient 

 *0429 being again used, and the readings being converted to 

 the vertical position. The second column gives the reading 

 according to the Baudin thermometer, after the proper cali- 

 bration correction had been applied and the reading also 

 reduced to the vertical position. The third column gives the 

 difference between the numbers in the two first. 



2 L2 



