OF ARTS AND SCIENCES. 81 



not further discuss the matter, but will use the first definition, as 

 being the simplest. 



In the above formula I have implicitly assumed that the apparent 

 expansion is only a function of the temperature; but in solid bodies 

 like glass there seems to be a progressive change in the volume as 

 time advances, and especially after it lias been heated. And hence in 

 mercurial and alcohol thermometers, and probably in general in all 

 thermometers which depend more or less on the expansion of solid 

 bodies, we lind that the reading of the thermometer depends, not only 

 on its present temperature, but also on that to which it has been sub- 

 jected within a short time ; so that, on heating a thermometer up to a 

 certain temperature, it does not stand at the same point as if it had 

 been cooled from a higher temperature to the given temperature. As 

 these effects are without doubt due to the glass envelope, we might 

 greatly diminish them by using thermometers filled with liquids which 

 expand more than mercury : there are many of these which expand 

 six or eight times as much, and so the irregularity might be dimin- 

 ished in this ratio. But in this case we should find that the correction 

 for that part of the stem which was outside the vessel whose tem- 

 perature we were determining would be increased in the same propor- 

 tion ; and besides, as all the liquids are quite volatile, or at least wet 

 the glass, there would be an irregularity introduced on that account. 

 A thermometer with liquid in the bulb and mercury in the stem would 

 obviate these inconveniences ; but even in this case the stem would 

 have to be calibrated before the thermometer was made. By a com- 

 parison with the air-thermometer, a proper formula could be obtained 

 for finding the temperature. 



But I hardly believe that any thermometer superior to the mer- 

 curial can at present be made, — that is, any thermometer within the 

 same compass as a mercurial thermometer, — and I think that the 

 best result for small ranges of temperature can be obtained with it by 

 studying and avoiding all its sources of error. 



To judge somewhat of the laws of the change of zero within the 

 limits of temperature which I wished to use, I took thermometer 

 No. 61 G3, which had lain in its case during four months at an average 

 temperature of about 20° or 25° C, and observed the zero point, after 

 heating to various temperatures, with the following result. The time 

 of heating was only a few minutes, and the zero point was taken 

 immediately after ; some fifteen minutes, however, being necessary for 

 the thermometer to entirely cool. 



vol. xv. (n. 8. VII.) 6 



