C s ] 



II. On the irregular Indications of Thermometers* . By 

 John Herapath, Esq.\ 



HPHE first time I noticed any irregularity of thermometrical 

 -*- indication, was in the summer of 1820. Holding the 

 thermometer of a friend obliquely, with its bulb in boiling 

 water, and then raising the stem to a vertical position, I re- 

 peatedly observed that the latter position gave a higher indi- 

 cation than the former. This, as far as I remember, happened 

 only when the thermometer was first put in the water obliquely, 

 and then brought to a vertical position ; but I do not recollect 

 whether any difference happened when the position was first 

 vertical and then oblique; nor do I indeed remember that 

 such a case was tried. My friend having at the time treated 

 my observation as accidental, and due to the greater influence 

 of the ascending steam on the vertical than on the oblique 

 tube, turned my attention to a different object ; though the 

 least consideration of the thickness of the glass and the very 

 slow transmission of heat by this body, ought to have con- 

 vinced us that such a reason was incorrect. I have no pre- 

 cise recollection of the obliquity of the tube, or of the amount 

 of the difference of the indications ; but I rather think the de- 

 clination from the vertex was about 30°, and the difference of 

 the indications a degree or two. The thermometer, as far as 

 I remember, was rather large in the calibre. 



Some months after this I observed, in the course of some 

 experiments on the resulting temperatures of equal portions of 

 mercury differently heated, that the thermometer with which 

 I took the higher temperature often sunk, in taking imme- 

 diately afterwards that of the mixture, from a half to a whole 

 degree lower than that to which it quickly rose and settled. 



My attention being shortly afterwards more particularly di- 

 rected to the correction of thermometers, I resolved to under- 

 take a few experiments for the purpose of discovering a guide 

 to a more practically accurate correction than that usually 

 given. 



Exp. 1. — I put the bulb of a thermometer which was about 



* It had been imagined by some scientific gentlemen into whose hand, 

 f. friend put this paper, that my object is to support the theory of Bellani. 

 Tais opinion they formed from my having made, as they say, an erroneous 

 qi otation of his experiment ; owing to my relying on a hasty mental im- 

 pression enfeebled by a considerable lapse of time. I beg however to ob- 

 serve, that I never had an idea of supporting Bellani's views. My present 

 ideas of thermometrical corrections had occurred to me some years before 

 Bellani published* or perhaps made, his experiment; and my allusion 

 to it was for the purpose of showing, that, from the impression I had of the 

 experiment, Bellani erred in discovering the cause of his phenomenon. 



■) Communicated by the Author. 



50° 



