538 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1779. 



with a small pocket thermometer of about 6 inches long, made by Mr. Nairne; 

 an instrument, it must be confessed, not very accurate for such an examination, 

 but with which I thought I could 'observe to within a quarter of a degree; 

 my object at that time, amidst a variety of other philosophical pursuits, being 

 to assure myself that the variation took place, rather than critically to examine 

 the quantity of it. I shall relate these observations, as the result on my return 

 to England led me to some more accurate. 



Observations of the boiling-point, made in a journey over the Alps. 



Place of obser- 

 vation. 



'Height of the 

 barometer. 



Bologna. . . . , 



Geneva 



Modane .... 

 Lannebourg . 

 Mount Cenis, 

 Ditto 



Inches. 

 30.21 

 28.60 

 26.61 

 25.75 

 24.03 

 23.91 



Heat of boiling 

 water by ob- 

 servation. 



213.5 

 210.4 

 207.3 

 205.1 

 201.2 

 201.1 



Heat of boiling 

 water by Mr. 

 De Luc's rule. 



213.5 

 210.9 

 207.4 

 205.9 

 202.6 

 202.4 



Difference. 



Difference in 

 ^^ of total. 



Mean -r^ = JL. 



Hence it might be concluded, that the motion or variation of the boiling 

 point, with a given variation in the pressure of the atmosphere, was -j-^ or -^\ 

 greater than by the theory alluded to.* These were indeed but gross experi- 

 ments, and perhaps unworthy of such a competition. They induced me how- 

 ever to make the following. In the beginning of last year (1778), with the 

 assistance of Mr. Ramsden, I procured a most excellent thermometer, every 

 way adapted for this purpose. It was about 14 inches long, but the interval 

 between freezing and boiling only %\ inches,-|- and though every degree was 

 something less than the 20th of an inch, yet by means of a semi-transparent 

 piece of ivory, which applied itself close behind the glass tube, sliding up and 

 down in a groove cut in the brass scale for that purpose, carrying a hair-line 



* The same instrument immersed in snow just melting at the top of Mount Cenis fell to 32°, the 

 point of freezing observed at the level of the sea. — Orig. 



+ It may possibly be suggested, that if this interval had been greater, viz. 20, 30, or 40 in. 1 should 

 have had a much larger scale and more convenient instrument ; but in this, as ui most otlier mechanical 

 contrivances, our progress beyond certain limits is prevented ; for if the perpendicular height of the 

 column of quicksilver be much increased, the weight of it will be such as to distend the ball, and 

 the instrument may differ from itself in a vertical and horizontal position by half a degree, as I have 

 seen in a tube only 15 inches long ; and if this circumstance be endeavoured to be corrected by 

 making the bulb of the thermometer tJiicker, its sensibility will be proportionably diminished. \i 

 my experience were to lead me to conclude any thing, 1 should consider a tube of a foot long as a 

 maximum, and the bore of such a diameter as to admit a ball of a 4th or a 5tli of an inch. — Orig. 



