VOL. LXVII.] I'HILOSOPHICAL TRANSACTIONS. 259 



all the quicksilver in the tube will be surrounded by the steam of the boiling 

 water, and consequently will be nearly of the same heat as the water itself: we 

 therefore made some experiments to determine how regular the boiling point 

 - would be when tried in such vessels, both when the ball was immersed in the 

 water, and when it was exposed only to the steam as recommended by 

 Mr. Cavendish, in these abridgments, vol. 14, p. 5 J. 



The vessel used in these experiments is represented in fig. 11, pi. 2, Asba is 

 the pot containing the boiling water; od is the cover; e is a chimney for carrying 

 off the steam; Mm is the thermometer fastened to a brass frame; this ther- 

 mometer is passed through a hole pf in the cover, and rests on it by a circular 

 brass plate eg fastened to its frame, a piece of woollen cloth being placed 

 between eg and the cover, the better to prevent the escape of the vapours. 

 Two pots of this kind were used by us; one 5 inches in diameter and Q deep; 

 the other, 4-1- in diameter and 23 deep. Two of the thermometers principally 

 used were short ones, the brass plate (eg) being placed only 34 inches above the 

 top of the ball, and the boiling point rising not much above that plate: the 3d 

 thermometer was much longer, the plate (eg) being 17 inches above the ball. 

 They were all 3 quick; the 1st containing only '2^ degrees to an inch; the 2d 

 5°; and the 3d 10°. The 1st had a cylinder instead of a ball, ]i inch long and 

 -jSg- in diameter;* the two others had spherical balls, about ^ of an inch in 

 diameter. 



On trying these thermometers in the abovementioned vessels, with the v\'ater 

 rising 2 or 3 inches above the top of the ball, we found some variations in the 

 height, according to tlie different manner of making the experiment, but not 

 very considerable ; for the most part there was very little difi^erence whether the 

 water boiled fast or very gently; and what difference there was, was not always 

 the same way, as the thermometer sometimes stood higher when the water boiled 

 fast, and sometimes lower. The difference however seldom amounted to more 

 than -rV of a degree, unless a considerable part of the sides of the pot were 

 exposed to the fire; but in some trials which we made with the short ther- 

 mometers in the short pot, with near 4 inches of the side of the vessel exposed 

 to the fire,-|- they constantly stood lower when the water boiled fast than when 

 slow, and the height was in general greater than when only the bottom of the 



* In the two short thermometers the quicksilver would have descended into die ball when cold, 

 had not the tube been swelled a little, close to the ball, in order to prevent it. — Orig 



t In all our experiments, the water was boiled over a portable black-lead fiimace, covered with an 

 iron plate, which had a hole cut in it just large enough to receive the bottom of the pot ; so that, by 

 passing the bottom through this hole to a greater or less depth, we could expose more or less of the 

 sides to the fire. In the other experiments, not more than one inch of the sides was ever exposed to 

 the fire. — Orig. 



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