VOh. LXIX.] PHILOSOPHICAL TRANSACTIONS. 53t) 



division, at the extremity of which was a vernier dividing each degree into 10; 

 with a lens of an inch focus: this apparatus being made moveable, first by the 

 hand, and more delicately by means of a micrometer screw, whose head was 

 divided into 25 divisions, each equal to the 40th of a degree (for so truly cylin- 

 drical was the tube, which had been with care expressly selected from a great 

 quantity of glass, that the divisions in the neighbourhood of the freezing point 

 did not differ from those near the boiling point by so much as a 40th of a 

 degree, and this variation appeared in other parts of the tube strictly uniform, 

 as was found by breaking the column of mercury); by means then of this 

 apparatus I was enabled to read off any height of the thermometer to within the 

 50th of a degree. The vessel, in which the water was boiled, which was always 

 spring water, was a cylindrical tin pot, 13 inches high and 4J- inches wide, with 

 a top something resembling that described in Mr. De Luc's work, contrived to 

 carry off the steam without incommoding the observer, with a waste pipe for 

 the superfluous water in boiling, which might otherwise fall on the fire and 

 extinguish it. The ball of the thermometer was immersed to within 2^ inches 

 of the bottom of the vessel, and lOi inches below the surface of the water, so 

 that as near as might be the whole column of mercury was exposed to the heat 

 of boiling water, there being only 15° or 20" of the scale, equal to about the 

 50th part rising out of the water exposed to the temperature of the steam, which 

 in 1 or 2 experiments was found to be 180" or IQO", so that the correction for 

 this defect of heat would only amount to a very few hundredths of a degree, 

 perhaps about .04 or .05, which, as the instrument was exposed to the same 

 circumstances as near as might be in all the observations, I have taken no notice 

 of. I thought it necessary to say thus much respecting the precision of the 

 instrument and the apparatus, and shall now relate the observations at length. 

 A long table of the observations is then given, containing a series of the 

 heights of the barometer all at the heat of 50°, between 26.498 inches, the 

 lowest, and 30. 967 the highest, with the annexed correspondent regular series 

 of the heat of the boiling point, from 207°.07 the lowest, up to 2 14°. 15 the 

 highest. 



Having collected this series of experiments, I was anxious to see how far the\ 

 corresponded with Mr. De Luc's, and on comparison of N° 1 and N° 15, I 

 found that the decrease of the boiling heat was '-— greater than the rules ad- 

 mitted of from an alteration of the pressure of the atmosphere of 4-i. inches. 

 This difference led me into an examination of all my observations, to see how 

 far they were consistent with themselves ; how far they disagreed from Mr. De 

 Luc's; and lastly what general conclusion might be drawn from them. The 

 result was, that the mean motion of the boiling point for 1 inch of the baro- 

 meter when the mercury stands at 28.886 inches, is = 1°.743; according to 



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