234 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



quicksilver, corresponding to every 20 degrees of change of temperature in the 

 thermometer, vvere successively marked on a deal rod applied to the manometer. 

 It is to be observed, that both instruments, while in the water, were in circum- 

 stances perfectly similar; that is, the ball and bulb were at the bottom of the 

 vessel. ' ! i 



It is easy to conceive, in experiments of this very delicate nature, part of which, 

 namelv, those on air less dense than the atmosphere, were extremely difficult and 

 even laborious, that mathematical exactness was not to be looked for; and that, 

 notwithstanding every possible precaution was taken, irregularities would occur. 

 These however were not so numerous as might have been expected, nor any way 

 so great as to render the research fruitless: for a few of that kind being thrown 

 out of the total number, the mean of the others, which were very consistent 

 among themselves, served to prove beyond the possibility of doubt, that the ex- 

 pansions of common air did not keep pace with the dilatations of quicksilver. 

 The manometrical space, answering to the 20° of the thermometer between 52° 

 and 72°, was always found to be greater than any other 20° of the scale. Here 

 it is to be understood, that I do not pretend to have ascertained the exact point 

 in that space where the maximum falls: the 

 mean corresponds to the 62d degree, and yet 

 I am inclined to think that it is somewhat 

 lower, perhaps it may be about the 57th: 

 from this point, the condensation of air down- 

 wards, and its expansions upwards, follow a 

 diminishing progression, compared with the 

 condensations and dilatations of quicksilver. 

 The manometrical are equal to the thermome- 

 trical spaces, in 2 points of the scale ; namely, 

 at or near the freezing temperature on one 

 side, and between the 1 12° and 132d degrees 

 of the scale on the other. At the zero and 

 boiling point they are less than the thermo- 

 metrical spaces. The thermometrical, com- 

 pared with the manometrical spaces, will 

 therefore appear as in the annexed table. 



Experiments for determining the victual Expansion of Common Air, in the 

 Manometer, affected by the heat of 212°. — For this purpose it became necessary 

 to ascertain, in every manometer, the exact proportion between the capacity of 

 the tube and that of its bulb. This was done, by weighing the quicksilver that 

 filled them respectively, in a balance that was sensible to a very small fraction of 

 a grain. The contents of the bulb, and that part of the tube between it and 



