374 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



difFeretice, that the thermometer was open at the top ; so that the diminution 

 of external pressure could not affect the dimensions of the bulb ; and the result 

 was the same, the mercury in the thermometer sunk 2 or 3 degrees, and gra- 

 dually rose again. Does not this show, that the air in the receiver, being ex- 

 panded during the exhaustion, attracted or absorbed heat from the mercury in 

 the thermometer ? Both during the exhaustion, and during the re-admission of 

 the air into the receiver, a steam was regularly observed to be condensed on the 

 sides of the glass, which in both cases was in a few minutes re-absorbed. This 

 steam must have been precipitated by its being deprived of its heat by the ex- 

 panded air : if it could have happened from any other cause, the vapour could 

 not, in both situations, viz. of exhaustion, and of re-admission, have been 

 taken up again. 



3. In December 1784, with the assistance of Mr. Fox, the following experi- 

 ment was carefully made. A hole, about the size of a crow-quill, was bored 

 into a large air-vessel, placed at the commencement of the principal pipe in the 

 water-works which supply the town of Derby. The water from 4 pumps, 

 which are worked by a water-wheel, is first thrown into the lower part of this 

 air-vessel, and thence rises to the top of St. Michael's Church into a reservoir, 

 which may be about 35 or 40 feet above the level of the air-vessel. Two ther- 

 mometers were previously suspended on the leaden air-vessel, that they might 

 become of the same temperature with it ; and, as soon as the hole was opened, 

 had their bulbs reciprocally applied so as to receive the stream of air ; and the 

 mercury in both of them sunk 2 divisions, or 4 degrees. This sinking of the 

 mercury in the thermometers could not be ascribed to any evaporation of mois- 

 ture from their surfaces, because it was seen, both in exhausting and re-admit- 

 ting the air into the exhausted receiver, that the vapour which it previously con- 

 tained was deposited during its expansion. 



4. There is a very curious phenomenon observed in the fountain of Hiero, 

 constructed on a very large scale in the Chemnicensian mines in Hungary, 

 which is very similar to the experiments above related. In this machine the air, 

 in a large vessel, is compressed by a colunm of water 260 feet high : a stop- 

 cock is then opened, and as the air issues out with great vehemence, and, in 

 consequence of its previous condensation, becomes immediately much expanded, 

 the moisture it contained is not only precipitated, as in the exhausted receiver 

 above-mentioned, but falls down in a shower of snow, with icicles, adhering to 

 the nosel of the cock. This remarkable circumstance is described at large, with 

 a plate of the machine, in the Philos. Trans, for 1761, vol. 52. 



5. From the 4 experiments already related ; first, of the mercury sinking in 

 the thermometer, by being exposed to the stream of air from an air-gun ; 2dly, 

 from its sinking in the receiver of an air-pump, during the time of exhausting 



