cn the mechanical Expanfion cf dir. 4 - 
contained ; and if this flream of air had previously been more 
condenfed, or in greater quantity, fo as not inflantly to acquire 
heat from the common atmofphere in its vicinity, it would 
probably have fallen in Show, as in the fountain of Hiero, 
mentioned below. 
2. About twelve or fourteen years ago, by the affiflance of 
Mr. Waltire, a celebrated itinerant teacher of philofophy, 
a thermometer was placed in the receiver of an air-pump, and 
fome time being allowed, that it might accurately adapt itfelf 
to the heat of the receiver, the air was haftily exhaufled ; 
during which the mercury of the thermometer funk two or 
three degrees, and after fome minutes regained its previous 
height. In November 1 787, by the aflifrance of my very ingenious- 
friend Mr. Forester French, the above experiment was re- 
peated ; but with this difference, that the thermometer was open 
at the top; fo that the diminution of external preffure could 
not affect the dimensions of the bulb; and the reiult was the 
fame, the mercury in the thermometer funk two or three tie- 
grees, and gradually rofe again. Does not this Shew, that the 
air in the receiver, being expanded during the exhauftion, 
attra&ed or abforbed heat from the mercury in the thermo- 
meter ? 
Both during the exhauflion, and during the re-admiffion of 
the air into the receiver, a fleam was regularly obferved to be 
condenfed on the fides of the glafs, which in both cafes was 
in a few minutes re-abforbed. This fleam mufl have been 
precipitated by its being deprived of its heat by the expanded 
air : if it could have happened from any other caufe, the 
vapour could not, in both fituations, viz. of exhauflion, and 
of re-admiffion, have been taken up again. 
I 
3. lit- 
