4-6 Dr. Darwin’s f rigor ific Experiments 
X. In December 1 784, with the afliflance of Mr. Fox, the 
following experiment was carefully made. A hole, about the 
flze of a crow-quill, was bored into a large air-veflel, placed at 
the commencement of the principal pipe in the waterworks 
which fupply the town of Derby. The water from four 
pumps, which are worked by a water-wheel, is fl rd thrown 
into the lower part of this air-veflel, and from thence rifes to 
the top of St. Michael’s Church into a refervoir, which may be 
about thirty-five or forty feet above the level of the air-veflel. 
Two thermometers were previoufly fu fpended on the leaden 
air-veflel, that they might become of the fame temperature 
with it; and, as foon as the hole was opened, had their bulbs 
reciprocally applied fo as to receive the dream of air ; and the 
mercury in both of them funk two divifions, or four degrees. 
This fluking of the mercury in the thermometers could not 
be aferibed to any evaporation of moidure from their furfaces, 
becaufe it was feen, both in exhauding and re-admitting the 
air into the exhauded receiver, that the vapour which it pre- 
vioufly contained was depolited during its expanfion. 
4. There is a very curious phenomenon obferved in the 
fountain of Hiero, condrubted on a very large fcale in the 
Chemnicenflan mines in Hungary, which is very fimilar to the 
experiments above related. In this machine the air, in a 
large veflel, is comprefled by a column of water 260 feet high : 
a dop-cock is then opened, and as the air iflues out with great 
vehemence, and, in confequence of its previous condenfation, 
becomes immediately much expanded, the moidure it contained 
is not only precipitated, as in the exhauded receiver above- 
mentioned, but falls down in a (hower of fnow, wflth icicles, 
adhering to the nofel of the cock. This remarkable circum- 
dance 
