meafuring Heights with the Barometer. 675 
fame temperature. The confequence of this, application 
was, that the column fhortened about -^ths of an inch; 
which feems to prove, that the quicklilver vapour now 
reached the fummit of the tube, and, acting againft it, 
overcame, by fo much, the preffure of the atmofphere. 
Ifhould nowproceed to give feme account of the fourth 
fet of this laft clafs of experiments, made on the con- 
denfation of the quicklilver, by means of artificial cold, 
below the temperature of the air. Previoully however 
to this, it may not be improper to take notice, in a more 
general way, of fome others that were made on expan- 
fion; as tending, with certain circumftances yet to be 
mentioned, not only to confirm thofe already deferibed, 
but likewife to account for many irregularities that occur 
in operating with barometers. 
In the courfe of the preceding experiments, from ac- 
cidents of various kinds, it was often necefiary to reboil 
the quicklilver; and in that operation, many tubes were 
broken. The frequent removal of the focket from the 
bottom of the veflel, in order to get others ground for it, 
became at laft very troublefome ; and made more caution 
neceflary, in boiling fuch as were ground, efpecially in 
frofty weather, which happened to be the cafe in the laft 
days of March, 1775: wherefore it was thought bell in the 
interim to try, what might be the expanfion of a column 
4 S 2 of 
