meafuring Heights with the Barometer. 669 
and difplaced frequently, during the courfe of every ex- 
periment ; for the heat was very expeditioufly commu- 
nicated to the iron cittern, and thence to the quickfilver 
it contained ; and both were found to cool very faft, after 
the lamp was removed. Such was the ftate of the appa- 
ratus, when the firft fet of this third clafs of experiment* 
was made. In thofe of the fecond fet, its height was far- 
ther augmented by tin foldered to the top, that a tube of 
the ordinary length might be wholly immerfed in boil- 
ing water. The third and laft alteration confifted in the 
occalional application of a detached tin cafe, equal in 
diameter to the upper part of the veffel, having a hole in 
its bottom to admit the top of a long tube to pafs. This 
cafe was fo contrived, that its bottom flood two inches 
and a half higher than the lid of the veffel, thereby al- 
lowing room for the hand to move the index up or 
down. In this ftate the apparatus is reprefented in the 
view; and its various ufes will be beft underftood from 
the account of the experiments, which were fubdivided 
into four lets. 
Thofe of the firft fet were made with tubes of a large 
bore, upwards of three-tenths of an inch in diameter, of 
the ordinary length, with a vacuum over the quickfilver 
of two inches and a half or three inches, part of which 
reached above the top of the veffel. The mean of three 
expert- 
