meafuring Heights with the Barometer . 671 
its fides and fummit. Thefe globules were very fmall 
near the furface of the water, augmenting gradually as 
they approached the top of the tube, where they were 
greateft : their bulk increafed with the heat ; and when 
the water was at or near boiling, they would fometimes 
unite, and defcend by their own gravity, along the fides of 
the tube, into the general mafs. Hence the progreffive di- 
minution of the rate of expanfion of the column of quick- 
filver in the barometer, perceptible even in the firft clafs 
of experiments, is eafily accounted for by the refiftance 
of the elaftic vapour^, acting againft the top of the tube^ 
which was here colder than the reft. 
But in the application of the barometer to the mea- 
furement of heights, the whole inftrument is of the fame 
temperature ; wherefore, in the fecond fet of this third 
clafs of experiments, the tin veflel was heightened, that; 
tubes of the ordinary length, placed in it, might be wholly 
immerfed in boiling water. The mean of four experi- 
ments, which agreed very nearly among themfelves,., 
gave .5117 for the total expanfion between freezing and 
boiling; for the a o°, between 112 0 and 132^059; and 
for the laft ao°, between 192 0 and 21a 0 . 046. In thefe- 
(c) Having mentioned the circumftances to Mr. ramsden, it firft occurred- 
to him, that the refiftance of the elaftic vapour was the caufe of the diminu- 
tion in the rate of expanfion. 
expert- -4 
