meafuring Heights with the Barometer. 673 
the 20 0 in the middle of the fcale .058 ; and for the 11 p- 
permoft 20°. 065 : whence the mean rate for every 20% 
is nearly .0605 (e K In this fet, the condenfation in the va- 
cuum of the tube was particularly attended to: it began, 
as in thofe of the firft fet, immediately above the furface 
of the boiling water, which was always kept an inch or 
two above the top of the column : the lowermoft glo- 
bules were very fmall, increafing gradually till they got 
without the lid of the veflel, where they were the largeft; 
thence they diminifhed uniformly upwards, and dilap- 
peared entirely three or four inches below the top of the 
tube. Though the rate for the middlemoft ao°, in thefe 
laid experiments, be below the mean, probably from fome 
inaccuracy in obfervation ; yet, being compared with the 
former fets, they ftill ferve to corroborate each other : for 
in thefe with the long tube, the vacuum feems to have 
been either completely maintained, or nearly fo; and we 
accordingly find the maximum of expanfion increafed, 
and its rate rendered nearly uniform, as will be farther 
confirmed from what follows. 
(e) Mr. cavendish, who affifted in the firft part of the experiments with 
the open tube, informed me, that, in thofe made by his father Lord Charles,- 
the difference between the expanfion of quickfilver and glafs, from 180° of heat, 
was .469. If to this we add Mr. smeaton’s dilatation of giafs, the total 
expanfion of 30 inches of quickfilver will be .544, which agrees with the 
experiments in the long tube, and gives a rate of only .003022 for each degree, 
4 S 
Vol. LXVIL 
I have 
