meafuring Heights with the Barometer . 665 
of the deal rod ; whereas the dilatation of the tubes, by 
the fame degree of heat, was always very vifible lb) . 
Finding from the fecond clafs of experiments, whereof 
the general refult hath now been given, that glafs rods 
feemed not to lengthen more than deal; and that tubes 
of different bores, and probably too of different forts of 
glafs, were fufceptible of different degrees of extenfion, 
which rendered it impoflible, by this means, to afcertain 
the longitudinal expanfion of the quickfilver they con- 
tained; I thought it neceffary to recur once more to the 
barometer, and to try whether it could not be fo con- 
trived as to a£t in water of different temperatures, from 
freezing to boiling. This led me to the experiments of 
the third clafs : and in order to comprehend them tho- 
roughly, it feems neceffary to point out fome few altera- 
tions which the apparatus underwent. 
The center of the bottom being pierced on purpofe, a 
brafs focket was prepared for it, wherein a hole was bored 
conically, to receive the ground-end of a barometer tube, 
of the ordinary length of 3 3^ inches ; the tube having 
been firfl ground in a feparate piece of brafs, and ulti- 
(b) Since thefe experiments were made, tlie relative expanfion of 18 inches 
©f one of the tubes and one of the rods that had been formerly employed, was 
found to be, by Mr. cumming’s pyrometer, nearly as 4 to 1, from a heat 
approaching to that of boiling oil. 
Vol. LXVII. 4 R mately 
