698 Col. roy’s Experiments for 
Experiments , for determining the actual expanjion of com- 
mon air , in the manometer , affected by the heat of 2 1 2 0 . 
For this purpofe it became neceffary to afcertain, in 
every manometer, the exaft proportion between the ca- 
pacity of the tube and that of its bulb. This was done, 
by weighing the quickfilver that filled them refpedlivelv, 
in a balance that was fenfible to a very fmall fraction of 
a grain. The contents of the bulb, and that part of the 
tube between it and zero, exprefled in grains, w r as called 
the air in experiment. The apparent expanfion of that air 
was meafured, by the grains that filled the feveral feftions 
of the tube between zero and the boiling point ; the fum 
being the total expanfion or increafe of volume, from a 
heat of 2 1 2 0 . The apparent expanfion, thus found, was 
again augmented for the dilatation of the tube, on the 
following principles. 
In the firft part of this paper I have fhewn, that folid 
glafs rods dilate much lefs than barometer tubes. The 
mean betw'eenMr. smeaton’s and my experiments, gives 
of an inch for the longitudinal extenfion of every 
foot of thefe tubes, by 2 1 2 0 . From the rate of going of 
a clock, for near a year, whofe pendulum rod is folid 
glafs, its dilatation feems to be one-third part of a fteel rod, 
or 
