662 Col. roy’s 'Experiments for 
merit. This was of courfe to be done, either by fuch dif- 
ferences as I could difcern and meafure, or by thole that 
had refulted from the experience of others. 
The nature of the apparatus, employed in this clafs of 
experiments, will be ealily underftood from plate XVII. 
where it is reprefented, as it was ufed in tbofe of the third 
clafs. In its firft ftate it was not quite lb long, and a 
chafing-dilh with a charcoal fire, occupied the place of 
the ciftern holding the quicklilver below By means of 
a circular bit of tin, foldered edgeways in the center of 
the bottom, and an aperture in the middle of the lid, the 
tube was kept lleadily in the axis of the vefiel. Other open- 
ings in the lid, ferved for the admiffion of the thermo- 
meter, and the application of a deal rod clofe to the fide 
of the tube, when its height was to be meafured. The 
longitudinal expanfion of the glafs was marked by a 
fcratch thereon with a fine edged file at the top of the 
deal rod, when refpectively at the temperatures of freez- 
ing and boiling. The apparent dilatation of the quick- 
filver was in like manner marked, by the coincidence of 
its furface with the lower edge of a brafs ring embracing 
the tube. 
It having been found impoffible to procure tubes 
whofe bores were truly cylindrical, or of any uniform 
figure, the experiment was repeated, as often as poffible, 
in 
