Meafurement of a Bafe on Hon nflow- Heath. 43 1 
At this time Lieut, Colonel Calderwood could not conve- 
niently lend us his affiftance in the field ; but he vifited us 
occasionally, and 011 one of thefe vifits propofed tome, that 
glafs rods Should he made ufe of inftead of deal ; putting me in 
mind of another experiment.* that I had made, which feemed to 
Ihew that folid glafs rods expanded lefs than tubes. This pro- 
pofition the Lieutenant Colonel, before he came to the heath, 
had made to Mr. Ramsden, who appeared averfe from making 
the trial, becaufe of the great length of the rods, and the 
brittlenefs of the material. Neverthelefs, it being fufhciently 
obvious, that glafs rods or tubes of the full length, or fome- . 
thing approaching towards it, would be much fooner provided 
than any metal rods whatever, and the faving of time being 
a point of confequence ; Lieut. Colonel Calderwood was ac- 
cordingly requeued to make the trial at the glafs-houfe,- as foon 
as poffible after his return to town. Next day he fucceeded in 
getting a fine tube drawn, eighteen feet long, and about one 
inch in diameter; and there feemed to be no longer any 
doubt, that thofe of the proper length might be obtained. It 
was found, that folid glafs rods of fuch extraordinary dimenfions 
.could not be had, it being impoffible to take at once a fu-fficient 
* The experiment here alluded towns made with Mr. Cumming’s pyrometer, 
which from its conllrudtion did not admit of a very accurate eftimation of the 
heat communicated to the ftandard bar, the rod, and tube refpedtively. Either, 
therefore, the natures of the glafs rod and tube, made ufe of at. that time, mult 
have been very different, to caufe the difference of expanfion ; or fome circum- 
ftance in the inllrument unattended to had occafioned the fallacious appearance : 
for it will be found, from the experiments hereafter to be given in detail, that a 
folid glafs pendulum rod expands fully as much as, nay in this particular inftance 
even more than a tube ; but different glafs, having different fpecific gravities, 
will no doubt be fufceptible of different degrees of expanfibility, 
Vol, LXXVe LI 1 quantity 
