Meafurement of a Bafe on Honnflow-Heath. 401 



Kt this time Llent. Colonel Calderwood ccjld not conve- 

 niently lend us bis afiiftance in the field ; but he viiited us 

 occaiional]y, and on one of thefe vitits propoled to me, that 

 glafs rods (hould be made uie of inftead of deal ; pvitting n'le in 

 mind of another experiment * that I had made, which feemed to 

 fhew that folid giafs rods expanded lels than tubes. This pro- 

 pontion the Lieutenant Colonel, betore 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 fufliciently 

 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 iine tube drawn, eighteen feet long^ and about one 

 inch in diameter; and there leemed 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 dimeniions 

 could not be had, it being impoffible to take at once a fufficient 



* The experiment here aIIu<Jed to was made with Mr. Cumming's pyrometer,, 

 "which from its conftru6tion did not admit of a very accurate eftimation of the 

 heat communicated to the ilandard bar, the rod, and tube refpeftively. Either, 

 thereforti, the natures of the glafs rod and tube, made life of at that time, muft. 

 h'ave been ret)' diflferenr,, to caiile the d-ifferenee of expanfion ; or fomc circum. 

 ftance- in the inftrument uaattended to had occafioned the fallacious appearance ;■ 

 i&c it will be found, froHH 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, LXXV, L 1 I quantity 



