Meafurement of a Bafe on Hou n flow- He a t h . 461 
The laft day of Auguff was employed in difcharging the 
party, and removing the various parts of the apparatus to 
Spring- Grove Houfe. 
Bejcription of the Microfcopic Pyrometer , made ufe of for deter- 
mining by experiment the expanf on of the metals concerned in 
the meafurement of the Bafe. Tab. XX. 
Having, in the preceding part of this Paper, given a very mi- 
nute account of the aTual operations in the field, that the 
Public, being thus informed of every cireumftance, might be 
the better enabled to judge of the accuracy of the refult, it 
remains yet to point out, in what manner the equations for 
the expanfions of the ftandard feale, heel chain, and glafs rods, 
applied to the apparent meafurement of the bafe, in feveral of. 
the preceding notes, have been obtained by means of experi- 
ments with the pyrometer. 
It is .Efficiently well known, that many years ago, a very' 
ingenious and valuable Member of this Society did publifh in 
the Philofophical Tranfa&ions (vol. XLVIII. 1754, N° 79.) 
an account of experiments made with a pyrometer of his in- 
vention. No doubt was entertained of the accuracy of the 
experiments here alluded to on the contrary, they will be 
confirmed; by the account now to be given of thefe recently 
ma de, with which, they very nearly agree. But as different 
pieces of metal of the fame kind are certainly fufceptible of 
different degrees of expanfion, it was judged beff, on the pre- 
fent occafion, to put rods to the teff of thofe very metals that 
had been made ufe of in the actual meafurement of the bale,. 
7 For, a 
