Meafurement of a Bafe on Hounflow-Heath. 461. 



The laft day of Augnft was employed in dlfcharghig the 

 party, and removing the various parts of the apparatus to 

 Spring-Grove Houfe. 



"Dejcrtption of the Microfcopic Pyrometer, made tife of for deter- 

 mining by experiment the expanfion 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 adual operations in the field, that the 

 Public, being thus informed of every circumftance, might be 

 the better enabled to judge of the accuracy of the reiult, it 

 remains yet to paint out, in what manner the equations for 

 the expanlions of the flandard fcale, fteel 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 fufiiciently well known, that many years ago, a very 



ingenious and valuable Member of this Society did publiih in 



,the Phiiofbphical Tranfaaions (vo-l. XLVIII. 1754, N^ 79.) 



.an account of experiments nciade 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 



•made, with which they very nearly agree. But as .different 



pieces of metal of the fame kiad are certainly fufceptible of 



different degrees of expanfion, it was judged heft, on the pre- 



fent occafion, to put rods to the tell: of thofe very ipetals that 



had been made ufe of in the adual meafurement of the bafe. 



7 For, 



