47^ Major-General Roy's Account of the 



dulum rod, whofe expanfions were only tried once. The dif- 

 ference of a few divifions between the mean and extremes oti 

 the heat of i8o° being, in things of this fort, of no importance, 

 it was judged wholly unneceiTary to aim at a greater degree of 

 precifion in repeating them oftener. By referring to the table^ 

 particularly that cdlumn containing the expanfions on one foot 

 by 1 80°, it will be perceived, that they are uniformly a fmall 

 iKatter lefs than what has been affigned to the fame metals 

 refpeflively, in the experiments formerly alluded to, 



Ullhnale deferm'mation of the length of the Bafe on Hounflow- 



Heath. 



'In the former part of this paper, we have had occafioii 

 to fpeak of the feven firft columns of the general table 

 of the bafe; and the titles at the lops of the others re* 

 fpedtively ferve fufficiently to explain thofe towards the right- 

 hand ; the expanfion of glafs above,' and its contraction below 

 62°, contained in the eleventh Jind twelfth columns, being 



• deduced from the recent experiments with the pyrometer. 



Feet, 



The hypothenufal length of the bafe, as mea- 

 fured by 1369.925521 glafs rods of twenty feet 

 each +4.31 feet, being the -diftance between the ^ 

 lafl: rod and th-e center of the north- weft pipe, has 

 -been lliewn to be - - - 2"^4b2.S2b4 



The reduction ^contained in the feventh column 

 of the general table to be dedu<5led is ; . 0*0714 



Hence the apparent length of the bafe, reduced 



toL-the level of the foiith-ea ft extremity, becomes 274-02^7490 



