Major-General Roy’s Account of the 
dukim rod, whofe expanlions were only tried once. The dif* 
ference of a few divilions between the mean and extremes on 
the heat of 1 8o° being, in things of this fort, of no importance, 
it was judged wholly uniieceflary to aim at a greater degree of 
precilion in repeating them oftener. By referring to the table, 
particularly that column containing the expanlions on one foot 
by i8o°, it will be perceived, that they are uniformly a fmall 
matter lefs than what has been afligned to the fame metals 
refpeCtively, in the experiments formerly alluded to. 
if 
Ultimate determination of the length of the Bafe on Hounflow- 
Heath. 
In the former part of this paper, we have had occalion 
to fpeak of the feven firft columns of the general table 
of the bafe; and the titles at the tops of the others re- 
fbeCtively ferve fufficiently to explain thofe towards the right- 
hand ; the expanlion of glafs above, and its contraction below 
62°, contained in the eleventh and 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 difhnce between the 
laffc rod and the center of the north-weft pipe, has 
been (hewn to be - - - 27402.8204 
The reduction contained in the feventh column 
of the general table to be deducted is ; » 0.0714 
Hence the apparent length of the bafe, reduced 
to the level of the fouth-eaft extremity, becomes 
27402.7490' 
The 
