Trigonometrical Survey. 507 
mometer was at 55 0 *, but the temperature is omitted when 
those points in the plank were transferred from the brass stand- 
ard. The “original points” must be those alluded to in the 
General’s account of the Hounslow Heath base (Phil. Trans. 
Vol. LXXV. p. 403), which were fixed in the plank from the 
brass standard in the temperature of 6 3 0 ; but it is probable 
that General Roy supposed them to have been transferred in 
62°, and, through mistake, subtracted the sum of the two first 
corrections in page 131, instead of their difference, which in 
that case would have been the true correction for the contrac-, 
tion of the chain. The error however, is about 33 inches : 
for since the chain in the temperature of 55 0 was equal to loo 
feet of the brass standard in that of 63°, it follows, from the 
table- of expansions in the General’s account of the Hounslow 
Heath base, that its length in S 3 °fo was e( l ua l to 100 f eet 
the brass standard in 62°; and therefore 53V0 the tempera- 
ture to which the measurement by the chain should be re- 
duced. Now the apparent length being 258,36736 chains, and 
68290,5 the sum of all the degrees shown by the thermometers 
in the table, page 134, we -have 285,36736 x 53^- — oS 2 p . ’ 5 . x 
*00763 inches = 12,8 inches, the contraction below S 3 °fo'^ 
this, with the other corrections applied to the apparent length, 
give 28535 feet 8 inches, instead of 28532 feet 11 inches. 
To determine the distance from Hollingbourn Hill to Fair- 
light Down from this base (28535,66 feet) by means of the 
fewest triangles, we suppose, according to General Roy (page 
* That this was the temperature, appears in a great degree from various comparisons 
we made with the chain and the two new ones on Hounslow Heath : Sir J. Banks 
very obligingly favoured us with the Society’s chain, for the purpose of trying its 
length with the new chains. 
