Meafurement of a Bafe on Moiin How-Heath . 423 
taken as before-mentioned, the rough meafurement of that 
feflion with the chain was completed, and found to contain 
nineteen hypothenufal diRanCes of 600 feet each, and oile of 
404.55, making in the whole 11804,55 feet, between the 
twentyffixth Ration at the Staines Road and the center of the 
pipe near King’s Arbour, the mean temperature being 6 2°§. 
Here it is to be obferved, that this laid fection was only mea- 
fured once with the chain, the tradt not being yet fufficiently 
cleared to admit of its being done to the heft advantage ; and, 
when completed, it was judged to be better to proceed diredtly 
in the operation with the rods, than to lofe time in the ufual 
repetition, lince the merits of the chain, in this way of ap- 
plying it, were already fufficiently well eRabliffied ; and any 
future teRs to which it was to be put were propofed to be of 
a more rigid nature. 
When the length of the chain, in its original Rate, was 1 
afcertained by the dots on the brafs pins in the New-England 
plank, it was found, in the then temperature of 74% to ex- 
ceed the 1 00 feet by near one quarter of an inch, or 0.245 hich. 
Therefore, in the temperature of 63°, being that in which 
the lengths of the deal rods were laid off, and differing very 
little from what was likewife the mean heat of the air, when 
applied upon the Heath, the chain, according to the experi- 
ments on the expanlion of the very fame Reel, would exceed 
the too feet by 0.161 inch, or 0.0134 foot. Hence the 
fum of the three fe&ions of the bafe, 274 chains, being mul- 
tiplied by 0.0134 foot, we ffiall have 3.67 feet for the equation 
of the chain 4-4.55 feet, to be added to its length, which will 
then become 27408.22 feet from the center of one pipe to the 
center of the other : and this would have been the true length 
of the bafe, as given Jby the rough meafurement with, the 
VoL. LXXV. K k k chain. 
