Meajiirement of a Bafe on Houn (low-Heath. 423 



taken as before-mentioned, the rough meafurement of that 

 ietSlioii with the chain was completed, and found to contain 

 nineteen hypothenufal diftances of 600 feet each, and one of 

 404.55, making in the whole 11804,55 feet, between the 

 twenty-lixth fration at the Staines Road and the center of the 

 pipe near King's Arbour, the mean temperature being 62"|, 

 Here it is to be obferved, that this laft feftion was only mea- 

 fured once with the chain, the tracl not being yet fafficiently 

 cleared to admit of its being done to the beft advantage ; and., 

 when completed, it was judged to be better to proceed diredWy 

 in the operation with the rods, than to lofe time in the ufual 

 repetition, (ince the merits of the chain, in this way of ap- 

 plying it, were already fufficiently well eftabiiihed ; and any 

 future tefts 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 ftate-, was 

 tifcertained 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 100 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 iikewife the mean heat e^ the air, whea 

 applied upon the Heath, the chaiii> according to the experi- 

 ments on the expanfion of the very fame freel, would exceed 

 the lod feet by 0.161 inch, or 0.0134 foot. Hence the 

 fum of the three fedions of the bafe, 274 chains, being mul- 

 tiplied by 0.0134 foot, wefliall have 3.67 feet forthe equation 

 of the chain 4-4.55 ^^^^' ^^ 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 

 HJf the bafe, as given by the rough meafurement with the 



Vol. LXXV, K k k chain. 



