/ 
460 Major- Genera /Roy’s Account of the 
mkted, which is the leak of all probable, it could only have 
happened, at the place of the tripod, by bringing a wrong 
point of the idem over it when the operation was relumed. 
But it is well known, how much care and pains were taken to 
prevent any thing of that fort. Indeed the hypothenu'fal diff 
tances, as given by the chain, agreed fo nearly among them- 
felves, that even a foot or ten inches would have made fo re- 
markable a difference in the lituation of the next picket as 
could not have palled unobferved. Befides, in returning with 
the glafs rods, after palling the Staines Road, the meafure- 
ment was gradually found (without any leap whatever) to 
over-fhoot the pickets, and at laid over-reached the fouth-eaft 
pipe by 17.875 inches. I am therefore inclined to believe, 
that the difference arifes partly from what may have been loft 
by conftantly butting one rod agaitift the other, ■whereby the 
end of the 1370th did not reach fo near to the north- weft pipe 
.as it ought to, and would have done, if the rods had been 
applied to each other by coincident lines. It muft, however, 
be con felled, that the near agreement between the glafs and 
deal rods in the upper part of the heath feems not perfectly 
reconcileable to this fuppolition. Neverthelefs, the defcent 
being quickeft, and the irregularities of the furface much 
more conliderable in the lower than the upper part, might 
produce fome effect in one which did not take place in the 
other. But the chief part of the difference I take to have pro- 
ceeded from over-rated expanlion.; that is to fay, the rods, 
when brought into ufe, contracted fooner than we imagined, 
and thereby gave a fhorter meafure than what was affignable 
to them from the mean of any two or more comparifons. 
The 
