460 Major-General Roy*s Account of the 



mitted, whicli is the leaft of all probable, it could only ha\^ 

 happened at the place of the tripod, by bringing a wrong" 

 point of the fceiTi over it when the operation was refumed. 

 But it i^ well known, how much care and pains were taken to 

 prevent any thing of that fort. Indeed the hypothenufal dif* 

 tances, as given by the chain, agreed fo nearly among them- 

 j elves, that even a foot or ten inches would have made fo re- 

 markable a difference in the fituation of the next picket as 

 could not have paffed unobferved. Befides, in returning with 

 the glafs rods, after paffing the Staines Road, the meafure- 

 ment was gradually found (without any leap whatever) to 

 over-fhoot the pickets, and at laft over-reached the fouth-eafl 

 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 againfl 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 confefted, 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 confiderable in the lower than the upper part, might 

 produce fome effe6l 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 expanfion ; that is to fay, the rods, 

 when brought into ufe, contracfled fooner than ave imagined, 

 and thereby gave a fhorier meafure than what was affignabk 

 to them from the mean of any two or moreromparifons. 



The. 



