4 o 6 Major- General R o y *s Account of the 



thereby difcovered refling on the common bottom, which has 

 bands laid acrofs it for the purpofe, a few inches below what 

 has now become the'furface of the cheft. It was necef- 

 fiary that the ftandard fhould reft thus high, both that the 

 light might come freely upon it, and that, being fupported by 

 the deep fides of the cheft, it might be prevented from twift- 

 ing, for it will be remembered that it is only trufled laterally. 

 By means of a fmall brafs fpring fixed to each end of the 

 ftandard, a fine filk thread, as being lefs liable to accident than 

 filver wire, is ftretched along its ftem, which by fmall wedges 

 prepared for the purpofe, and flipped in between it and the 

 bands on which it refts, is always brought into the fame poii- 

 tion. This being done, the filk thread is turned off, fo as to 

 permit the meafuring rods to be laid on the ftandard for com- 

 parifon. With regard to the fmaller cheft, fuch a one was 

 aftually made, and lent down to the heath, towards the clofe 

 of the operation with the deal rods ; but from fome miftake in 

 its dimenfions, it would not admit the third rod. 



Stands for the Meafuring Rods. Tab. XVIII. and XIX. 



From the extraordinary levelnefs of Hounflow-Heath, the 

 afcent from the fouth-eaft towards the north-weft being little 

 more than one foot in a thoufand in the diftance of five miles, 

 it was eafiiy (Qtn, that the computed bafe-line, or that actually 

 forming a curve parallel to the furface of the fea, at that height 

 above it, would fall fo little fhort of the hypothenufal diftance, 

 meafured on, or parallel to, the furface of the Heath, as 

 Icarcely to deferve notice, had it not been thought neceflliry to 

 3 fliew 



