4 o 6 , Major-General Roy’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- 
fary that the ftandard fhould reft thus high, both that the 
light might come freely upon it, and that, being fupported by 
the deep hides of the cheft, it might be prevented from twill- 
ing, for it will be remembered that it is only truffed 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 pofi- 
tion. This being done, the filk thread is turned oft', 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 
adtually made, and fent 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 eafily feen, that the computed bale-line, or that aflually 
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 lurface of the Heath, as 
icarcely to deferve notice, had it not been thought neceflary to. 
3 Ihew 
i 
