Mefurement of a Bafe on Hounflow-Heath. 409 
.of the horizontal clamp, according to the ficfe on which the 
rod lies that is to be moved on into coincidence ; and the other 
-{hews it a&ual.ly in its place, with the clamp itfelf detached in 
elevation along-fide of it. Thus .from the plan it may be 
perceived, that the firft, or adjufted rod, lies towards the far- 
ther fide of the table, and is there fecured by the vertical 
clamp. The fecond, or moveable rod, lies on the hither fide, 
and therefore the horizontal clamp is placed in the farther 
groove, where it is firmly pinched by the nut underneath. The 
rod has been brought to coincidence by working with the two 
milled-headed fcrews againft the oppofite fides of the crofs foot. 
This apparatus, although perfectly good in theory, was found 
to be much too confined in its nature to anfwefi well in prac- 
tice, requiring the hands to be placed with a degree of preci- 
fion, which could not be effected in the field without great lofs 
of time ; and this was the real caufe, as will be feen hereafter, 
that the meafurement by coincidences with the deal rods was 
given up, and that by contacts adhered to. 
Towards the left-hand of tab. XVIII. the plan of one of 
the fquare tables is reprefented with the ends of the fecond and 
third rods upon it in contact. In this operation it will be 
perceived, that only one crofs-foot of each rod could now reft 
on and be clamped to the hand, the tables having been inad- 
vertently cut too fmall to admit of both; and although this 
has the appearance of imperfection, yet no inconveniency 
whatever was found to refult from it in practice, experience 
having fhewn, that the clamping of either end fufficed to keep 
the rod heady. Along-fide of the table, the vertical clamp, 
being that now folely made ufe of, is like wife reprefented in 
elevation. 
H h h 2 
On 
