[ 54*5 ] 
which was moveable, defigned to take the Lengths 
of fhch Standards as confift of hollow Beds or Ma- 
trices, like thofe already fpoken of at the Exchequer, 
and the others, to be prefently mentioned, at Guild - 
hall : And a Third Beam-Compafs was fitted in the 
common way, with fine Points* for the taking off, or 
laying down, fuch Meafures as are marked out by the 
Diftance of Points or Lines, on any plane flat Super- 
ficies. All which Compafles were feverally fo con- 
trived, as to be lengthened by the turning of a fine 
Screw, one of whofe Revolutions anfwered accu- 
rately to the 40th Part of an Inch , and to which there 
was applied an Index, fhewing, on a fmall circular 
Plate with 20 Divifions, the broken Part of a Revo- 
lution ; and whereon the Place of the Index might, by 
the Eye, be eftimated to about the 10th Part of a 
Divifion ; whereby the Motion of the moveable 
Cheek, End, or Point, might confequently be judged 
of, to about the 8000th Part of an Inch. 
But Mr. Graham , when he determined by thefe 
Inftruments the following Particulars, defired it 
might be obferved, that although the Alterations of 
the Compafles were fenfible to fo fmall a Quantity ; 
it was not to be fuppofed the Meafures here taken 
with them, could be eftimated to the fame Exa&nefs. 
The Hand cannot judge with fo much Nicety, of the 
Shake of a Rod, when applied between the Cheeks, 
or when let into one of the hollow Beds or Matrices 
above-mentioned : Neither can the Eye, though 
aflifted with a Magnifying-glafs, pretend to fee, with 
that Accuracy, the Place of the Compafs-points, 
when applied to the taking off a Meafure, fet out by 
Points or Lines, on the plane Surface of a Rod or 
Rule. 
