( ?<5 ) 
IV, An Account of ObJerVations made of the Vari- 
ation of the Horizontal Needle at London, in 
the latter Tart of the Year \ 722, and beginning 
of 1723. By Mr. George Graham, Watch- 
maker , F S. 
T H E Figure of the three Needles, with which the 
Experiments were made, was prifmatick ; their 
Lengths were nearly iz,z Inches; their Ends, which 
pointed to the Divifions, being filed to an Edge, 
which made a fine Line perpendicular to the Horizon. 
The Caps of two were of Chryftal, the other of 
Glafs ; they were well pohfhed on the Infide, in that 
Part which touched the Pin they moved upon. The 
Box was Brafs, and of a Breadth fufficient to admit of 
zo° on each Side the middle Line, and covered with 
a piece of ground Glafs. The circular Arches at the 
Ends were raifed fo much above the Bottom of the 
Box, as to have their upper Surfaces, upon which the 
Divifions were cut, lie in the fame Plane with the 
Needle, and at fuch a Diftance from each other, that 
the Needle might play freely between them. A few 
of the Degrees at the North End were divided into 
fix equal Parts, each Divifion being ic/. It was eafy, 
by the help of a Convex Glafs, to determine the 
pointing of the Needle to lefs than a Quarter of thefe 
Divifions, or to about 2 7 of a Degree. The Pin, upon 
which the Needle moved, was of Steel hardned, and 
ground to a fine Point ; and by a Spring placed in the 
Box, the Needle might be railed from off the Point, 
and let down again at Pleafure, without removing the 
1 Glals, 
