[ 476 ] 
XXXV. April 21, 1772. Experiments on 
two ■ Dipping-Needles , Dipping- 
Needles were made agreeable to a Plan of 
the Reverend Mr. Mitchell, F. R. S. 
ReElor ofl. hornhill in Yorkshire, and exe- 
cuted for the Board of Longitude , by Mr . 
-Edward Nairne, o/'Cornhill, London. 
Read July 9, f a '^HE magnetic needles were twelve 
I// ~' JL inches long, and their axes (the 
ends of which were of gold allayed with copper) 
reded on fridtion-wheels of four inches diameter, 
each end on two fridtion-wheels, which wheels were 
balanced with great care. The ends of the axes of 
the fridtion-wheels were likewife of gold allayed 
with copper, and moved in fmall holes made in 
bell-metal ; and oppofite the ends of the axes of the 
needles, and the fri&ion-wheels, were flat agates, 
finely polifhed. Each magnetic needle vibrated in 
a circle of bell- metal, divided into degrees and half- 
degrees, and a line paffing through the middle of 
the needle to the ends pointed to the diviflons. The 
minutes fet down in the experiments were, by efti- 
maticn, as the third of half a degree is counted ten 
minutes. The inflruments were carefully placed, fo 
that the needles vibrated exadfly in the magnetic 
meridian. 
