the Royal Society’s Injiruments. 391 
drawn the lines ferving by way ot index on the needle 
itfelf, or to have obferved by both ends, either of which 
would have been attended with, a confiderable increafe 
of trouble to the obferver; but Mr. Nairn e, the artift 
who made the inftrument, has ftnce ground i'ome him- 
felf, which are perfectly free from this fault, the concave 
furface being of an extremely regular lliape and well 
polifhed, and alfo of a very fmall radius of curvature; 
which is a matter of confiderable confequence, as other- 
wife the point of the pin will not eafily flip fufhciently 
near to the bottom of the hollow. 
Care was taken to place the variation compafs in a 
part of the houfe where it is as little likely to be affeCted 
by the attraction of the iron work as in any that could 
be found. As it leemed, however, to be not intirely out 
of the reach of the influence of that metal, I -took the 
following method to examine how much it was influ- 
enced thereby. The inftrument was removed into a 
large garden belonging to a houfe in Marlborough Stieet, 
diftant from the Society’s Houfe about one mile and a 
quarter towards the Weft, where there feemed no danger 
of its being affeCted by any iron-work. Here it was 
placed exadtly in the meridian, and compared for a few 
days with a very exadl compafs, placed in an adjoining 
room, and kept fixed conftantlv in the fame iituation. 
It was then removed back to the Society’s Houfe, and 
compared again with the fame compafs. The obferva- 
tions were as follow : 
Obfervations 
