[ 385 3 
XX. Observations on the Variation , and on the Dip of the 
magnetic Needle , made at the Apartments of the Royal Society , 
between the Tears 178 6 and 1805 inclusive . By Mr. George 
Gilpin. Communicated by Henry Cavendish, Esq. F. R. S. 
Read June 19, 1806. 
Of the Variation Compass. 
The variation compass used in making the following obser- 
vations is the same instrument used in former observations of 
the variation, and published by the Society in several volumes 
of their Transactions: and as a particular and accurate descrip- 
tion of its construction was given' by Henry Cavendish, Esq. 
F.R.S. in the LX Vlth volume, it will not be necessary to say 
any thing here on that subjects But these observations being 
the first that have been communicated since the compass was 
put up in the Society’s apartments in Somerset Place, it may 
not be amiss to point out its situation in the house at the time 
of observation, and the method pursued to attain such allow- 
ances as were proper to be made in deducing the results here 
given. 
1. The compass in the house, at the time of observation, 
was placed in the middle window, on the south side of the 
Society’s meeting-room, upon a strong mahogany board i|- 
inch thick. Against the opposite building the dial-plate of a 
watch is fixed, making an angle with the true meridian of 
31 0 8',8 to the eastward, as a mark to which the telescope of 
3 D 2 
