fp^f Variation of the compass. 



VI. 



Observations on the Variation, a?id on the Dip of the Mcignetit 

 Needle, made at the apartments of the Boi/al Society, between: 

 the Years 17S6 and. 1805 inclusive. By Mr. Georgs 

 GlLFIN,* 



Of the Variation Compass, 



Variation com- JL HE variation compass, used in making the following obser- 



pass of the vations, is the same instrument used in former observations of 

 Ro} alSociety. ' 



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 LXVIth volume, it will not be necessary to say 

 any thing here on the subject. But these observations being 

 the first that have been communicated since the compass was 

 pat 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 her* 

 given. 

 JtiS situation, 1. The Compass in the house, at the time of observation, 



and the posi- ^5^g placed in the middle window, on the south side of the 

 Cibn ot the azi- -,.^, ,. , , ij.r 



muth mark dc- ^<^^"'6ty s meetmg-room, upon a strong mahogany hoard I^ 



scribed. inch thick. Against the opposite building the dial plate of a 



watch is fixed, making an angle with the true meridian of 

 3P 8',S to the eastward, as a mark to which the telescope of 

 the compass was adjusted. To obtain the angle that this 

 mark made with the true meridian, I fixed a transit-instru- 

 ment on the mahogany board above mentioned, precisely in 

 the same place where the compass had been placed, and hav- 

 ing adjusted iis telescope to the same mark, the transits of the 

 sun and stars over a vertical circle passing through the zenith 

 •nd this mark, were observed; and the angle contained be- 

 tween 

 * Philosophical Transacti^ons, 1*06, 



