VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS. 55 



will Stand higher, and sometimes the other, which causes a difficulty in observ- 

 ing. Another inconvenience also attends it when the indices of the needle are 

 on a level with the point of the pin, which is of more consequence ; namely, 

 that it causes the two indices not to agree, and consequently makes a sensible 

 error, when only one index is made use of, at least in nice observations : but 

 when the lines, serving by way of index, are drawn on the needle itself, and 

 therefore are nearly on a level with its centre of gravity, it can cause very little 

 error. The agate cap, which was first made for this instrument, was of this 

 kind ; and was so fault)', that if no better could have been procured, it would 

 have been necessary either to have drawn the lines serving by way of index on 

 the needle itself, or to have observed by both ends, either of which would have 

 been attended with a considerable increase of trouble to the observer ; but Mr. 

 Nairne, the artist who made the instrument, has since ground some himself, 

 which are perfectly free from this fault, the concave surface being of an ex- 

 tremely regular shape and well polished, and also of a very small radius of 

 curvature ; which is a matter of considerable consequence, as otherwise the 

 point of the pin will not easily slip sufficiently near to the bottom of the 

 hollow. 



Care was taken to place the variation compass in a part of the house 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 seemed however to be not entirely out of the reach of 

 the influence of that metal, I took the following method to examine how much 

 it was influenced by it. The instrument was removed into a large garden 

 belonging to a house in Marlborough-street, distant from the Society's house 

 about one mile and a quarter towards the west, where there seemed no danger of 

 its being affected by any iron-work. Here it was placed exactly in the meridian, 

 and compared for a few days with a very exact compass, placed in an adjoining 

 room, and kept fixed constantly in the same situation. It was tlien removed 

 back to the Society's house, and compared again with the same compass. By a 

 mean of the observations, the variation shown by the compass in the room, is 

 l'.3 greater than by the Society's instrument in the garden, and 14'. 1 less than 

 by the same instrument placed in its proper situation ; so that the variation ap- 

 pears to be 13'.4 greater in that part of the Society's house where the compass 

 is placed, than in the abovementioned garden ; and therefore, as tiiere is no 

 likelihood of its being aftected by any iron in the latter place, the needle seems 

 to be drawn aside 1 5^' towards the n. w. by the iron work of the house and 

 adjacent buildings. 



On comparing the observations of the last two years together, the variation 

 appears, after allowing for the error of the instrument, to have been 27' greater 

 in 1775 than in 1774 ; though I have been informed by Dr. Heberden, who has 



