56 PHILOSOPHICAL TRANSACTIONS. [aNNO 1776. 



made observations of this kind for several years past, that the annual alteration 

 of the variation has, in general, been not more than lO'; and in particular, 

 that the alteration in the last year appears to be only llV; so that the great 

 difference observed at the Society's house seems to be owing, not solely to the 

 real alteration in the variation, but partly to some other cause ; though what that 

 should be I cannot conceive, unless some change was made in the iron work 

 either of this or the adjoining houses between the two periods ; but I do not find 

 that any such change has been made. During the last year indeed there have 

 been two large magnets in the house, each consisting of several great bars joined 

 together, being what the late Dr. Knight used for making artificial magnets, and 

 at the time of the observations in 1774 there was only one ; but their distance 

 from the compass is above 30 feet : and I am well assured, that in the situation 

 in which they are actually placed, they cannot draw the needle aside more than 

 3', and not more than 15', when the line joining their poles is placed in such a 

 direction as to act with most force.* The single magnet in the year 1774 was 

 placed nearly in the same situation and direction that the two were in 1775, so 

 that the difference of their effect in these 2 years can hardly have been so much 

 as 3' ; and therefore the great apparent alteration of the variation, between the 

 two periods, cannot have been owing to them. Neither can it have been owino- 

 to the fault of the agate cap used in the year 1774, as the error proceeding from 

 thence could hardly be more than 2 or 3'. It is intended that, for the future, 

 the abovementioned magnets shall be kept always in the same situation and 

 direction that they are in at present, and in which they were in 1775. 



Of the Dipping Needle. — In this instrument the ends of the axis of the 

 needle roll on horizontal agate planes, a contrivance being a[)plied, by which 

 the needle is at pleasure lilted off from the planes, and let down on them again, 

 in such manner as to be supported always by the same points of the axis resting 

 on the same parts of the agate planes ; and the motion with which it is let down 

 is very gradual and without shake. The general form of the instrument, the size 

 and shape of the needle, and the cross used for balancing it, are the same as in 

 the dipping needle described in Phil. Trans., vol. 6'2, p. 476. It is also made by 

 the same artist, Mr. Nairne. It may be seen in the Meteorological Journal, that 

 the dip was observed first with the front of the instrument to the west, and then 

 to the east ; after which the poles of the needle were reversed, and the dip ob- 



* The principle by which this was determined is, tliat it" a magnet is placed near a \ariation com- 

 pass, witli its poles equi-distant from it, and situated so tliat each shall act equally oblique to tlie 

 length of tlie needle, it can have no tendency to alter the variation; and that tlie situation in which 

 it alters it most, except when placed nearly north or south of tlie compass, is when the line joining 

 its poles points almost directly towards the needle. This experiment I tried purposely on tlie occa- 

 sion, and found it answer ; but I believe any one skilled in magnetism would ha\e granted the truth 

 of the position without that precaution. — Orig. 



