14 Improvements in Physical Science [Jan. 
other two. This internal magnet he conceived to move, and this 
motion occasioned the declination. I think it possible that an 
internal magnet with four poles might occasion the variation without 
itself being subjected to motion; for we know that the two poles 
would act upon each other, and vary each other’s intensity. But if 
such be the cause of the annual variation, a time will come when 
it will cease; though this time may be far distant. 
Dr. Halley conceived the position of the principal north pole to 
be not far from Baffin’s Bay. Mr. Churchman placed it in north 
latitude 58°, and longitude 134° west from Greenwich ; but the 
following observations by Captain Brown, for which the world is 
indebted to Colonel Beaufoy, who furnished the compass by means 
of which they were made, shows that this position is not accurate. 
{Annals of Philosophy, vol. v. p. 368) 
Variation. Latitude. Longitude, 
79° 42W | 72° 46°N | —9? ~ 7 
79 00 W e972 «SN foe 
BS STB Ve a7 261N ia 2 
738 44 W —_- — liga om 3 
74 00 W 70 58N 54°14 W : 
73 40 W 70. 55 Nol | eon zg 
F200; Worse k AO ie DN fncip—a oes : 
71. 00 W 66 59 N 57 4W a 
70 40 W 65 44N | 59 31 W || 
70 00 W 63 40 N 59 22 W 
68 00 W 3 34N | 58 33 W [J 
Were the position of the principal north magnetical poles of the 
earth so far south as latitude 58°, the needle in the preceding table 
_ would have always pointed to the south of west ; whereas its direc- 
tion is northerly, even in latitude 72° 46’. This might perhaps be 
accounted for, indeed, by supposing another magnetic north pole 
to exert some influence on the needle; but Churchman, who only 
admitted the existence of one magnetic north pole, could not avail 
himself of such a supposition. 
4. But the most important set of magnetical observations are 
those of Colonel Beaufoy to determine the variation of the needle. 
The diurnal variation of the needle was discovered by Mr. George 
Graham ; and afterwards a set of experiments was made upon it by 
Mr. Canton, and by M. Van Swinden, in order to determine its 
yate at different seasons of the year. ‘The result of these was, 
that the diurnal variation is greatest in summer and least in winter ; 
and that it increases from eight in the morning till two, when it 
gradually returns to its original position. 
Colonel Beaufoy’s observations were made with a much better 
instrament than had been formerly employed, and they were con- 
