of the Magnetic Needle at Fort Marlborough. 343 
heat acting on magnets in the earth, as its efficient cause. This 
was first observed in Europe in 1756, by Mr. Canton.; and the 
results of the foregoing observation being diametrically oppo- 
site to his, with similar effects, afford not a small confirmation 
of the essential part of Halley's theory. The sciences of 
electricity and magnetism (in my opinion intimately con- 
nected) are at present in their infancy. We observe effects 
which we cannot trace up to any satisfactory cause. If ob- 
servations similar to these, and to those made by Mr. Canton, 
could be made in equal latitudes and longitudes, and in cor- 
responding situations in the torrid zone, by collating the re- 
sults, I think a rationale of this wonderful pheenomenon might 
be obtained. 
From the greatness of the angle of dip of the needle, I am 
led to suppose that the magnetic poles are fixed within the 
magnetic nucleus far within the earth's surface, and that some 
of these poles are more powerful in their action than others, 
from the variation observed in various places of the globe. I 
shall be happy if these observations (I mean the figured ones) 
will enable you to elucidate this subject. 
I remain, with much esteem, &c. 
JOHN MACDONALD. 
24th of June. A meridian was drawn by means of the fol- 
lowing apparatus. A plate of brass was cast, and reduced to 
a level surface in a lathe, in which concentric circles were ac- 
curately cut on it. A short hollow cylinder was screwed on 
the centre. This tube received a brass cylinder, pointed at the 
upper end. An iron-wood post was fixed in the ground. The 
brass plate was fixed level on the post by means of a gunner's 
mdccxcvi. Y y 
