352 
Mr. Christie on the magnetism of 
experiments which I had previously made, and in those which 
I proposed making with this apparatus, I conceived a sphere 
to be described about the centre of the needle, referring 
the situation of the iron to a plane, in which, according 
to the hypothesis I had adopted, it should equally affect the 
north and south ends of the needle. The line in which the 
needle would place itself, if freely suspended by its centre of 
gravity, I considered as the magnetic axis ; the points where 
this axis cuts the sphere, the poles, the upper being the 
south, and the lower the north pole ; and the great circle at 
right angles to the axis, the equator, being the plane above 
mentioned. The position of the iron was thus determined 
by its latitude and longitude ; the longitude being always 
measured from the eastern intersection of the equator with 
the horizon. The angle which the axis makes with the 
horizon I considered to be, according to the most accurate 
observations, very nearly 70° 30'.* 
As I shall have frequently to refer to different adjustments 
* In 1818 Captains Kater and Sabine found the dip to be 70^34 in the 
Regent’s Park; and in 1819, in the same place. Captain Sabine found it to be 
70° 3 3'. 27. Since making the greater part of these experiments, I have had oppor- 
tunities of observing the dip at this place. With a very good instrument, by 
T. Jokes of Charing Cross, having a 7-inch needle, consisting of two circular arcs, 
on Captain Kater’s construction, the mean of 40 observations, 10 with the face of 
the instrument east, 10 with the face west, and the same with the poles reversed, 
gave the dip 70° 15.25 on the 23d December, 1821, between the hours of i and 
4 P. M. the observations being made in my garden. With another instrument, also 
by T. Jones, having an 8-inch rectangular needle, the mean of 40 observations 
made in my garden, (about a mile from the former place of observation) near noon 
on the 5th and 6th May, 1824, gave 70^06.5 for the dip. With the same instru- 
meot, but using a needle on Meyer’s construction, the mean of 40 observations 
near noon on the 8th May, 1824, gave the dip on the same spot 70° io'.5. 
