352 
Mr. Christie’s theory of the 
whatever point of the equator heat was applied, bears imme- 
diately on the subject of the daily variation, and besides, 
coincides perfectly with the results obtained with the plate 
of copper and bismuth, I shall state it. 
The sphere was adjusted on an axis, of which that part in 
contact with the copper was of wood, and this axis being 
adapted to the instrument to which I have already referred, 
could be inclined to the horizon at any angle, and could like- 
wise be placed in any azimuth. When the axis was in the 
plane of the magnetic meridian, if the centre of the needle 
was placed vertically over the centre of the sphere, its posi- 
tion would correspond to that of a needle on the surface of 
the earth at a place having the latitude equal to the elevation 
of the pole of the sphere, and where the mean variation 
vanished. The angle of elevation of the pole of the sphere 
was 50°, as most convenient for making observations at a 
distance from the equator. A lamp being applied so that its 
flame was as nearly as possible on a point in the equator, the 
sphere was turned on its axis until the equatorial regions 
became nearly uniformly heated ; and the lamp was allowed 
to remain for 5 minutes after the sphere was brought to rest; 
so that the point during this time over it became the place of 
greatest heat, and the poles of rotation the points of greatest 
cold. Now, whatever point in the equator was made the place 
of heat, I invariably found, that when the elevated pole was 
towards the north, the deviation of the north end of the 
needle was towards the west, when the place of heat was on 
the meridian above the horizon, that is to the south ; and 
towards the east, when the place of heat was on the meridian 
below the horizon. This is precisely the character of the 
