lo Mr, Christie on the effects of temperature on 
projections nearest to the needle from the corresponding 
end of the magnet within the pan, was exactly 3 inches ; I 
could therefore, in any instance, determine very accurately 
the distance of the centre of the magnet from that of the 
needle. The pans were placed on the table, so that the 
indexes on the pieces of brass coincided with the meridian 
line. Water was now poured into the pans, and the tem- 
perature of the magnets was varied by varying the tempera- 
ture of the water. The temperature of each magnet was 
ascertained by a thermometer placed in the water, with its 
bulb resting on that pole of the magnet which was nearest 
to the centre of the needle. In my first observations I how- 
ever made use of only one thermometer, which was moved, 
during them, from one magnet to the other. In Plate I. 
Fig.i , an apparatus of the same nature, which I subsequently 
made use of, is represented. This differs from that employed 
in these experiments only in having the boxes containing 
the magnets made to slide on a ruler, whose axis being in 
the magnetic meridian, and the axes of the magnets adjusted 
in the boxes also in the meridian, they can be made to ap- 
proach or recede from the needle, in that line, which saves 
considerable trouble in the adjustments when observations 
are to be made at different distances. Fig. 2 and 3 repre- 
sent the plan and elevation of another apparatus which I had 
constructed for Mr. Foster, and which he has taken with 
him on the North-western Expedition, to enable him to make 
observations on the daily variation, particularly with a view 
of ascertaining the times of maximum east and west, and 
also of zero, should any of the stations at which he may find 
himself be favourable to the employment of such an ap- 
