of intensity in the dipping and horizontal needles , &c. 1 79 
magnetism, it ought to be constant in all parts of the world, 
which is contrary to observation. 
In making these experiments, a dipping apparatus by 
Dollond, belonging to the Board of Longitude, was used. 
This instrument had a needle n|- inches in length, of an ob- 
long shape, and rounded at its extremities ; it was placed in 
the magnetic meridian, on a pedestal built of stones, and thus 
afforded the means for ascertaining the variations of intensity 
in the earth’s magnetism, as indicated by the vibrations of the 
dipping needle. But as I had not a suitable apparatus for 
ascertaining the variations in horizontal intensity with the 
same needle; a cubical box 12 inches high was prepared, for 
which I was indebted to the kindness of Captain Hoppner. 
This box had glass ends, to admit of the vibrations of the 
needle being: observed, and contained at the bottom a hori- 
zontal circle, divided to every 5 degrees, for the purpose of 
measuring the arc of vibration ; it was likewise fitted with a 
contrivance, by which the needle could be made to vibrate in 
any arc at pleasure, and the top was so constructed as to 
allow the suspension of the needle, to be placed directly over 
the centre of the circle. The suspension consisted of a few 
fibres of floss silk, attached to one of the extremities of the 
axis of the needle, just sufficient to sustain its weight, and 
several inches in length, to lessen the effects of torsion. 
This box was also mounted on a pedestal, similar to the one 
on which the dipping apparatus stood, and both were pro- 
tected from the weather by being placed in a house built of 
snow. For observing the horizontal vibrations of this needle, 
a small telescope, having a vertical wire fixed in the focus 
of the eye-piece, was placed on a stand firmly frozen to the 
