514 DR. LAMONT ON THE MAGNETIC OBSERVATORY 
2. The differential apparatus for horizontal intensity differs — 
very much from the modes of construction hitherto employed. 
Experiments have convinced me that a magnet can never be — 
placed in a position perpendicular to the meridian, either by fixed 
magnets, or by the force of torsion, (whether by a steel spring 
or by the bifilar suspension,) without the necessity of a careful 
check being kept by frequent examination; and that even with 
great care some uncertainty will always remain. 
For this reason I have preferred a constant deflection made 
in the following manner :— 
S 
ns is amagnet freely suspended by a silk fibre about 3 inches 
in length, bearing in the middle a mirror, and inclosed in a 
suitable case; x! s! and 2s!’ are two deflecting magnets attached 
to a brass bar; the magnets are of equal strength and equi- 
distant from the suspended magnet, and in the same horizontal 
plane. The deflectors are perpendicular to the suspended mag- 
net ns, and cause its direction to keep the angle $ with the 
magnetic meridian NS. If » be the moment wherewith the 
deflectors strive to turn the suspended magnet, we have 
p = Xsin 9, 
where X denotes the horizontal intensity of the earth’s mag- 
netic force. Let the change of the horizontal intensity be © 
A X, the change of the declination (i. e. of the line N 8) be A8, 
the change of the suspended magnet n s, Ai (so that the change 
of the angle ¢ = Ai — Ad), the change of yu for 1° increase of 
temperature = — a and the temperature = ¢, we have 
Np Oe ee ee (ae) 
x = (Ai — A8) cot. > di be 
If we make ¢ =— 45°, then 
AX , d 
a = — (Ai Ad) (2*) t. 
In order to obtain the change of intensity simultaneous (or 
nearly simultaneous) readings of the two differential instruments 
