AND INSTRUMENTS AT MUNICH. 519 
angles of deflection with precision. If, still adhering to M. 
Gauss’s method, large angles are taken, other circumstances 
equally disadvantageous are originated. 
I have succeeded in finding a simple method by which all the 
previous difficulties appear to me to be avoided. I will endea- 
your, in a brief description, to make the material points relating 
to it intelligible to those who have occupied themselves with 
magnetic determinations. I reserve for a future opportunity an 
exact development both of the theory and of the practical 
method, with reference to some circumstances which are here 
passed over for the sake of brevity *. 
On the circle a! a, d!.b, which is fixed, is a moveable brass piece 
AB, having two verniers, aand 0; a frame containing the freely 
suspended magnet n s is placed on and attached to the middle 
of the brass piece; the magnet bears a mirror parallel to its 
magnetic axis, so that when the magnet is in the position ms in 
the magnetic meridian, the optical axis of the telescope A! is 
perpendicular to the plane of the mirror, and hence the image 
of the wire in the eye-piece coincides with the wire itself. Ifa 
small magnet x" s!' is placed on the brass support, it exercises a 
certain moment in turning the suspended magnet, and the brass 
support must be moved into the position A B, when the optical 
* This memoir has been since published, entitled Bestimmung der Horizontal 
Intensitaet des Erdmagnetismus nach absolutem Maase. The dimensions of the 
defiection-bar, recommended by Dr. Lamont in this memoir, are 36™™ long, 
6mm broad, and 1™m thick; or in British inches 1-42 long, 0°24 broad, and 0-04 
thick, nearly. The moveable brass piece on which the deflection-bar is laid 
should have a length of half a metre, or 19-7 British inches. 
. 
