a 
Further development of a method of observing the Dip and the Magnetic Intensity at 
the same time, and with the same Instrument. By the Rev. Humpurey Lioyp, 
M.A., F.R.S., M.R.LA. Fellow of Trinity College, and Professor of Natural 
and Experimental Philosophy in the University of Dublin. 
Read December 28, 1835. 
Ow a former occasion I had the honour of submitting to the Academy a new me- 
thod of observing, as applied to Terrestrial Magnetism, in which the dip and the 
intensity of the magnetic force were determined with the same instrument, and by one 
observation. As this method has fully realized the expectations which I ventured 
at that time to entertain respecting it, I feel it my duty to enter somewhat more mi- 
nutely into its details, and to explain the modifications which experience has led me 
to adopt in the practice of it. The ordinary dipping needle is supported on an axle 
which is supposed to pass precisely through its centre of gravity ; and, consequently, 
the position which it assumes, when placed in the magnetic meridian, is the direction 
of the magnetic force. But if one of the arms of the needle be loaded with a weight, 
the needle will no longer rest in the line of the dip, but will assume a new position of 
equilibrium under the combined influence of magnetism and gravity ;—the inclination 
of the needle to the horizon being connected with the dip, the magnetic force, and the 
moment of the added weight, by a very simple relation. This is the simple principle 
of the method which has been already laid before the Academy. In order to apply 
it, let us supose two small weights to be attached in succession to the southern arm of 
the needle, at fixed distances from its centre ; and let the statical moments of these 
weights be » and vy, and the corresponding inclinations of the needle to the horizon 
~ and @; then, it has been shown* that 
uw cos Z=go sin (6—Z), (1) 
v cos 0=¢o sin (8—8) ; (2) 
in which 6 denotes the dip, ¢ the earth’s magnetic force, and « a constant depending 
on the distribution of magnetism in the needle itself. If therefore the angles Z and @ 
* Page 161. 
