III. 
On the Determination of the Horizontal Component of the 
Earth’s Magnetic Force. 
BY L. E. AKINS. 
The method I have used in the determination of H is the one 
used at the University of Glasgow. The attractive feature of the 
method is the simplicity with which the ratio of m to H, and the pro- 
duct of mH are determined, where m is the magnetic moment of a 
given bar magnet. The apparatus used is a small Bottomley magnet- 
ometer and a deflecting bar magnet. 
The cut, which is exactly one half full size, fully illustrates the 
magnetometer, except that the needle needs a little explanation. 
The one I used consists of two bar magnets about eight-tenths centi- 
metres long and one-tenth centimetre wide, and as thin as the steel 
can be conveniently worked, d'he magnets are fastened with shellac, 
like poles together, to the back of a small mirror and suspended by a 
single silk fiber as in the cut. I silver my own mirrors, using the short 
process given in Roscoe’s Chemistry, Vol. II. With care in cleaning 
and by keeping the glass quite warm while the silver is being precipi- 
tated, a very good mirror can be obtained. The mirrors I used were 
small microscope covers five-tenths centimetres in diameter. 
The first experiment is to find an expression for ~ which is 
accomplished as follows. The magnetometer is set up so that its nee- 
dle is in the magnetic meridian of the place and its centre directly 
over the point where a line, exactly at right angles to the N. and S. 
line, cuts that line. A very convenient arrangement for this consists 
of two boards, each eight inches wide, one five feet long, the other 
