SUE YET OF THE WEST OF FRANCE. 
39 
The Magnetic Intensity. 
The method invariably adopted for determining the horizontal component of the 
earth’s magnetic force was that of vibrations and deflections. 
The horizontal, vertical, and total forces are calculated to English measure, one foot, 
one second of mean solar time, and one grain being assumed as the units of space, of time, 
and of mass. 
The vertical and total forces are obtained from the absolute measure of the horizontal 
force and the dip. 
The observed times of vibration are entered in the Table without correction. 
The time of one vibration has been obtained from the mean of twelve determinations 
of the time of 100 or of 200 vibrations. 
In deducing from the observed vibrations and deflections the product and ratio of the 
magnetic moment of the magnet and the earth’s horizontal magnetic intensity, the 
induction and temperature corrections have always been applied, and the observed time 
of vibration has been corrected for the effect of torsion of the suspending thread. 
The induction coefficient g> is 0-000244. 
The temperature corrections have always been obtained from the formula 
q{t 0 — 35°) -hy f (# 0 — 35°) 2 , 
where t a is the observed temperature, and 35° Fahr. the adopted standard temperature. 
The values of the coefficients q and q' are respectively 0-0001128 and 0-000000436. 
The correction for error of graduation of the deflection bar at 1 foot is + 0-00004 ft., 
and at 1*3 foot +0-000064 ft. 
It has been found necessary to apply the correction ( l — for the rate of the 
chronometer at two stations only, i. e. at Laval and at Bordeaux, where the rate was 
respectively +2 S, 68 and +2 S T8 ; at the other stations it was always less than 2 s . 
In the calculations of the ratio the third and subsequent terms of the series 
P Q, 
l+p+^ 4 +--- have always been omitted. 
The value of the constant P was found to be —0-002797. 
The angular measure of one division of the scale in the vibration-apparatus was found 
to be =2'-26. 
The value of v'Tv at 90° is 1-71636; this was deduced by Mr. Welsh of Kew, from 
observations made with three inertia cylinders. 
No correction has been applied for semiarcs of vibration, which were always small. 
