SUEYET OF THE WEST OE FRANCE. 
49 
rections could be accurately determined, and any correction founded on the supposition 
of the simultaneous similar action of the disturbing forces in England and in France 
might appear somewhat arbitrary. That a correction, however, might be applied with 
advantage is rendered more than probable by the results of the comparison of the Kew 
and Lisbon magnetograms, and by the great similarity between the daily range in 
England and in Italy which I remarked in some of the Florence declination-curves sent 
by Signor Donate It would seem, from the comparisons made by Dr. Stewart and 
Senhor Capello, that the declination and horizontal-force disturbances at Kew and 
Lisbon are simultaneous, in the same direction, and in the proportion of LG to 1 for the 
declination, and 1-8 to 1 for the horizontal force ; whilst, on the other hand, Dr. Stewart 
remarks that there is “ very little likeness between the vertical-force curves.” 
We will therefore assume that the perturbations of the declination and horizontal 
force are simultaneous in England and in France, and in the proportion of 1*3 to 1 and 
1'4 to 1 respectively, and we will take the corrections from the magnetograms obtained 
at Stonyhurst Observatory during the survey, since the Kew and Stonyhurst curves may 
be considered as almost identical. 
From hourly measurements of the undisturbed portions of our declination-curves from 
Dec. 16th, 1868 to Jan. 16th, 1869, we obtain 2T47 as the mean reading of the ordinate 
for Jan. 1st, 1869; and taking the difference between this and the ordinate at the time 
of each observation, we obtain a number which, when divided by Do and multiplied 
into 28' 38" - 875, the coefficient of the declination magnetograms, gives us the correction 
in arc for reducing the French observations to their mean value on Jan. 1st, 1869. No 
correction for torsion of thread has been found necessary. 
The results obtained from the corrected observations compared with the values found 
above, will best show what is gained by this correction. 
From uncorrected observations. 
x= 0-010562 
y = — 0-003953 
w=— 20° 31' 16" 
r— 0-011278 
Error of Paris — 2'-28 
Probable error of any observation 6'*95 
From corrected observations. 
0-010385 
-0-003833 
-20° 15' 35" 
0-01107 
P-26 
7-15. 
These figures of themselves would scarcely justify us in giving much weight to this 
correction. 
4 he horizontal-force magnetogram seems to offer a surer means of improving our 
results. The standard ordinate has been obtained from hourly measurements of the 
curves on undisturbed days in Aug. and Sept. 1868, the gradual loss of magnetism of 
the suspended magnet being taken into account. The coefficient of the curve is 0-02689. 
In this case the amount of difference between the mean observed and the computed 
values at each station is generally diminished by the correction ; but, owing to the small- 
ness of these differences and to the trace of the magnetograph having been lost at the 
MDCCCLXX. H 
