50 
ON THE MAGNETIC STJEYET OF THE WEST OF FEANCE. 
time of the observations at Angers and Bordeaux, where the differences were greatest, 
the total change of tile values is inconsiderable. 
The following comparison will give an idea of the results : — 
From uncorrected observations. 
From corrected observations. 
X—- 
-0-00037605 
-0-00037562 
y= 
0-0013401 
0-0013398 
U — 
74° 19' 30" -4 
74° 20' 19"-5 
V — 
0-0013919 
0-0013914 
Error at Paris 
. • • 
0-0040 
0-0034 
Mean probable error 
. 
0-0067 
0-0065 
A similar correction might be applied to the Total Force and the Dip ; but since the 
simultaneous variations of the vertical force are less understood than those of the hori- 
zontal force or the declination, the attempt could scarcely be expected to lead to any 
satisfactory result in the present state of our knowledge. 
Appendix. 
The observations made with the same instruments at Loyola on Sept. 3 and 4 are not 
taken into account in the above reductions, since we wished to confine ourselves entirely 
to the West of France. It may, however, be of some use to subjoin a synopsis of the 
results obtained. 
Sept. 3rd, 3 h ll m 9 S, 26 G.M.T. Azimuth of sun 165" 35' 46"*7. Azimuth of mark A 
141° 53' 40'', read with the altazimuth. 
3 h 43 m Azimuth of magnet 229° 49' 5". Azimuth of mark A 
206" 42' 40", read with the unifilar. 
Correction for zero-point of scale of collimator magnet —15' 34"-6. 
The declination is not deduced from want of sufficiently reliable data as to latitude 
and longitude. 
Sept. 3rd, 6 h 26 m , temp. 71 0, 6 Fahr. Observed deflection 12° 7' 2". Distance between 
centres of magnets 1 ft. 
,, 4 h 54 m , „ 77 - 0 Fahr. Observed time of one vibration 4 S, 87359. 
Hence 
Moment of magnet =0 '48205 
Horizontal force ..... =4-5546 
Vertical force . . . . . . =8 -7245 
Total force =9-8419 
Sept. 4. No. 1 Needle. Observed dip 62° 26'. 
A 
