SURVEY OF THE WEST OF FRANCE. 
43 
The Magnetic Declination. 
The method adopted in the determination of this element was the following. 
The azimuth of a fixed horizontal mark, situated at a considerable distance, was first 
read with Cooke’s transit theodolite, and then a transit of both limbs of the sun taken 
with the same instrument, the time being noted by Frodsham’s chronometer. The azi- 
muth circle having been read, the theodolite was removed, and Jones’s unifilar placed on 
the same tripod-stand. The reading of the fixed mark and of the collimator magnet 
completed the observation. The torsion of the silk thread was removed entirely, as far 
as that Avas possible, before each observation. 
Brest was the only station at which the above method was departed from, and there, 
on account of the confined space in which the observations were taken, a distant mark 
could not be sighted. It was therefore considered that more accurate results would be 
obtained by observing the sun’s transit by reflection from the mirror of the unifilar, 
according to Dr. Lloyd’s method. The chief reason for not using this method at other 
stations was that the line of collimation of the telescope was not perpendicular to the 
axis of the mirror, and the motion in azimuth of the axis was not sufficient to correct 
this error. 
The correction to be applied to the azimuth reading of the sun on account of 
a • o ^ 
2m sirr- 
this position of the mirror is given by the formula x = — ■ where the constant 
1 * J cos a 
cos a! cos 
2 in — — , , i a , a', a" being the altitudes of the sun at the times of obser- 
. a' -f -a .a — u 1 1 ° 
sin — sin — — 
& 
vation. 
The formula cos a— 
sin h cos S 
sin A . 
was used for determining the altitudes. 
The determination of the meridian line, or of the azimuth of the sun, at each obser- 
vation depends mainly for its correctness upon the knowledge of the error and rate of 
the chronometer employed. No pains were therefore spared to prevent all jolting of 
the instrument during the voyage, and no opportunity was missed of comparing it with 
other chronometers. The following Table is the result of these comparisons, made 
always with the greatest courtesy by those in charge at the several observatories and 
depots de chronometres. 
The chronometer is a large-size marine instrument by Frodsiiam, No. 3148; it has 
been in constant use at Stonyhurst Observatory since the beginning of 1863, and its 
daily rate is found by the observation of clock stars on every favourable night. Its 
mean daily rate previous to the journey was 0 S, 57, and afterwards it increased to 0 S, 61. 
The rate is found to vary somewhat during the year. Mr. Feodsham kindly examined 
the instrument before it was taken to France and immediately after its return, and 
declared it to be in perfect order. 
G 
