No. 7.] DECLINATION. 17 
THE OBSERVATIONS. 
In addition to the direct readings of the position of the free declination 
needle, the deflection observations taken for the determination of the horizontal 
intensity, afford an opportunity for the calculation of the declination. The 
two magnets belonging to the apparatus, which are indicated by the Roman 
numerals V and VI, were employed as deflectors at two distances marked on 
the deflection-rod, HZ = 39°638 em. and e = 29°840 cm. By the four known 
positions of the deflecting-magnet perpendicular to the deflected declination 
needle, the meridian position of the needle is obtained, in addition to the 
angle of deflection, as the mean of the four readings. These declination 
determinations, which are not isolated, as readings of the position of the free 
needle were always taken both before and after the observations for deflection, 
I have included and duly entered in the series of observations. 
The following list contains in chronological order all the declination ob- 
servations taken during the expedition, with a statement of the manner in 
which they were made, and the addition of the hour of each single reading, 
by local time, to the nearest entire minute. In the column headed ‘Needle’, 
P,, P,, and L indicate that the observation was made respectively with the 
double declination needle in position P,, or with the same needle in position 
P,, or with the small declination-magnet. In the column headed M, will be 
found the mean of the two vernier readings, corrected for the error of colli- 
mation, for which, as previously mentioned, the following values are employed: 
for P, — 30-1’ 
» P, +3017 
nee 8-6) 
When the figure in column Mis the result of the calculation of deflection 
observations, a statement is added in the column “Needle” as to the kind of 
deflector that has been employed, and the distance. For instance, L.V. in- 
dicates that the small declination needle has been used as deflected magnet 
together with deflector V at the shorter distance; P,.VJ, that the double 
declination needle has been employed as deflected magnet together with de- 
flector VJ, at the longer distance. The double needle, as will be mentioned 
2] 
vo 
