NO. ?.] 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, E = 39'638 cm. 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 g , and L indicate that the observation was made respectively with the 

 double declination needle in position P t , 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: 



forP, -30-1' 



P, +30-1' 



L - 8-6' 



When the figure in column M is 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,.V!E, that the double 

 declination needle has been employed as deflected magnet together with de- 

 flector VI, at the longer distance. The double needle, as will be mentioned 



3 



