and Directive Force of the Magnetic Needle. 189 
1—Curve showing the Mean Diurnal Variations of the Declination at Philadel- 
j 844, 
phia, for the year 1 
556 \~/ 
= Ue Li 
O%123 45 6 7 8 9 101112131415 16 1718 19 20 21 22230, 
Increase of numbers at the side corresponds to decrease of declination: Oh answers 
to midnight. 
and from 24.m. to Sa.m.a greater easterly movement. Thus 
the declination has two maxima, viz., at lp.m, and at2 a.m, 
and two minima, at 8 a.m. and 10 p.m. The principal maximum 
sat | p.m., and the principal minimum at 8 a.m. The principal 
disturbance occurs during the day, and consists in an increase of 
declination or a westerly movement, from 8 a.m. to 1 p.m, and 
a decrease of declination, or an easterly movement, from 1 to 10 
P.M. The disturbance at night, is of the same nature, but much 
less in amount ; that is, the north end of the needle first moves 
toward the West and afterward toward the Hast. 
B 
M s A 
Ss’ 
Now let P (Fig. 2) be the point of the upper atmosphere di- 
tectly over the station of the needle, EW and PM the parallel of 
