and Directive Force of the Magnetic Needle. 193 
months, as the sun’s action is more in advance on the parallels of 
latitude to the north, and as the currents from these are nearer 
and inclined under a larger angle to the prime vertical than those 
from the parallel under the sun, it may very well happen that the 
needle will be deflected toward the east. We must take into ac- 
current from the effect of resistances as the distance traversed in- 
creases. Early in the forenoon the determining points fall to the 
West of the prime vertical (PA), and a decided westward move- 
ment should obtain. 
shall show in another connection (see p. 202) that the ecliptic 
currents play an important part in the forenoon ; tending to pro- 
uce an easterly movement in certain morning hours, (in the 
summer, ) and subsequently a displacement toward the west. The 
oscillation of the needle first to the east and then to the west, 
during the forenoon, is a result of the joint action of both sys- 
ems of currents. ; 
We have hitherto regarded the two component currents which, 
by their changes of intensity and direction, determine the varia- 
lions of the declination and horizontal force, as lying, respectively, 
in the geographical meridian, and in the line crossing the meridian 
at right angles. Strictly speaking, the primary currents should 
be decomposed into two currents following the magnetic merid- 
1an and traversing this meridian perpendicularly. In this region 
of the earth the two meridians are inclined to each other under a 
small angle. For the sake of simplicity I shall continue to con- 
sider them as coincident. It will be easy in any special case, 
'o allow for the effect of their actual inclination. 
Annual Variations. 
Under this head are com prehended all the inequalities of declin- 
ation, and of diurnal variation of declination, whose period is either 
an entire year or any fraction of a year. The laws of all such 
equalities may be deduced from the following table, taken from 
Vol. Il of the Toronto Observations. (See next page. 
€ most conspicuous inequality is that of the diurnal varia- 
tion. The diurnal range changes materially from one season to, 
another. Thus, in the year 1846, the mean daily range during 
