e 
and Directive Force of the Magnetic Needle. 205 
pends on these currents, should be at its maximum. During the 
afternoon the primary ecliptic current becomes less and Jess ob- 
lique to the meridian, in a direction, S. of E. to N. of W., and 
hence the needle tends to shift its position toward the east. The 
secondary current is opposed to this; but its effect will be weak- 
ened in. the latter part of the afternoon, by the progression south- 
ward of the point of meridian passage of the curreut. ‘The pri- 
mary ecliptic and the radial currents, therefore, unitedly impel the 
needle toward the east, while the secondary current urges it in the 
opposite direction. The hourly change, so far as it is due to the 
primary current, will be greatest early in the forenoon, and late in 
the afternoon, and least at noon; so far as it is due to the second- 
ary current, it will be greatest toward mid-day, and least at 6 
4.M.and6».m. From the effect of both currents it should be 
greatest, a short time after the middle of the forenoon. It should 
be less at the middle of the afternoon, because the secondary 
current is now opposed to the radial. Table II. (p. 208) gives 
the hourly change from 9 to 10 a.m. in September 3725, and 
from 2 to 3 p.m 2. ; 
_ At the vernal equinor the point of concentration of the ecli 
tic currents is on the Southern Tropic ; the secondary current, 
previous to 6 a.m. will be much less oblique to the meridian, and 
after that hour much more oblique than at the Autumnal Equi- 
nox. The primary current will now, in the earlier hours, run 
from S. of E. to N. of W.; its obliquity will diminish from hour 
to hour, and hence there will be a tendency toward the east. 
After 6 4. m. the current goes from N. of E. to S. of W., and its 
obliquity increases so that the tendency continues to be the same. 
In the’ present case the secondary current is opposed*to the pri- 
Is the least possible in the morning hours. On turning to 
the forenoon than in the afternoon. The actual deflection from 
se 
