, lie 
—— 
‘Stice, and S. 0 
and Directive Force of the Magnetic Needle. 201 
At the Solstices, at the hours of noon and midnight, the eclip- 
tic current will have no effect on the declination, since it wi 
cross the meridian of the station perpendicularly. But at 6 a. m. 
' and thereabouts it will tend to produce a deflection of the needle 
from East to West, in the interval from the northern to the 
southern solstice; and this in both hemispheres. For at the 
northern solstice, it will, at this hour, cross the meridian under 
an angle of 664°, and tend from north to south, but at the south- 
ern solstice it will cross the meridian under the same angle from 
‘South to north. The extratropical currents are at this hour more 
oblique to the meridian, than the intertropical (just represented by 
asingle ecliptic current); but their representative current wil 
have the same inclination to the meridian at the one solstice as at 
the other, passing from N. of E. to S. of W. at the northern sol- 
f E. to N. of W. at the southern solstice. These 
currents will therefore conspire with those that originate between 
the tropics in producing an annual oscillation of the needle, from 
the one solstice to the other. The observed oscillation of about 
5, at 7 to 8am. (see p. 195), is therefore to be regarded as the 
sum of the effects of the entire system of radial and ecliptic cur- 
tents. It is to be observed that each set of currents has traversed 
an arc of nearly 90° before it comes into operation at this hour. 
The same effect, though less in amount, should occur at each of 
the successive forenoon hours. In the afternoon the tendency is 
everywhere reversed ; the ecliptic currents are therefore now an- 
tagonistic to the radial (see p. 198), and tend to counteract these 
currents in their tendency to produce the same species of oscilla- 
tion as in the morning hours. In this way we may explain the 
comparatively small oscillation that occurs toward 6 p. m. (a fact 
which is conspicuously indicated by the curves of Fig. 6.) To- 
ward mid-day, and afterwards, the stronger meridional currents, 
When the sun is at the nearer solstice, has the effect to make the 
West declination in both hemispheres, greater at the northern than 
at the southern solstice. We shall have corresponding results if 
We compare any two months equally distant from the equinox. 
At intertropical stations they will also be similar if the com- 
Son is mad 
of the equinox, at which the sun is on the same side of the 
Zenith of the place. (This qualification is made only with re- 
g curves, constructed from the ob- 
answers to the former in- 
_Srcoxp Srntes, Vol. KIX, No. 56—March, 1955. 26 
