‘. nee omt IER = ee 
¢ 
and Directive Force of the Magnetic Needle. 207 
our of noon, the declination is greater in Angust than in June 
because of the greater obliquity of the ecliptic to the meridian; 
—the increase of this obliquity prevailing over the diminution of 
the action of the radial currents. In the winter months the mid-day 
declination should be the least in December, because the tendency 
of the radial currents to deflect the needle toward the west i 
then at its minimum. In November the declination at noon 
should be greater than in January or February, because the ecliptic 
crosses the meridian in November in a direction from S. o 
N: of W., and in January and February from N. of E. to 8. of W. 
It is a consequence of our theory of the combined action of 
radial and ecliptic currents, that in the middle of the day the dec- 
lination should be greater at the autumnal than at the vernal 
equinox. This fact has already been noticed (p.199). Jt fol- 
lows also that the declination should be nearly the same both at 
4.M. and 6 p.m. at the equinoxes. The actual difference at 6 
A. M. is only 0/25; at 6 p. m. the declination is 1/9 greater at the 
Vernal than at the autumnal equinox. This excess of 1/9 is 
probably in part due to the irregular disturbances, so called, for 
these are much greater at the autumnal than at the vernal equi- 
Nox, and at 6 p. mu. and for several hours after the easterly disturb- 
ance preponderates over the westerly. It also may be in part at- 
tributable to the more northerly position, in the latter part of the 
alternoon, of the point of meridian passage of the secondary eclip- 
ic current. Another consequence of our theory is that the increase 
of declination from 6 a.m. to noon should be greater at the au- 
tumnal than at the vernal equinox; because at the former the three 
Currents are combined in their action, whereas at the latter the 
re is opposed to the othertwo. The actual numbers 
and 
effect}, and y= primary current; then c+y=8"'65, z—y=95'67, 
%=7'-16, y=1-49, We have an opportunity of verifying this de- 
termihation of the effect of the primary current. The difference 
78. -7'-89—1°-49=6/-40= rad. — sec: ; 278+1'-49= 
4°27 =rad, — sec. With Table II, we get the results, 4-63, 4-38, 
