and Directive Force of the Magnetic Needle. 211 
is considerably to the north of the equator, and in coincidence 
of action when the sun gets to the south of the equator. (See 
ig. 7.) 
Let us take now the special case represented in Fig. 7. 
same effects will occur that we have just considered; they will 
only be less in amount. But we may also discern effects pro- 
duced by the primary ecliptic current. Before and after the equi- 
nox this current crosses’ the meridian under a large angle, early 
in the morning, and by the rapid shifting of its position tends to 
deflect the needle more and more toward the west during the fore- 
noon. During the afternoon the tendency is reversed. It is thus 
Opposed to the intertropical radial current during the greater part 
of the day both before and after the equinox. At the earlier date, 
at which the sun is north of the equator, the primary current 
crosses the meridian early in the morning in a direction from N. 
of E. to S. of W.; and at the later date it runs at the same hour 
in a direction from §. of E. to N. of W. The needle ought 
therefore to have a greater west declination early in the morning 
after than before the equinox. That such is the fact will be 
Seen on inspecting Fig. 7. 
It is to be observed that the easterly movement of the needle 
from 6 to 7 a. m., when the sun is north of the equator, and west- 
erly movement at the same hour when the sun is south of the 
equator, is attributed to the action of the radial currents. When 
the sun has a south declination the westerly tendency resulting 
from the action: of the radial- currents, from 6 r Sa. M., 
should be relatively greater at St. Helena than at Toronto. Ac- 
cordingly the radial may prevail over the ecliptic currents at that 
hour, at St. Helena, and the ecliptic over the radial at Toronto ; 
compare Figs. 6 and 7. y 
t may be stated in general terms that the peculiar phenomena 
of the diurnal variation of the declination which have place at 
St. Helena and the Cape of Good Hope, and probably at inter- 
tropical stations generally, receive their explanation in the alternate 
preponderance of the extratropical currents of the Northern or 
Southern Hemisphere, according as the sun is north or south of 
the equator; together with the modifying action of the ecliptic 
and radial currents that are developed between the tropics. 
(To be continued.) 
‘ 
