and Directive Force of the Magnetic Needle. 37 
photosphere, ae pass near the zenith on opposite sides, ih neu- 
’ tralize each o in their action on the vertical force, since they 
will be of ales intensity. But the ecliptic currents hai lie on 
take place in this difference will accordingly be attended with va- 
riations in the intensity of the vertical force. If we consider the 
entire set of ecliptic currents above the horizon it will be seen — 
that the perturbations of the vertical force will be chiefly pro= 
duced by the currents which are remote from the zenith ; for the 
proper tangential force of any single current will lie in the plane 
of the horizon if the current passes through the zenith, will 
lie in the vertical line if the GHirent is in the el fie. “and bh 
those which are above it, but aoe “ae ie effect. In the case 
of the horizontal force the pines above e and be elow the horizon 
have opposite tendencies. If it should s improbable that the 
differences of intensity of the a ia should be the oc- 
casion of sensible disturbanaaper se vertical force, when the di- 
rect effects of these currents on the horizontal force are slight, it 
is to be observed, iadarcon to the distinction just noticed, that 
the effect on the vertical force of two currents on opposite 
~ sides of the zenith, at the distance of 30°, is one half of the en- 
tire force due to the difference of the currents ; also that the sur- 
ace of the atmosphere crosses the plane of the horizon under a 
small angle, and hence that the portion of the photosphere which 
is in most effective action is very large in comparison with that 
in the vicinity of the zenith which is principally concerned 
in modifying she horizontal force. If we suppose the height of 
the atmosphere to be about 60 miles 61 (miles) its surface will 
cross the plane of the horizon under an angle of 10°, and the 
length of an are extending 45° from the zenith will be about 60 
wine while the length of its continuation down to the horizon 
will be about 640 miles. 
In studying the effects of the ecliptic currents ti 
€ must observe that any current which passes to the south of the 
pie tends to augment the vertical force, and any current which 
to the north to diminish it; again that any current w hich, 
coming from the north, passes to the east of the zenith tends to aug- 
ment the vertical force, and any current which, coming from the 
same quarter, passes to the west of the zenith tends to diminish it ; 
also that in the case of currents coming from the south the effects 
are just reve ersed. The variations — by the vertical force 
on any particular day mnst depend on the law of variation of the 
ecliptic currents along the circle of ieetement (p. 200), on the 
ft 
