and Directive Force of the Magnetic Needle. 43 
As to the exciting cause of these irregular currents, the occa- 
sional ecliptic currents can only be referred, it would seem, to par- 
oxysmal disturbances of the sun’s magnetic force ; which disturb- 
ances, from what we have already seen (p. 185), must be in some 
way connected with those mighty processes going on at the sun’s 
surface that reveal their action by giving rise to the appearance of 
dark spots on the disk. The irregular radial currents cannot be 
ascribed to the sudden angmentation of some impulsive force em- 
anating from the sun, for such a force would have its greatest ef- 
fect on the meridian over which the sun was, and thus the max- 
imum of westerly disturbance of declination would be at noon, — 
instead of 7 or 8 a.m. They may be supposed to have their ori- 
We 
might possibly be attributed 
to variations of atmospheric humidity. But there is another and 
more plausible supposition that may be made; it is that the irregu- 
lar radial currents have their origin in the sudden arrival at certain 
points of the upper atmosphere, from time to time, of some mate- 
rial emanation radiating from the sun’s surface ; and either by its 
‘force of impact or by an electric action, developing electric cur- 
rents diverging in every direction over the photosphey. -If we 
. this meridian. 
when it reaches the earth, the same velocity as the ®arth in its 
orbit, then, confining our attention for the moment .to the circle 
of intersection of the plane of the ecliptic with the surface of 
