: a 
and Directive Force of the Magnetic Needle. 199 
We must now direct attention to the existence of another form 
af electric current in the photosphere of the earth, already briefly 
_ alluded to, from which several interesting eflects ensue. Thisisa 
current, or rather system of currents, induced by the sun’s mag- 
netic action on the photosphere, and running in a direction paral- 
lel or nearly so to the ecliptic and from east to west. I conceive 
them to be developed by the inductive action (the simple propaga- 
tion of an impulse probably) of currents traversing the sun’s sur- 
ace in a direction parallel, or nearly parallel to hisequator. The 
physical theory of their excitation does not fall within our pres- 
ent inquiry. The following discussion will, I think, serve com- 
etely to establish the fact of their existence. To have a clear 
conception of their diurnal and annual change of position, we 
may regard them (or at least those which originate in the lower 
latitudes) as represented by a single current followiug the direc- 
tion of the ecliptic traced on the earth, and observe’ that this is 
carried around with the sun during the day; and at a given hour 
of the day goes through in the course of a year the same changes 
of position that it does during aday. It is also to be observed 
that the force of this current will be the greatest at or near the 
point directly underneath the sun. At either equinox, and at the 
hour of noon, this current will be inclined 234° to the meridian 
of the station ; at the vernal equinox passing from the north to 
the south to the north side. The meridional component of this 
current will then be directed from north to south at the vernal 
and from south to north at the autumnal equinox. The north 
of the needle onght therefore, to stand farther to the west at 
the autumnal than at the vernal equinox, at.the hour of noon and 
thereabouts, at all stations. In fact there isan excess at Toronto, 
at noon, of 3/-23.* (See Table I.) In what precedes I have 
only considered the action of the ecliptic currents near the equa- 
t. In point of fact, the sun acts upon the high as well as the 
low latitudes, and develops at each point of its action a current 
Which sets out in a direction parallel to the ecliptic and follows 
the course of a great circle. The more northerly currents may 
mM general be approximately represented by a single current pass- 
ing through the zenith of the station.t Throughout the year, 
* This i equality of declination, which has its positive maximum at the autum- 
nal and negative maximum at the ve 
Served that, other thines being the same, the effect of a current will be the greatest 
‘when it nan..: 41 ae ea ve ek ge 
E through 
ra 
