190 W. A. Norton on the Variations of the Declination 
latitude and meridian traced through this point, S the peint of 
the photosphere directly underneath the sun, at a certain hour in 
the forenoon, AB the parallel of latitude followed by the sun 
in its diurnal ‘motion, and SP the are of a great circle followed 
by the electric current flowing from § to P. “e this current be 
decomposed into the two equivalent currents Pd and Ph, the one 
lying in the meridian MP, and the other in the parallel of lati- 
tude EW. The variations of the current Pd, as the point § 
moves westward along the parallel AB, serve to explain the prin- 
cipal westerly and easterly movement of the needle, while those 
of the current PA serve to explain the variations of the horizon- 
tal force during the same hours To understand the effect of a 
current in its action on a magnetic needle, we have only to recall 
to mind Ampeére’s rule, viz., to suppose ourselves lying along the 
gaient circulating from the head toward the feet , and facing the 
needle, then the north end will move toward the right ; or we 
may, in the present case, rea bir ourselves to be standing, (in a 
Vertical position,) on the line of the current, and to be looking 
toward the needle from the sida from which the current flows, 
then the circular action of the current on the north end of the 
needle will be in the direction of the motion of the hands of a 
watch. It follows that when the meridional current in the upper 
South, it will be impelled toward the East. It is manifest that 
tHe meridional current Pd will increase enter the hour of noon, 
noon. The Eylestion < the fact that the turning ee ae 
current will be directed toward the south; but as the sun, OF 
rather the point of excitement underneath - it, moves westward 
this current will decrease, and vanish altogether when the sun is 
over A, and hence from the cause under consideration the motion 
should ‘be westward. After the sun has passed B, the meridional 
current will again be directed toward the south, but it will now 
be on the increase, and hence the Range: motion should con- 
tinue. The reason of the morning easterly movement will ap- 
pear in the 
The similar Secéascal movements of the needle may be ex- 
plained in a similar vg if we suppose that there is another 
point of excitement on the photosphere from which electric cur- 
