70 W. A. Norton on Molecular Physics. 
of these increases, as we have seen, from the vernal to the 
autumnal equinox, and an action to augment the horizontal 
force, each day, is followed by one to diminish it, the tendency 
of the daily accumulation of residual currents should be to di- 
minish its mean daily value, from the vernal to the autumnal 
equinox. Such a tendency does in fact manifest itself. As the 
result of the observations at Philadelphia, already referred to, 
the mean monthly value of the horizontal force was 0°0018 of 
its absolute value less in September than in March.° : 
In the former memoir it was maintained that the Irregular Dis- 
turbances of the magnetic needle might be satisfactorily explained 
if we admit the existence of occasional photospheric currents 
proceeding from various points over the preceding and following 
hemispheres of the earth, and that the ordinary region of max- 
imum excitation lies in the plane of the ecliptic, from 60° to 90 
to the west of the point of the earth’s surface that has the sun 
in the zenith ; and that the region diametrically opposite to this 
is a secondary region of special excitation. In special in- 
stances the point of maximum excitation may have other posi- 
t es that 
the ecliptic currents, whether developed in the crust of the earth or its photos 
ne general causes are: (1 
its orbit; (2), a variation in the directi 
on of currents e 3 (8) 8 o 
the extent of the portion of each parallel of latitude that is exposed to the imping- — 
ing action of the ether, or auroral matter; (4), a change in the direction of the 
y em, as compared wi: i i 
gressive motion of the solar th the di of the _— 
tion of th. The epo i and minimum, dependent upon the first 
e solstices ; and those dependent fourth 
causes should fall near the effects of the second cause will vary 
at TS gg Pe Nae Rn Oe Ns rend et 
d 
hanges that occur annually in the effective action of — 
Feel Da 
with the locality. The currents due to the general motion of the solar system are 
~The conjoint aabwite ct thet 7 ‘ of . Je : 
sidering those which occur at the intertropical station of St. Helena. ota 
wn. ee oe 
that h b considera-_ 
