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32  W.A. Norton on the Variations of the Declination 
tide following the flood at an interval of about 6 hours. But it is 
to be observed that the fall and rise during the night are compar- 
atively slight, and that in the summer months the nocturnal} rise 
peach is altogether wanting ;—the decrease continuing from 
Ap.m.to 104. ™. é principal minimum is at 10 a. m. to 12 ™. ; 
the principal maximum is at 4 p. m. in the summer, and generally 
at 4toG a.m. in the winter. These phenomena may be referred 
to the sacs action of the two systems of electric currents, 
radial and ecliptic, by which the corresponding variations of the 
declination have been explained. In the present case we have 
the effects of the components of these currents, which cross the 
meridian at right angles, but in the case of the variations of de- 
clination, the effects of the components running in the direction 
of the meridian. ‘To consider first the effects of the perpendic- 
ular components of the radial currents; let us confine our atten- 
tion for the moment, to the sped current — from the 
point immediately underneath the sun. This is represented by 
Ph in Fig. 2 (p. : ; S being sipposid to ‘ay the point of the 
photosphere which has the sun in its zenith. During the whole 
of the forenoon it will = toward the west and the hori- 
zontal force will be diminished, and during the whole of the af- 
ternoon it will be directed toward the east and the horizontal _ 
force will be augmented. If we suppose Tapisasisy of she cur- 
rent arriving i P from the point S to be the same for all posi- A 
tions of S the parallel of latitude AB, PA will have? i kead | 
imum vale aie the sun is on the prime vertical at A, and again 
when it is on the same circle, at B, in the afternoon. Daring the 
forenoon it will decrease, and vanish altogether at noon. In the 
afternoon it will go through the same changes in the inverse or- 
der. The minimum of horizontal force should therefore oceur 
early i in the forenoon and the maximum late in the afternoon. 
ut in point of fact while the sun is moving from A to M the 
distance SP is ae and therefore the current propagated 
from 8 increasin he component Ph is also proportional to 
cos APS, and tiverelore at first varies very slowly. It may very 
_ well happen therefore that the minimum will occur sometime 
after the sun is at A, and the maximum sometime before he is at 
B. Again, instead of a single current proceeding from S, there 
is actually, according to our theory, a similar current emanating 
from all the points of the photosphere which are exposed to the 
suns’s action, and the minimum of horizontal force obtains when 
the resultant of all the actions of these innumerable currents, di- 
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