68 W. A. Norton on Molecular Physics. 
val of time after the moment of most intense excitation ; because, 
doubtless, of the residual currents that continue in action, with 
diminishing energy. | 
two sets of currents, that have been specified, afford a 
complete explanation of the observed periodical variations of 
the declination and directive force of the magnetic needle. In 
considering their separate action it is to be distinctly observed: 
(1) that the currents produced in the earth’s crust by the impul- 
sive action of the ether of space are developed at each station, 
between the hours of midnight and noon—thongh the currents 
thus excited will be propagated on and produce a certain effect 
at other stations, before midnight and after noon;—(2) that the 
radial photospheric currents are chiefly effective between the 
ours of 6 A. M. and 6 P, M.—though their influence extends, 
especially during the summer, into the earlier and Jater hours of 
the night. In their effect upon the declination, the marked ten- 
dency of the first set of currents is to deflect the needle toward 
the east for a certain interval of time before and after 6 A.M, 
while the conspicuous tendency of the second is to deflect the 
needle toward the west for a certain interval about the middle of 
the day. Another effect of the latter set of currents is, when 
the sun is north of the equator, to augment the morning easterly 
deflection produced by the former currents. In their effect upon 
the horizontal force of the needle, the tendency of the radial 
ariations, we 
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