272 Prof. Norton on Molecular Physics. 



radial photospheric currents are chiefly effective between the 

 hours of 6 a.m. and 6 p.m., though their influence extends, 

 especially during the summer, into the earlier and later 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 

 photospheric currents is to diminish its intensity between mid- 

 night and noon, and increase it between noon and midnight ; 

 but these effects are especially produced during the forenoon 

 and afternoon. On the other hand, the other set of currents 

 tend especially to augment the horizontal force during the latter 

 half of the night, and to diminish it during the forenoon. The 

 morning increase of the horizontal force is more conspicuous 

 during the winter than during the summer months, for the rea- 

 son that the diminishing action of the radial currents in the 

 morning hours is greater in summer than in winter. 



In studying the Annual Variations, we must take note of any 

 changes that may occur during the year in the intensity of the 

 two sets of currents by which all the phenomena are conceived 

 to be produced. In fact both sets of currents have varying 

 effective intensities. In these latitudes the radial currents are 

 most effective toward the summer, and least effective toward the 

 winter solstice — as a natural result of the varying positions of 

 the point of the earth's photosphere directly underneath the 

 sun*. The other set of currents have a maximum of effective 

 action at the autumnal equinox, and, considered individually, a 

 minimum at the vernal equinoxf. For at the autumnal equinox 

 the most advanced point of the earth's surface upon which the 



* The precise epoch when the radial currents are most effective should 

 vary with the latitude of the station. It is plain that near the tropic of 

 Cancer it should be some weeks before or after the summer solstice ; for 

 at the summer solstice, at the hour of noon, the currents or waves that reach 

 the station from the different points of the photosphere that receive the 

 sun's rays should exactly neutralize each other. The epoch or epochs in 

 question it is obvious should approach the summer solstice as we recede 

 from the torrid zone. The observations made at Philadelphia on the hori- 

 zontal force indicate that the radial currents are most effective in determi- 

 ning the diurnal variation of the horizontal force about a month and a half 

 before and after the summer solstice. 



t Just as with the radial currents, the epoch of maximum effect must 

 vary with the latitude, and in the lower latitudes should occur before and 

 after the autumnal equinox. 



