DEPARTMENT OF TERRESTRIAL MAGNETISM. 349 



Instead of using the sun-spot numbers direct for comparison with magnetic 

 and electric variations, it is found (No. II) that a more satisfactory measure of 

 solar radiations and emanations affecting the Earth's magnetic and electric 

 conditions may be based upon the solar-rotation range of sun-spot frequency, 

 or upon some quantity indicative of the rate of change, or variability of sun- 

 spottedness during the Sun's rotation. It is found, in general, sufficient to 

 use for this purpose a series of monthly numbers which represent the average 

 departures of the daily sun-spot numbers from their respective monthly 

 means, regardless of sign. As a first attempt these numbers are regarded as a 

 measure of the energy expended, or work done, by the Sun during periods of 

 activity of interest here; they are briefly termed the "solar-energj'^ numbers" 

 and are designated by D, to distinguish them from the sun-spot numbers, N. 

 For short periods especially, as for example a month, the solar energy numbers 

 are found decidedly superior to the sun-spot numbers, or other measures of 

 solar activity hitherto used. Further improvement in the measure D may 

 be found possible by taking into account certain types of solar prominences. 



A simple and rapidly computed quantity is deduced in No. Ill which is 

 satisfactory, in general, as a measure of the energy-change, w, experienced by 

 the Earth during a magnetic change, or variation, v. This quantity as a 

 first approximation may be set equal to eHv, where H is the horizontal inten- 

 sity of the observing station, v is the observed magnetic variation, or range in 

 the variation, and e is a numerical factor. The values of w are found so closely 

 similar at stations widely distributed over the Earth that quite satisfactory 

 values for comparison with solar changes, for example, may be obtained at a 

 single magnetic observatory situated in moderate latitudes. 



Regarding some of the results obtained under the heading "Solar Activity," 

 see abstract on page 350. The chief magnetic and electric results obtained 

 were as follows: 



a. The Earth's magnetic energy and average intensity of magnetization, as 

 well as the strength of the normal electric currents circulating in the Earth's 

 crust, suffer a diminution during increased solar activity. The electric cur- 

 rents induced in the Earth during periods of increased solar activity are in 

 general reversed in direction to the normal currents, the strength of these 

 superposed currents increasing with increased solar activity. 



b. The diurnal range of the strength of earth-currents, as in the case of the 

 diurnal ranges of the Earth's magnetic elements, increases with increased 

 solar activity ; at time of maximum activity the range, as shown by the obser- 

 vations at the Observatorio del Ebro, Tortosa, Spain, 1910-1919, was about 

 50 per cent higher than during the time of minimum solar activity. 



c. The magnetic effect running a concomitant course with the solar-activity 

 cycle is retarded, on the average, about one year, so that there is a residual, or 

 an acyclic, effect at the end of the cycle. The actual amount of retardation, 

 in general, increases with intensity of the Sun's activity or energy. This lag in 

 the magnetic effect may be accounted for by the fact that the electric currents 

 generated inside the Earth during magnetic storms and magnetic variations 

 continue for some time after the apparent cessation or diminution of solar 

 activity, or after the period of the variation experienced. The same lag is 

 shown by polar-light frequencies at times of maximum solar activity. 



d. On fine-weather or electrically calm days the atmospheric potential- 

 gradient, or the deduced negative charge on the surface of the Earth, increases 

 with increased solar activity, the range in the variation between minimum and 

 maximum solar activity being about 20 per cent. The electric conductivity 

 of the atmosphere, on the other hand, shows but little, if any, systematic 

 variation during the solar cycle. Accordingly, since the vertical conduction- 



