proceedings: Washington academy of sciences 507 



The prime object of the present investigation is to ascertain to what 

 extent the earth's magnetic state at any time may be dependent upon 

 solar conditions, i.e., upon causes exterior to the earth. The purpose 

 is not to determine anew the relation between fluctuations in the 

 earth's magnetism about some base line or normal value and fluctua- 

 tions in the sun's activity, but rather to find out whether the base 

 line or normal value itself varies with solar activity, and, if so, how. 

 Thus during periods of intense sun-spot activity, violent fluctuations 

 in the earth's magnetism are known to occur. These fluctuations may 

 continue for a few hours or for a few days and then subside. There is 

 often found to be an after effect, of a quiescent, persistent character, 

 as the result of which the earth's magnetization remains below par for 

 several months, or more, after the apparent cessation of the magnetic 

 storm. 



It is a matter of no little interest to know whether the earth's mag- 

 netic state ever completely returns to a former state after having ex- 

 perienced such magnetic-storm effects as described. Some previous 

 investigations bearing on this question had been made and reported 

 upon, by the author on the basis of data extending over a few years at 

 the most. Now, however, the investigation apphes to data extending 

 over a period of about 2| sun-spot cycles. 



It is shown that the absolute values of the magnetic elements that 

 define the earth's magnetic state at any time vary in a definite and 

 in an appreciable manner with change in the sun's activity, as revealed 

 to us by sun-spottedness and by values of the solar constant of radia- 

 tion. The two sets of measures of solar activity serve to supplement 

 each other in determining the causes for the anomalous changes in the 

 earth's magnetic state from year to year. 



The conclusions indicate that in the selection of a common epoch to 

 which the magnetic elements resulting from a magnetic survey shall be 

 reduced, consideration may have to be paid to the position of the epoch 

 with reference to the years of maximum and minimum solar activity. 



The conclusions also confirm those reached by the author in a paper 

 presented before the Society in October, 1904, viz, that the secular 

 variation of the earth's magnetism, besides being caused by a sys- 

 tem of forces below the earth's surface, is also appreciably caused by a 

 system above the earth's surface, and that the secular variation results 

 not only from changes in the direction of magnetization, but likewise 

 from changes in the intensity of magnetization of the earth. 



H. L. Curtis, Recording Secretary. 



