I9I0.] TERRESTRIAL MAGNETIC DISTURBANCES. 133 



average diurnal range for a fairly long interval of time, so that one 

 set of phenomena could be almost directly deduced from the other, 

 a similar relation has not been found to hold between the magnitude 

 of a magnetic disturbance and any measure of solar activity thus 

 far proposed. 



Fresh interest was shown in magnetic storms a year ago by 

 Hale's discovery of magnetic fields in sun-spots and some persons 

 immediately jumped to the conclusion that the origin of our 

 magnetic storms had been discovered. However, the rapid de- 

 crease in the strength of the sun-spot fields with elevation, observed 

 by Hale, shows that they, at the distance of the earth from the sun, 

 could not by direct action produce an eflfect to be detected by the 

 most sensitive modern magnetic apparatus, whereas the effects 

 actually observed during a magnetic storm exceed by lOO and even 

 1,000 times the limit of measurement. It ought to be stated that 

 Hale himself never, as far as I know, expressed any opinion as to 

 the possibility of a direct magnetic action of sun-spots. 



In order to pave the way towards a solution of some of the 

 difficulties mentioned, the following investigation was originally 

 begun, partly as the result of a desire on the part of Professor Hale 

 to ascertain as quickly as possible what solar phenomena should be 

 given chief attention in the proposed study. He wished to know, 

 for example, how closely the curves resulting from his new method 

 of measuring the solar activity — by the total area of the bright 

 calcium flocculi seen on the sun's disc — corresponded with the well- 

 known fluctuations in the earth's magnetic activity. He divided the 

 sun's disc into zones 10° wide and determined the area of all the 

 flocculi present in each zone. The sum of all the zones is the total 

 area for the period in question and the solar activity was taken to 

 be directly proportional to this area. Since the rotation period of 

 the sun varies with solar latitude, means are taken for each zone 

 corresponding to the rotation period for that zone in order to elimi- 

 nate the effect of the solar rotation. 



In my paper presented to the society a year ago I was enabled 

 to communicate some preliminary results obtained from our com- 

 parative study of the variations in the sun's activity, as shown by the 

 calcium flocculi measurements at the Mount Wilson Solar Observa- 



