44 



SCIENCE 



[N. S. Vol. XXXIII. No. 837 



fact appears ever to be present in some de- 

 gree, at least, in the prevailing magnetic 

 condition of the earth, is hardly to be 

 treated as an outcast or as an "abnormal" 

 feature. I very much question whether it 

 will ever be possible to set up any rules 

 which will separate scientifically the sup- 

 posedly abnormal from the supposedly 

 normal fluctiiations of the earth's magnet- 

 ism. I am rather inclined to believe that 

 as progress is made there will be fewer and 

 fewer attempts in this direction. On the 

 contrary, instead of eliminating a disturb- 

 ance because it will not fit in with a partic- 

 ular mold or pattern, we shall learn that 

 knowledge is mostly to he advanced ty re- 

 taining it. 



In order to have something definite with 

 which to deal I am going to ask you to con- 

 fine your attention to-day chiefly to the 

 abruptly beginning magnetic disturbances, 

 bearing in mind, however, that not all mag- 

 netic storms begin thus. The first interest- 

 ing question which arises regarding these 

 sudden storms is whether, if referred to 

 universal time — say, Greenwich mean time 

 — they begin, within the limits of measure- 

 ment, at the very same instant over the en- 

 tire globe, or at least over a great portion 

 of it. Up to the beginning of this year it 

 was, indeed, generally believed that there 

 was no measurable interval between the 

 times of beginning of sudden disturbances 

 even for very distant stations. The opin- 

 ion was that the time differences which 

 were revealed were only apparent ones, 

 not real — that, if there were no errors of 

 observation, they would vanish. So the 

 belief arose that it might be possible to 

 find the longitude between two distant 

 stations by determining with the utmo,st 

 accuracy the local mean times of identical 

 perturbations, for, be it remembered, even 

 the very smallest of these find their coun- 

 terparts frequently at very remote stations. 



A magnetic effect propagated with the 

 velocity of electromagnetic waves wotdd 

 require but one eighth of a second to pass 

 completely around the earth; if with the 

 speed of a cathode ray, the circuit time 

 would be on the order of a half or three 

 fourths of a second. If then magnetic dis- 

 turbances were propagated over the earth 

 with such velocities, our present instru- 

 mental means would not suffice to detect 

 any time differences between stations, 

 though on opposite sides of the earth. If, 

 however, differences of whole minutes are 

 found instead of only fractions of a second, 

 it will at once be seen that a most valuable 

 clue has been obtained towards disclosing 

 the nature and origin of our magnetic 

 storms. It is my belief, furthermore, that 

 a long step forward will have been taken 

 towards the solution of the origin of the 

 earth's magnetism when once we have 

 found out what causes it to vary — what it 

 is that can derange the magnetization of 

 our entire planet to the extent of ten to 

 twenty per cent, in the brief interval of a 

 quarter of an hour, as actually occurred 

 during the magnetic storm of September 

 25, 1909, the most notable of which we have 

 any record. Referring to changes in the 

 earth's magnetism, insignificant alongside 

 of those just mentioned, viz., the secular 

 changes, which only reach a comparable 

 amount when integrated over a period of 

 several centuries, Maxwell says: "When 

 we consider that the intensity of the mag- 

 netization of the great globe of the earth is 

 quite comparable with that which we pro- 

 duce with much difficulty in our steel mag- 

 nets, these immense changes in so large a 

 body force us to conclude that we are not 

 yet acquainted with one of the most power- 

 ful agents in nature." 



I would now like to call your attention to 

 some interesting facts found with regard 

 to 38 of these sudden disturbances which 



