1881.] on Magnetic Disturbance, Anwra', a)i(l Earth Currrnfs. G65 



power of the earth at widely distant places aro related to ono another. 

 lu one or two cases Senor Capello aud Professor Balfour Stewart 

 had compared the Lishon and Kew curves for a i)articular disturb- 

 ance, but the photographic magnetic records have never before been 

 Collected from other stations, and there has been no opportunity of 

 comparing them. From the precise similarity of the forms of the 

 curves in many cases we may say tliat the rate of change of magnetic 

 disturbances at widely distant stations is the same. Tliere is nothing 

 fitful or flashing in such disturbances as these of ]\Iarcli 2Gth. We 

 might imagine a current in the crust of the earth, or a current or 

 transfer of electricity in the air, near to, i. e. within 20 or 30 miles of 

 each of these observatories, but to imagine the same current and the 

 same variations of the current at so many diflercnt stations all 

 changing in the same way at tlie same instant is difficult, unless it 

 can be shown in what way all these changes are connected with the 

 cause of such a regular electric discharge. It seems easier to imagine 

 that such changes as these are due to a change produced by induction 

 in the magnetism of the earth itself by some distant body. It is easy 

 to show that the magnetism produced by a current in a magnetic sub- 

 stance round which it flows is greater in its action on a small magnetic 

 needle than the direct action of the current itself. Hence a current 

 flowing in the crust of the earth should produce its principal eftect 

 on a magnetic needle by the magnetic induction which the current 

 induces in the earth itself. 



Sometimes disturbances occur where at the same instant there are 

 similar deflections of the declination needles at stations wide apart, 

 and suddenly at one of the stations the needle no longer goes with the 

 others, but is deflected for a considerable period in the 0}»posite 

 direction to the others, turning when they turn, and tracing out a 

 similar curve, but turned always in the opposite direction. Such 

 cases occurred frequently during March 1879, and especially on 

 March 23rd, about 1.30 and about 7 r.M., Kew time, and on March 

 29th about 9 p.m. 



An examination of the principal disturbances at Kew and at 

 St. Petersburg seems to show that : — 



(1) A diminution in tlie liorizontal force is accompanied by greater 

 easterly deflections of the declination needle at St. Petersburg than at 

 Kew. 



(2) Increase of the horizontal force is accompanied by greater 

 westerly deflections at St. Petersburg than at Kew, or is sometimes 

 accompanied by a westerly deflection at St. Petersburg and an 

 easterly deflection at Kew. 



These cases which I have taken will be sufiicicnt to show how 

 important it is that there should be additional magnetic observations, 

 especially in the southern hemisphere, where photographic records 

 should be taken, so that we may learn something about the magnetism 

 of the earth. Practically we have to rely on one excellent observatory 

 (Melbourne) for the whole of the southern hemisi^here. Surely the 



