1881.] on Ma'jnctic Dixturhanrc, Aur(»\T, awl lunlli Carrcntn. GG3 



A preliminary account of a comparison of tlio declination curves 

 from the European stations was brou^^lit before tlie British Associa- 

 tion last year at Swansea, and tliis evminf^ I luive to bring bcforo 

 you some further points which come out of tlieso comparisons. 



It must be remeTubercd that at nortliern stations the liorizontal 

 force is smaller in proportion to the wliole force tlian it is at stations 

 nearer to the equator, so that tlio same disturbance will produce less 

 effect on the horizontal force or on the declination nccdlo in latitudes 

 near the ecpiator. 



Also the needles at different stations are by no means in the same 

 state of sensibility, and even at the same station they chanf^o with 

 time, so that they are not always equally sensitive, and when they 

 lose their magnetism they have to be remagnetised. 



Let us take the disturbances on March 15-lG, 1879 (see Plate 1), 

 which will illustrate some of the points which I wish to bring out 

 prominently. 



Not only do magnetic changes occur at the same time at diflcrent 

 stations, but there is a great similarity between them. 



We see that soon after 10 a.m., Greenwich time, on March 15, 

 1879, there is a disturbance wave showing first a diminution and then 

 an increase in the horizontal force at St. Petersburg, Vienna, Kew, 

 and Lisbon. At INIelbourne, in Australia, there is a similar disturb- 

 ance at the same time both in the declination and in the horizontal 

 force. 



Again between 2 and 3 r.M., and between 4 and 5, there are very 

 small disturbances showing themselves at the same absolute time in 

 the horizontal force and declination curves. About 5.20 r.M. there 

 is a well-marked increase in the horizontal force and eastward 

 deflection of the declination needles. 



About 9.30 P.M., Greenwich time, a storm begins which lasts for 

 about an hour. It is felt in the northern and in the southern hemi- 

 spheres, near to and on both sides of the equator. At all European 

 stations the horizontal force is increased during the first part of tlio 

 storm and then diminished. At Lisbon the vertical force is first 

 increased and then diminished, and at St. Petersburg and Stonyhurst 

 there is a diminution in the vertical force at the same time; as at Lisbon. 

 If we regard the declination needles, we find that at St. Petersburg, Zi- 

 ka-Wei and Melbourne, and at Bond)ay the declination westward is 

 first increased and then diminished, whereas at Kew and Lisbon the 

 motions are in the opposite direction. The declination at Vienna 

 seems to be intermediate between Kew and St. Petersburg, but the 

 curve is incomplete. At Bombay and the Mauritius, near to, but on 

 opposite sides of the equator, the declination needles are deflected 

 opposite ways. The local time at these places was from 1 to 2 o'clock 

 at night. Now in what way can we account for such magnetic 

 disturbances as these ? If we assume that by magnetic induction from 

 some cause or other the earth's magnetism is altered, then the position 

 of the magnet which would produce the disturbance must be such that 



