ON MAGNETIC DISTDEBANCES AND EARTH-CUllRENTS. 467 



Now in what way can we accotmt for siicli magnetic disturbances ? 

 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 its pole, which 

 attracts the marked end of our needle, must lie at the beginning of the 

 disturbance to the E. of Kew and Lisbon, to the N. of Vienna, and to 

 the N.W. of St. Petersburg ; the Lisbon vertical force curve also shows 

 it to be below the surface of the earth. Hence an inductive action 

 equivalent to a change of position of the north magnetic pole, towards 

 the geog. pole, would account for these changes. The strengthening 

 and weakening of a magnet, with its N. pole to the N., on the meridian 

 of Vienna, might possibly account for the magnetic changes observed 

 between 9.30 and 10.30 at night, Greenwich time, on March 15, 1879. 



If we attempt to explain this disturbance by currents of electricity or 

 discharges of statical electricity in the air above the needles, then we 

 must imagine that at first there is a strong current from the S.W., over 

 St. Petersburg, from the W., over Vienna, and from the N.W., over 

 Kew and Lisbon ; the vertical force needle at Lisbon showing that the 

 current from the N.W. lies somewhat to the east of Lisbon. That at 

 the Mauritius this current is from the north, and at Bombay from the 

 sou.th. 



Hence we must imagine that a current of electricity passes down 

 from the N.W. to the S.E., going on towards the E. over Vienna, and 4 

 towards the N.E. over St. Petersburg. This must be kept up very much 

 along the same line throughout the first part of the disturbance, and then 

 the current or currents must be altered in strength in the same manner 

 at all stations. 



We will next consider what would hardly be called a magnetic 

 storm, but a few very small deviations of the magnetic needle lasting from 

 about 5.30 to 7.30 p.m. on March 26, 1879. (See Plate IX.) Only the 

 comparison of the originals will give the closeness of the similaiity of 

 the curves, and the declination curves of Vienna and Kew are absolutely 

 coincident. 



When the declination needle is deflected to the west, the horizontal 

 force-needle is deflected with its marked end towards the south, so that 

 in this disturbance the two needles are drawn towards the S.W. at the 

 same time with greater or less power, and twelve similar curves are clearly 

 traced out in the Vienna and Kew curves during the two hours. These 

 disturbances are all so small that, but for the comparison of photographs, 

 they would probably be lost sight of, yet we see that the same deflections 

 occur at the same instant at Kew and at Vienna, at St. Petersburg and 

 at Melbourne. From the remarkable similarity in these disturbances, and 

 their occurrence at the same time, we should expect that the cause of dis- 

 turbance is so far removed from the places of observation, that the difier- 

 ence of their distances from it need not be considered. This might not 

 unreasonably be urged as an argument in support of a theory that such 

 disturbances are due directly to the action of the sun regarded as a mag- 

 netic body. The numerical comparisons of observations made every five 

 minutes on certain days previously fixed upon, would probably never have 

 shown the way in which these minute changes of the magnetic power of 

 the earth at widely distant places are related to one another. 



In one or two cases Sefior Oapello and Professor Balfour Stewart had 

 Compared the Lisbon and Kew curves for a particular disturbance, but 



hh2 



