470 



HEPORT — 1881. 



points of the earth's surface at the same instant are so large as to be quite 

 comparable with the earth's total magnetic force ; and in order that any 

 cause may be a true and sufficient one, it must be capable of producing 

 these changes rapidly. They are so large that any cause which is shown 

 to be incompetent to produce the earth's magnetism can hardly be held to 

 account for such terrestrial magnetic changes. Since the large distur- 

 bances and the small disturbances do not follow totally different laws, but 

 agree equally well all over the earth, in so far as they agree we must 

 attribute them to the same cause. 



In the third storm, on August 13th (see Plate XIII.), the general 

 character of the disturbances is the same at St. Petersburg, at Kew, at 

 Lisbon, and at Vienna, as shown by a comparison of the horizontal force 

 as well as of the declination curves, and the vertical force needles are 

 similarly affected at St. Petersburg and at Stonyhurst ; but about 11.30 

 p.m., when the horizontal force at other European stations is considerably 

 increased, the horizontal force at St. Petersburg is very greatly diminished, 

 so that the record passes off the edge of the paper. The horizontal force 

 at Zi-ka-Wei is greatly diminished, and at Melbourne is also diminished 

 at the same time, the vertical force at St. Petersburg and at Stonyhurst 

 being at the same time diminished to its lowest value during the storm. 

 There are some very remarkable points of agreement between the St. 

 Petersburg horizontal force and the Zi-ka-Wei declination and horizontal 

 force curves, considering that the two places of observation differ nearly 

 30° in latitude, and are 6 hours apart in time. The Melbourne horizontal 

 force curve also bears a very striking resemblance to the Zi-ka-Wei hori- 

 zontal force curve, more especially from 11 p.m. until the end of the 

 storm. The Melbourne declination curve inverted and the Zi-ka-Wei 

 declination curve also have many characteristics in common. Taking the 

 Mauritius declination curve which is only slightly disturbed, probably 

 because the needle is not sufficiently sensitive, it is seen that the greatest 

 disturbances are similar to and coincident in time with disturbances in 

 the Zi-ka-Wei horizontal force curve. The similarity between the vertical 

 force curves at St. Petersburg and at Stonyhurst is very striking, espe- 

 cially when it is considered that at Lisbon and Stonyhurst, two places 

 nearer together, the vertical force curves have very few coincident and 

 similar disturbances. 



The following table contains the Comparative Scale Values for the 

 curves drawn in the Plates. 



