Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/186

Rh 176 METEOROLOGY [TERRESTRIAL MAGNETISM. than to increase the westerly declination, while the effect of numer ous sun-spots is to increase it. Again, at the Cape the tendency of disturbances is to increase the westerly declination, while that of numerous sun-spots is to decrease it. At Trevandrum again (if we judge of it by Bombay) the effect of disturbances will be to increase the easterly declination, while that of sun-spots is to decrease it. Again, it is believed that at Kew and Toronto the supposed disturb ance effect on the dip agrees in character with the sun-spot effect. On the whole, therefore, there is no definite relation between the two effects. Now if we take Hobart Town, the Cape, and Trevandrum in the above table, we find from Table XIX. that these stations seem to indicate that the magnetic state of the earth is most powerful at times of maximum sun-spots. Kew and Toronto, however, so far as declination and dip are concerned, appear to go the other way. If, however, we suppose that during the several years of maximum sun-spots the American pole as well as the Siberian is affected, and that on such occasions of long continuance the former has more influence than the latter, we shall be able to reconcile our results with the hypothesis of increased solar action. We can understand too that time must be an important element in any influence communicated to the American pole, and that, although such influence might be apparent at Toronto and Kew, which are comparatively near the pole, it would not be apparent at the other stations of Table XXI. We shall recur to this subject when dis cussing secular change. VARIOUS PHENOMENA CONNECTED WITH THE SUN AND WITH TERRESTRIAL MAGNETISM. 83. Closeness in Time between Solar Changes and Magnetic Disturbances. Loomis (American Journal of Science, vol. 1.) has registered the extent of sun-spots for the six days preceding and following each of the great magnetic disturbances at Green wich, and has compared these values with that for the very day of the disturbance. In this manner he has treated all the days of great magnetic disturbance at Greenwich for a period of twenty- three years, with the exception of those cases in which very few observations of sun-spots were made. The cases of disturbance thus treated amount to one hundred and thirty-five, and the following result has been obtained : TABLE XXII. Extent of Spotted Solar Surface. Mean of 135 Days. Days before Storm. Storm. 54321 50-1 54-8 53-5 52 3 48 9 57-9 Days after Storm. 123456 49-0 45-1 49-3 45-6 45 2 45 3 From this result Loomis draws the following conclusions: (1) great disturbances of the earth s magnetism are accompanied by unusual disturbances of the sun s surface on the very day of the magnetic storm ; (2) the great disturbance of the sun s surface which accompanies a terrestrial magnetic storm is generally heralded by a smaller disturbance three or four days previous, succeeded by a comparative calm which immediately precedes the magnetic storm. 84. There is one instance on record of a sudden solar change which was practically simultaneous with a magnetic disturbance. l On September 1, 1859, a little before noon, R. C. Carrington was observing by means of a telescope a large sun-spot, when, to quote his own words &quot;Within the area of the great north group (the size of which had previously excited general remark) two patches of intensely blight and white light broke out. ... I noted down the time by the chronometer, and, seeing the outburst to be very rapidly on the increase, and being somewhat flurried by the surprise, I hastily ran to call some one to witness the exhibition with me, and on returning within sixty seconds was mortified to find that it was already much changed and enfeebled. Very shortly afterwards the last trace was gone ; and, although I maintained a strict watcli for nearly an hour, no recurrence took place. . . . The instant of the first outburst was not fifteen seconds different from ll h 18 m Green wich mean time, and ll h 23 was taken for the time of disappearance. In this lapse of five minutes, the two patches of light traversed a space of about 35,000 miles. ... It was impossible, on first witnessing an appearance so similar to a sudden conflagration, not to expect a considerable result in the way of alteration of the details of the group in which it occurred; and I was certainly surprised, on referring lo the sketch which I had carefully and satisfactorily finished before the occurrence, at finding myself unable to recognize any change whatever as having taken place. The impression left upon me is that the phenomenon took place at an elevation considerably above the general surface of the sun, and accordingly altogether above and over the great group in which it was seen pro jected. &quot; It has been very gratifying to me to learn that Mr Hodgson chanced to be observing the sun at his house at Holloway on the same day, and to hear that he was a witness of what he also considered a very remarkable phenomenon.&quot; At the very moment when Carrington observed this pheno menon the three magnetic elements at Kew were simultaneously disturbed. This disturbance occurred as nearly as possible at ll h 15 m A.M., affecting all the elements simultaneously, and commencing quite abruptly. The first or most abrupt portion of the disturbance lasted only about three minutes for all the elements ; but after that there was a more gradual change in the 1 Phil. Trans., November 21, 1861. same direction before the curve turned. This more gradual con tinuation of the first sudden movement lasted about seven minutes for all the elements. This magnetic disturbance was, however, in reality a small one, and was followed by a very great disturbance which took place not many hours afterwards. 85. Simultaneity of Changes of Horizontal Force at Various Parts of the Earth. We have already ( 79) alluded to the superiority of the horizontal force in indicating by its changes what is taking place in the magnetic system of the earth. If this system be strengthened as a whole we shall no doubt find the horizontal force increased in value at our various stations, while if the earth s power be weakened as a whole we shall find the horizontal force diminished. Broun has discussed this subject at great length in a memoir already mentioned, and has embodied his observations in numerical results from which the following table has been extracted : TABLE XXIII. Daily Means of Horizontal Force at Makcrstoun, (M. ), Trevandrum (T. ), Singapore (S.), and Hobart Town (H.). 1844. M. T. S. IT. 1844. M. T. S. H. March 1 21-40 07-42 16-40 16-46 ; March 17 21-30 11-54 19-74 19-12 3 18-34 05-25 14-18 13-98 18 20-11 07-62 17-65 17-08 4 16-53 06-42 14-59 14-49 ,, 19 21-73 08-55 16-26 13-59 ,, 5 11-43 04-43 12-79 10-04 20 22-00 07-83 17-24 16-20 6 13-94 01-85 10-42 9-66 ,, 21 23-15 09-06 18-35 17-06 7 12-99 04-12 12-37 9-98 ! ,, 22 22-47 08-14 18-35 19-12 ,, 8 16-94 03-81 13-62 11-50 ,, 24 22-30 09-58 18-76 18-99 10 19-12 04-33 14-87 14-07 25 23-71 10-09 18-76 19-25 n 17-64 05-87 14-87 13-97 ,, 26 25-22 09-48 20-29 20-10 12 18-05 06-28 15-01 14-22 ,, 27 22-24 10-81 18-63 19-33 13 23-25 09-58 17-65 16-18 ,, 28 22-16 07-52 17-79 16-73 14 23-66 10-61 18-90 17-81 ,, 29 -5-97 03-50 11-54 7-82 ,, 15 22-12 10-09 18-63 16-85 ,, 31 17-06 02-27 11-81 10-27 This table shows a considerable likeness between the daily changes of the horizontal force at the four stations. For instance, we have a minimum which occurs on March 5 at Makerstoun and March (5 at the other stations ; we have likewise a well-defined maximum occurring at all stations on March 14, and another occurring at, Trevandrum on March 25, and at the other stations on March 26. Finally we have a well-defined minimum occurring at Trevandrum on March 31, and at the other stations a day earlier. Broun has extended a similar treatment to daily means for every hour, and fig. 38 conveys a good idea of the amount of simul taneity which obtains in the changes of such values of horizontal force at stations far apart. 86. Recurrence of Disturbances at Intervals of about Ticcnty-six Days. Broun 2 and likewise Hornstein 3 have observed that there is a tendency in large magnetic changes to recur at intervals of about twenty-six days. At first it was natural to suppose that we have here a magnetical indication of the true time of the sun s synodical rotation, the interval between two disturbances denoting that which elapses between two presentations to the earth of a peculiarly powerful solar meridian. It seems unlikely, however, that there is a really permanent one-sidedness of this kind in our luminary; but the result of observation seems to show that for a limited period, say two or three years, certain meridians of the sun appear to be peculiarly powerful. The cause of this we shall not here discuss, but simply treat the phenomenon as a fact derived from observation. Broun in his paper above quoted (Phil. Trans., 1876) makes the following remarks : &quot; We have seen that when one side of the sun is presented to the earth the magnetic force of the latter is greater than when the other side is turned towards us; we may even say that the intensity is greatest for a given solar meridian ; this, however, may be simply an integral effect resulting from the actions due to all the meridians. But can we suppose when a great and sudden increase or diminution of the earth s magnetic force occurs that this is produced by some change occurring on a particular solar meridian? This does not seem at all improbable. &quot; In order to examine the facts, all the cases were noted during the years 1844 and 1845 in which the daily mean horizontal force diminished one-thousandth of its whole value within an interval of three days; they were found to be twenty- eight in number. If we call the solar meridian presented to us on the 1st January 1844 the zero meridian (0), and consider the time of rotation to be twenty-six days, and that there are twenty-six meridians, we find that the solar meridiai s presented to us when these great movements occurred may be arranged in a ftw groups, as in the following table&quot; [Table XXIV., p. 177]. &quot; An examination,&quot; continues Broun, &quot; of this table will show that m arly half of the great changes began when the eighth meridian after the zero had passed, while five began near the twelfth after, and five near the zero itself. ... If any doubt existed as to the possibility of these being mere accidental coincidences, it would be removed, I think, by a consideration of the marked succession occur ring between July 31 (No. 18) and December 11, 1845 (No. 28). ... If we neglect the two cases of July 31 and August 2C, which commence at 46 and +5 respectively, we have five cases of successive solar rotations in which the diminu tions of intensity began on the +8 day. This exact recurrence at the end if twenty-six days of a marked diminution of force proves, it seems to me, that the actions are all due to the sun, whose time of rotation must be nearly twenty-six days. &quot; An examination will show that the sudden diminutions of terrestrial magnetic force are in nearly every case preceded by a sudden increase.&quot; In the above extract we have given the author s exact words, but, while thinking with him that these actions are due to the sun, it does not appear to us to follow that the time of the sun s rotation 2 Phil. Mag., August 1858; Phil. Trans., 1876. 3 Vienna Acad. S., June 15, 1871.