TERRESTRIAL MAGNETISM.] METEOROLOGY 177 must be nearly twenty-six days. This assumes that the meridian of peculiar power is fixed on the solar surface. It does not, however, seem impossible to imagine that such a meridian may have a proper motion of its own, and indeed the planetary hypothesis of the origin of sun-spots would rather lead to this conclusion. But if this be the case we shall be unable to deduce from recurrent magnetic dis turbances the true value of the period of solar rotation. 87. Repetitions of Magnetic Changes. J. B. Capello, director of the Lisbon observatory (Proc. Roy. Soc., October 1868), has remarked that at periods of dis turbance there are nearly synchronous movements of the de clination needle during corresponding hours for two, three, or more days. He thus describes these phenomena: " In some cases the re petition is only in two or three parallel movements; in others there are true pei iotls of repel ition of some hours in duration. The repeated periods are not entirely similar, their phases being so modified that in some cases their identity can only be recognized by a very minute investigation. The same periods, when re peated, have not always the s.ime total duration ; nor do they recommence at the same precise hour, but sometimes earlier and some times later, the differences varying from a few minutes to two or three hours. We also see that the greatest number of repetitions be long to the night hours, that is to say, those hours when the movements of the needle arc easterly. In the morning hours there do not appear to be any well- marked repetitions. There are twenty-four examples now given, fifteen of which show repetition on two days, eight on three days, and only one where the curve appears repeated for four days. It appears that all .the facts exhibited in these examples agree with the cosmical theory; the cause (existing in the sun or in space) appears to con tinue sometimes during two, three, or several days with out undergoing remarkable transformations. The re petition, being sometimes earlier sometimes later, seems also to indicate that the cause possesses a proper movement; the cause per sists, but only comes again into operation when the earth by its diurnal rota tion is placed in a similar position or conjunction to that of the preceding days." Stewart, having com pared Capello s curves with the corresponding traces of the declina tion at Kew, found that the Lisbon disturb ances are almost in variably reproduced at Kew at the same time, only to a greater ex tent, and also that the same amount of simi larity which the various Fig. 38. Lisbon curves exhibit is exhibited in the corresponding Kew curves. Ihe strongest point in favour of the hypothesis is, he thinks, " not so much the repetition of a single disturbance as the repetition of a complicated disturbance in most if not all of its sinuosities." several examples of this occur in the diagrams. It would seem that something of the above nature was suspected by Humboldt, the earliest investigator of disturbances. Humboldt was astonished
- o discover the frequency with which nocturnal perturbations
occurred, sometimes recurring at the same hour on several successive TABLE XXIV. Cases in which the Earth s Magnetic Force dimin ished One-thousandth of its Value or more in 1844-45 ( 86). No. of Case. Date (Jan. 1, 1844=0). Change of Force in Hun- dred- thou- sandths. Solar Meridians. 5 to 10. 1 to 14. -3 to +1 and others. 1 87 to 89 360 +8 to +10 2 no to lie -104 + 11 to +12 3 141 to 143 -107 + 11 to +12 4 189 to 190 -116 + 7 to + 8 5 213 to 214 -175 +5 to + 6 6 221 to 22-2 -135 + 13 to +14 7 267 to 270 -115 +7 to +10 8 273 to 274 -104 + 13 to +14 9 292 to 294 -268 + 6 to + 8 10 323 to 325 130 + 13 to +14 11 361 to 364 165 3 to 12 373 to 375 -210 +9 to +11 13 14 383 to 385 41C to 417 -163 -118 (-6 to -4) to +1 15 467 to 469 -350 -1 to +1 1C 526 to 528 -110 + 6 to+ 8 17 570 to 571 154 -2 to -1 18 677 to 580 102 + 6 to+ 9 19 603 to 604 101 + 5 to + 6 20 006 to 607 159 + 8 to + 9 21 632 to C33 153 +8 to + 9 22 23 646 to 648 658 to 659 -126 118 +8 to + 9 (-4 to -2) 24 25 668 to 670 684 to 687 -100 -100 + 8 to +10 (-8 to -6) 26 27 696 to 60S 702 to 703 -110 -291 (-6 to -4) -0 to +1 28 710 to 712 -122 +8 to +10 nights (Walker s Magnetism, p. 80). We would make two sug gestions before dismissing this subject. (1) If we imagine that these changes are caused by the solar influence acting vertically on some susceptible region of the earth, then, inasmuch as they occur at the evening or early night hours, this region must lie considerably to the west. (2) The region must also have a proper motion of its own (see Capello s remark). Is it possible that this proper motion is on the whole from west to cast, a motion which we know is pursued by meteorological weather, and in which it is imagined ( 52) that magnetical weather as defined by us likewise participates ? 88. Comparison of Declination Changes at Stations near each other. Messrs Sidgreaves and Stewart (Pro. Roy. Soc., October 1868) have compared together certain curves of the Kew and Stonyhurst declination magnetographs. These magnetographs are of the same pattern, and it was found that on ordinary occasions the declination traces at both stations were precisely alike. This was confirmed by placing the curves the one over the other, when they were found to coincide even in their most minute features. In times of dis turbance, however, it was found that the motions exhibited by the Stonyhurst curves were greater than those at Kew, and this excess of Stonyhurst over Kew depended not so much on the absolute size of the disturbance as on its abruptness. This feature of the comparison is exhibited in the following table (XXV.), in which the excess of Stonyhurst over Kew in scale divisions is compared with the abruptness of the disturbance, this element being measured by the changes occurring in unit of time:- Group I. Group II. Group III. Group IV. Excess (under 5). Abrupt ness. Excess (under 10). Abrupt ness. Excess (under 20). Abrupt ness. Excess (above 20). Abrupt ness. 2 3-7 G 4-2 10 .5 21 7-3 2 6-4 6 2-6 10
25 2-9 -3 4-0 8 6-3 11 7 25 10-7
3-1 5 3-3 10
20 7-0
3-1 8 8-7 10 8 21 6-6 4 2-9 5 3-5 15 6-4 21 11-2 1 1-8 7 6-3 11 4-9 22 9-6 4 3-3 9 4-7 13 7-4 24 7-8 3 5-2 5 4-1 Means 1 5 3-7 6-6 4-9 11 6-5 22 7-9 It is very desirable that further comparisons of this nature should be made. 89. Auroral Displays. These are very frequent if not continuous near the magnetic poles, while in middle latitudes they are the in variable accompaniments of all considerable magnetic storms. Near the equator they hardly ever occur. There is a considerable variety in the forms assumed by these displays, and it is possible that this may denote a corresponding variety in the cause or causes which give rise to this phenomenon. Loomis (SmitJisonian Report for 1865) specifies five such varieties: (1) a horizontal light like the morning aurora or break of day; (2) an arch of light which frequently extends entirely across the heavens from east to west and cuts the magnetic meridian
XVI. 23