Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/194

Rh 184 METEOKOLOGY [TERRESTRIAL MAGNETISM. the following manner. In a magnetic disturbance we have fre quently a general displacement of the various elements the hori zontal force, for instance ; now on the curve which represents this slow but considerable displacement a large number of comparatively small but very abrupt changes are superimposed. These latter appearances are invariably accompanied by quick and strong alter nations from positive to negative of the earth currents, while the former slow motion, although it may be of large range, hardly appears to have any galvanic equivalent at all. This would appear to favour the induction hypothesis, according to which small but abrupt magnetic changes should give rise to strong earth currents alternately positive and negative without reference to the position of the magnet above or below its normal at the time. 136. Another fact bearing upon this hypothesis is that mentioned in 88. From this it would appear that on ordinary occasions the curves recording the progress of the declination needle at Kew and Stonyhurst are as nearly as possible identical, but on occasions of disturbance the range at Stonyhurst is greater than that at Kew by an amount not apparently depending so much on the magnitude of the disturbance as on its abruptness. The introduction of the element of abruptness would appear to be in favour of the mix ing up to some extent of induced currents with the phenomena in question. 137. Sir George Airy has not been able to detect any resemblance in form between the regular diurnal progress of the magnet and that of the earth currents. It seems, however, possible that the peaks and hollows alluded to in 73 may form an important and integral part of the daily magnetic movement, and there even appears to be some evidence that the diurnal progress of the earth currents bears a nearer resemblance to that of the peaks and hollows than it does to the progress of the smoother curve which is usually held to represent the diurnal variation. But this is a question which can only be decided by more prolonged investigations. 138. To conclude, there can be no doubt that at times of great magnetic disturbance we have currents in the upper atmospheric regions and in the crnst of the earth which, so far as we can see, must either be due to atmospherical electricity or to induction, or to a mixture of both. The proportions of this mixture can only be decided by further inquiry and by the multiplication of stations where atmospherical electricity and earth currents maybe observed. It ought to be mentioned that the experience of the Kew observers, as far as this extends, seems unfavourable to the hypothesis of a connexion between auroras and atmospheric electricity. 139. Lunar -Semidiurnal Variation. From the fact observed by Broun ( 98) that the moon s magnetic influence is as nearly as possible inversely proportional to the cube of the moon s distance from the earth, it is impossible to refrain from associating it either directly or indirectly with something having the type of tidal action, but in what way this influence operates we cannot tell. Is it possible that the earth currents observed by A. Adams ( 101) are induction currents generated in the conducting crust of the earth by the magnetic change caused by the moon, inasmuch as these currents were found by him to be strongest in one direction about the lunar hours 3 and 15, when the lunar-diurnal magnetic effect is changing most rapidly in one direction ( 95), while they were found to be strongest in an opposite direction about the lunar hours 9 and 21, when the lunar-diurnal magnetic effect is changing most rapidly in an opposite direction ? 140. We might perhaps expect from the analogy of the tides that the sun should possess a semidiurnal magnetic effect similar in type to that of the moon. Now Sir George Airy in his analysis of the earth currents observed at Greenwich (Phil. Trans., 1870) during days of tranquil magnetism has detected in such currents a semi diurnal inequality having maxima in one direction at solar hours 3 and 15, while it has maxima in the opposite direction at solar hours 9 and 21. The reference to solar hours in this inequality is thus precisely similar to that which the inequality observed by Adams bears to lunar hours. 141. If there are induced currents of this nature in the crust of the earth, we might naturally suppose that there will be correspond ing currents in the upper regions of the earth s atmosphere, and in accordance with the suggestion made by Professor Stokes ( 132) we might perhaps suppose that these currents will be strongest when the upper atmospheric regions are heated by the sun and thereby rendered better conductors. Is it not possible to suppose that the influence of daylight upon the lunar magnetic effect discovered by Broun ( 97) may be due to this cause, and may it not also induce us to recognize the possibility of a maximum lunar influence ( 99) at times of maximum sun-spots, when there is reason to believe that solar radiation is most powerful ? 142. Secular Variation. Sabine and Walker are agreed in regarding this variation as cosmical in its origin, and they are apparently of opinion that it is caused by some change in the con dition of the sun. It seems difficult if not impossible to attribute it to anything else, since the terella of Halley cannot be now regarded as having a physical existence. Again it is more than possible it is probable that there arc solar variations of much longer period than eleven years. On the other hand the evidence given in 81, tending to show that an access of sun-spots produces a change in the magnetic state of the earth consistent with the hypo thesis that the magnetizing power of the sun has then been augmented, requires to be confirmed by more observations ; and even then it is certain that this magnetic change produced by a considerable change in spotted area is extremely small. We cannot therefore regard the very large secular magnetic change as due to a non-cumulative magnetic influence of some long-continued solar variation ; nor does it seem possible to attribute the change to solar influence at all unless we regard this influence as producing results of a cumulative nature. It is possible, however, to regard solar influence as producing a cumulative effect in one of two ways, or by a combination of both. For (1) time is necessarily an element in any influence acting upon the hard-iron system of the earth presuming the earth to possess such a system. There are in fact indications in the results of 82 that a system of this kind is perhaps connected with the American pole ; yet, even allowing the influence of time, it seems difficult to account for the peculiarities of the secular variation by an hypo thesis of this nature. But (2) any long-continued variation of solar power would no doubt act cumulatively in producing an increase or diminution of the large ice-fields round the poles of the earth. In the course of time this cumulative change in the extent and dis position of these might perceptibly alter the distribution of the convection currents of the earth and these, according to the views herein indicated, might in their turn perceptibly alter the earth s magnetic system. 143. Concluding Remarks. If we agree to look for an explanation of terrestrial magnetism and its changes to strictly terrestrial pro cesses, we may derive some assistance in our search from such con siderations connected with symmetry as enable us, for example, at once to perceive that when two perfectly similar tilings are rubbed together we cannot have electrical separation, because there is no reason why the one should be positively and the other negatively electrified. Suppose then that an observer stands at the equator and looks towards the north, and then turns his back upon the north and looks towards the south. In the first position let him regard the northern system of meteorological processes and motions, and in the second the southern. Now if symmetry obtained ( ibso- lutely in these systems that is to say, if the observer, whether he regarded the northern or the southern system of things, had in either case precisely similar phenomena at his right hand and at his left then we should see no reason why the earth should be a magnet, or why one hemisphere should be the seat of magnetism of the one kind rather than of the other. If then we regard meteoro logical processes and motions as being in some way the cause of terrestrial magnetism, we must direct our attention to that peculiar element which causes a want of perfect symmetry such as we have described in meteorological phenomena. This element can hardly be anything else than the rotation of the earth, which is from left to right to an observer facing the north, but from right to left to an observer facing the south. 144. Now if we look upon the terrestrial meteorological system modified by the earth s rotation as having produced somehow in the past the magnetic state of the earth, it seems most natural to regard the system which formerly produced this magnetic state as being likewise that which at present maintains it in its efficiency, and which also accounts for the various magnetic changes which take place. It would seem therefore that terrestrial meteorology and terrestrial magnetism are probably cognate subjects, and that they ought to be studied together in the well-founded hope that the phenomena of the one will help us to explain those of the other. Furthermore, if these meteorological processes deriving their one-sided character from the earth s rotation are to be regarded as accounting not only for the origin but for the maintenance of the earth s magnetic system, we can hardly fail to imagine that these processes must derive part of the energy which they exhibit from that of the earth s rotation. Tidal energy we know is derived from this source ; but we must likewise regard part of the energy displayed in convection currents whether in the air or in the ocean as derived no doubt from the same source. And we may perhaps allow that in the phenomena of tidal action, as well as in those of convection currents of the air and ocean, there may be, not merely a transmutation of actual energy directly through friction into heat, but likewise a transmutation of it, ultimately perhaps into heat, but first through the intermediate agency of electrical currents which serve to maintain the magnetic state of the earth and to produce magnetic changes. Now if this be the case, if there be a large and complicated system of tidal and convection currents all tending to change the rotative energy of the earth ultimately into heat, whether directly through friction or indirectly through the medium of electricity, it is surely impossible with the present state of our knowledge to calculate with the smallest pretensions to accuracy at what rate this transmutation is taking place, and hence at what rate the velocity of the earth s rotation is being slowly diminished. (B. S. )