Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/181

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 171 declination and either element of force, and (2) those tending to diminish the same. He finds that these two categories obey different laws, from which he argues that there are at least two sets of disturbing forces. In fact, if we have to give up the idea of a single force of constant type, it is natural to ask if the phenomena of dis turbance can be approximately represented as due to the united action of two independent types of force. It was probably some such idea that led Sabine to separate disturbances into these two categories above mentioned. Here there is no attempt to assert that these two types represent an ultimate and complete analysis of the forces concerned. We merely use the separation as the most convenient method at our disposal in the present state of our know ledge for ascertaining whether there be indications of a dual system. 54. Results in the Northern Hemisphere. Sabine s method of viewing the phenomena has enabled him to obtain the disturbance- diurnal variation for the following stations :- Kew 51 29 N. lat. Peking 39 54 N. Nertchinsk 51 19 N. Toronto 43 40 X. Port Kennedy 72 01 N. Point Barrow 71 21 N. The above stations have been so chosen that Kew may be regarded as on one side and Peking and Nertchinsk as probably on the other side of the Asiatic pole, while Toronto may be regarded as on one side and Port Kennedy and Point Barrow as on the other side of the American pole ( 29). The question as to what influence, if any, these poles have upon the disturbance-diurnal variation of declination is thus one which may be answered by examining the results obtained at these various stations. For this purpose, instead of recording the aggregate disturbances at the various hours, the result is expressed in ratios, the mean hourly ratio for the day being taken as unity, or in other words the whole body of disturbances for the twenty-four hours being reckoned as twenty-four. The results of this method are graphically represented in fig. 37, where in the left-hand curves Kew time is used, and in the right-hand curves local time, each starting at OM 8 W. long. 116 6 E. 114 9 E. 79 W. 94 20 W. 156 15 W. Fig. 37. 55. At all the various stations one curve exhibits unmistakably a single progression, while the other exhibits more or less dis tinctly a double progression. At Kew, Toronto, Port Kennedy, 1 If we refer to a paper by C. Chambers, director of Bombay Observatory (/ Ml. Trans., 1868), it will be seen that westerly disturbances at Bombay pre sent the same characteristics as westerly at Peking or Nertchinsk, the maximum being about twenty-two or twenty-three hours Bombay astronomical time. and Point Barrow it is the easterly disturbances which exhibit this single progression ; while, on the other hand, at Peking and Nertchinsk, stations which are oppositely related to the Asiatic magnetic centre, it is the westerly disturbances which do so. It is imagined by Sabine and others that this peculiar reversal is due to the fact that Kew and its associated stations may be regarded as on one side and Peking and Nertchinsk as on the other side of the movable magnetic system. Sabine has likewise remarked that the single-progression curves, whether denoting easterly or westerly disturbances, exhibit maxima which take place not far from the same absolute time. We have therefore plotted all the left-hand curves according to Kew time, that the eye may readily see the amount of simultaneity which their corresponding phases exhibit. It will be noticed that there is a very striking, simultaneity between the maxima of Kew, Toronto, Peking, and Nertchinsk, but that the maxima for Port Kennedy and Point Barrow, while both occurring about the same time, fall at a time decidedly if not very greatly different from that of the other maxima. Indeed the time of maximum for Port Kennedy and Point Barrow is not far from the time of minimum for the other stations. Now it has been noticed by Sabine that Port Kennedy and Point Barrow may be regarded as on one side of the American magnetic centre of intensity, while Toronto and the other asso ciated stations are on the other side. It seems therefore possible to connect this last fact with the change in the time of maximum. Sabine has likewise remarked that the aggregate amount of dis turbances is much greater at Point Barrow than at any other station. Now Point Barrow is likewise that spot where auroras are most frequent. Thus in the phenomena we are now discussing there is first of all a marked reference to the Asiatic pole ; secondly, a reference not so marked, perhaps, to the American pole ; and thirdly, a reference to the centre of auroral activity. Sabine, whose experi ence of such matters is very great, appears to think most of the reference of these phenomena to the Asiatic pole. He thinks that &quot;of the two magnetic systems which are distinctly recog nizable in the magnetism of the globe one has a terrestrial and the other a cosmical source,&quot; and that it is &quot;the latter of these two systems which, by its progressive translation, gives rise to the phenomena of secular change and to those magnetic cycles which owe their origin to the operation of the secular change,&quot; con curring with the conclusion of Walker that &quot;the magnetic influence at any point of the globe is the result of two distinct magnetic systems, the principal of which is the magnetism proper of the globe, having its (northern) point of greatest attraction in the north of the American continent, whilst the weaker system is that which results from the magnetism induced in the earth by cosmical action, and of which the northern point of greatest attraction is at present in the north of the Asiatic continent. Thus the direction of the magnet at any point results from the super position of these two systems, the nearest pole being always pre dominant over the more remote&quot; (Phil. Trans., 1868). While dis posed to think that something of this nature should be accepted as a working hypothesis, we would, however, point out that the Asiatic pole cannot be regarded as accounting for all the pheno mena of disturbances, but that the focus of disturbance is probably nearer the focus of auroras than it is to either of the foci of magnetic intensity. The right-hand curves representing these disturbance-diurnal variations which have two maxima are, except for Port Kennedy and Point Barrow, decidedly irregular. Sabine remarks also that, instead of having a reference to absolute time like those with one progression, their reference is rather to local time. We have therefore plotted all these curves according to local time ; nevertheless this reference does not come out with very great distinctness ; but it must be remembered that our analysis of disturbances into easterly and westerly, although, in the hands of Sabine, it has given us much new information, has no claim to be regarded as final and complete. 56. Results in the Southern Hemisjthcrc. Table VIII. shows the disturbance-diurnal variation of declination exhibited for St Helena, 15 56 7 S. lat., 5 40 5W. long.; Cape of Good Hope, 33 56 S. lat., 18 28 75 W. long.; Hobart Town, 42 52 5 S. lat, 147 27 5 E. long. At St Helena and the Cape the easterly disturbances present the appearance of a single progression, while the same remark slightly modified applies to the easterly disturbances at Hobart Town. Again the times of easterly maxima for St Helena and the Cape are very nearly simultaneous, while Hobart Town, which we may regard as situated on the opposite side of the chief southern magnetic centre from St Helena and the Cape, has its maximum nearly coincident in absolute time with the minimum of the other two stations. It would thus seem that the chief magnetic centre of the south is similar in its action as regards these phenomena to the chief mag netic centre of the north. Again the absolute time of single maxi mum for the south as determined by St Helena and the Cape is about twelve hours different from the corresponding time for the north as determined by Kew, Toronto, Peking, and Nertchinsk. All this is in favour of the working hypothesis already mentioned.