Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/177

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 167 for the secular change by imagining a solid globe or terclla, 1 con centric with the earth but rotating independently of the external shell and having a slightly different period of rotation, the shell having two poles and the terella two others. While continuing to admire Halley s sagacity, we shall not now be disposed to allow such a constitution of the interior of the earth, but will rather be led to look to some external influence as the cause of the secular variation. While we have strong evidence that the Siberian focus has changed its position, we cannot assert that the American focus has been absolutely stationary, or that neither focus has experienced any changes of force. On these points we must be content to be guided by observation alone. 34. It has been supposed by some magncticians that it is possible to compute with something like certainty the particulars of the motions of the magnetic foci. Hansteen more especially (1811-19) computed both the geographical positions and probable periods of revolution of this dual system of foci of force round the terrestrial pole. Sir Frederick Evans has discussed in connexion with the sub ject all the most recent observations, 2 and points out two objections to any such theory as that of Hansteen, viz., (1) that, while a mag netic turning point has been reached in certain regions, there are large portions of the earth in which this change has not yet been accomplished, and (2) that in certain districts of the earth very great changes in force have taken place. &quot; If we turn,&quot; he says, &quot; to the continent of South America and its adjacent seas, we shall find a diminution of the intensity of the earth s force now going on in a remarkable degree. An examination of the recent observations made by the Challenger officers at Valparaiso and Monte Video, compared with those made by preceding observers, shows that within half a century the whole force has respectively diminished one-sixth and one-seventh, at the Falkland Islands one-ninth.&quot; On the whole, while there is strong evidence that the Siberian focus has until recently been travelling eastwards, and its analogue west wards, and evidence less conclusive that recently a turning point in this motion has been reached, we are disposed to think with Sir Frederick Evans that a formal theory like that of Hansteen does not agree with recent observations. We shall revert to this subject. 35. In Tables III. and IV. certain yearly values of declination, dip, and horizontal force are given for various stations. INEQUALITIES IN OR CONNECTED WITH TERRESTRIAL MAGNETISM DEPENDING ON THE SUN. 36. As there is a marked likeness between the ways in which the sun dominates over the two great divisions of terrestrial phenomena, meteorology and magnetism, let us endeavour to describe the .sun s effect upon the latter by referring to its influence on the former, the chief peculiarities of which are well known to all. We find that the temperature of the air at a given station is subject to a diurnal fluctuation having its minimum value shortly before sun rise and its maximum early in the afternoon. We find likewise that the mean temperature for the day, as well as the amplitude of this diurnal oscillation, depends upon the season of the year, both being greatest about midsummer and least about midwinter. Now, if this were the only manifestation of solar influence upon this particular element, it would be possible to predict the temperature for any hour of any day once the mean temperature, the diurnal variation of temperature, and the modification of these for different seasons of the year had been well ascertained. But this amount of regularity is very far from taking place, the march of temperature being frequently interrupted, cloaked, perhaps even reversed, by the advent of peculiar weather. Thus we may have very cold weather in midsummer and very warm weather in midwinter, or we may have a very cold afternoon and a very warm early morning, by which means the ordinary conditions of temperature will be completely reversed. In like manner weather interferes even to a greater extent with the diurnal oscillation of the atmospheric pressure, so that, in British latitudes at least, it is only possible to obtain this correctly by means of a long series of observations. Weather, however, does not consist of a perfectly lawless inter ference with periodical phenomena, but is subject to laws of its own, some of which we are beginning to discover. Sometimes weather may exalt or depress the diurnal fluctuation of temperature without otherwise affecting its character ; but sometimes too the turning- points and the general appearance of this fluctuation are greatly influenced by peculiar weather. 37. Now it is believed that we have something of this- kind in those fluctuations depending on the sun to which the elements of terrestrial magnetism are subject. Let us take the declination as the most easily studied of the three magnetic elements, and suppose that we are engaged in considering the traces denoting the fluctuations of declination as derived from a set of self-recording magnetographs in Great Britain. Here we shall at once be able to recognize in an unmistakable manner the diurnal variation depend- ing upon the position of the sun, in virtue of which a freely- 1 See Walker s Terrestrial and Cosmieal Magnetism, where the subject is well discussed. 2 la his lecture to the Royal Geographical Society, March 11, 1878. suspended magnetic needle reaches the easterly extreme of its range about eight in the morning, and the westerly about two in the afternoon. We shall likewise perceive that the range of this diurnal fluctuation is greatest at midsummer and least at mid winter. In fine, the characteristics of this fluctuation, depending as they do upon the hour of the day and the season of the year, are not very different from those exhibited in the diurnal fluctua tion of atmospheric temperature. But, however thoroughly we may have ascertained the mean declination and its diurnal oscilla tion, as well as the modifications of these depending on the season of the year, we shall nevertheless find it impossible to predict the exact position of a freely-suspended magnet at any moment of a particular day. Here then too we have something which may be called magnetic weather, and which interferes with the regular progress of the systematic fluctuations of the magnet. Magnetic weather has, like its meteorological analogue, a set of laws of its own, some of which we are beginning to find out. Sometimes magnetic weather may exalt or depress the diurnal fluctuation of declination without affecting its character, but it is imagined that at other times the turning points and general appearance of this fluctuation may be greatly influenced by peculiar magnetic weather. 38. There is, however, a kind of magnetic change which, so far as we know at present, is not analogous to anything in meteorology, and introduces an additional element of complexity in any attempt to analyse the fluctuations of terrestrial magnetism. We mean the well-known magnetic disturbances or storms which occur simul taneously in places very widely apart. Under these circumstances it becomes a question how we can best deal in a practical manner with this complicated system of things. We do not think that with our present knowledge any better system can be adopted than that first introduced by Sir Edward Sabine in his discussion of the results of the colonial magnetic observatories. Suppose that we have hourly magnetic observations at a station, then first of all we should arrange these into monthly groups each hour by itself. We should then reject as disturbed observations all those which differ by more than a certain amount from their respective normals of the same month and hour, the normals being the hourly means in each month after the exclusion of all the disturbed observations. This method enables us, by its exclusion of disturbances, to ascertain with much accuracy the true form of the solar-diurnal variation of the magnetic elements at a given place corresponding to every month of every year, provided only that the observations are sufficiently numerous. On the other hand it will probably fail in accurately giving us the variations from day to day of the ranges of these diurnal fluctuations caused by the advent of peculiar magnetic weather, inasmuch as the records of the extreme effects of such weather will probably be cut off from the undisturbed observations and reckoned among the distiirbances. For instance, it is known that the solar influence on terrestrial magnetism varies from year to year, and it is suspected that there are also short-period fluctuations of solar influence. It would not, however, be a safe proceeding to attempt to estimate numerically this last-mentioned element of fluctuation by taking the successive diurnal ranges of those observations at any station, reckoned as undisturbed, by the above process, and plotting them as successive ordinates of a curve, and then supposing that this curve would give us a true graphical representation of solar changes. It would rather probably represent such changes with the tops and bottoms of the larger fluctuations cut off. But if the undisturbed observa tions fail in this respect we can hardly be wrong in supposing that there has been eliminated from them, as far as possible, all influence due to magnetic storms, and hence that they will afford us a much better means of estimating small fluctuations, such, for instance, as those due to the moon, than we could have had without their aid. Finally, with regard to that portion of the observations selected as disturbed, we are probably not certain that every such observa tion represents a true disturbance, or that the absolute times of occurrence of the various observations selected as disturbed at one station will be the same as those at another. Nevertheless Sir Edward Sabine has shown that at the Kew Observatory certain laws of disturbance deduced from the whole body of observations selected as disturbed are closely reproduced when this selection is made on a narrower basis ninety-five days of prominent disturbance being alone taken. With these prefatory remarks we shall now proceed to discuss the diurnal inequality of terrestrial magnetism. 39. Total Diurnal Inequality Defined. It will be seen further on that disturbed as well as undisturbed observations are subject to a diurnal variation, but these two variations are different, and the name diurnal inequality is generally given to the compound varia tion which is the joint resultant of the two. Solar-diurnal variation is that portion of the compound inequality which refers to undisturbed observations, while that which refers to disturb ances has received the name of disturbance-diurnal variation. It would appear that in the United Kingdom, and perhaps through out Europe, the total diurnal inequality is not very greatly different either in character or range from its most important component the solar-diurnal variation, at least so far as the