Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/184

Rh 174 METEOROLOGY [TERRESTRIAL MAGNETISM. (1) The diurnal ranges of the magnetic elements of declination and horizontal force are subject to a periodical variation, the duration of which is equal to that of the known eleven-year sun- spot period. (2) The epochs of minimum and maximum of magnetic and sun- spot effect are nearly coincident, the magnetic epochs on the whole occurring somewhat later than the corresponding sun-spot epochs. The variations of duration in different periods appear to be similar for both phenomena. (3) The occasional more sudden outbursts of magnetic and sun- spot energy, extending sometimes over periods of several months, appear to occur nearly simultaneously, and progress collaterally. 71. Disturbance- Diurnal Variation of Force Components. We may derive the following conclusions from the results obtained by Sabine for the observatories of Toronto, Kew, and St Helena. For each element there are two categories, namely, those disturbances which tend to increase and those which tend to diminish the element in question. (1) At Toronto the disturbances increasing both elements of force well represent single progressions with maxima occurring for both about 4 or 5 hours local time. Again the disturbances decreasing both elements represent fairly well single progressions with maxima occurring for both at about 14 or 15 hours local time. (2) At Kew the disturbances increasing both elements represent well single progressions with maxima occurring for both about 5 hours local time. On the other hand, the disturbances decreasing the horizontal force represent signs of a double progression and those decreasing the vertical force signs of a single progression, the maximum for the latter falling between the two maxima for the former, and occurring at 14 hours local time. (3) There is not the same close correspondence between the progress of the disturbances which tend to increase both elements nor between the progress of those which tend to decrease both elements at St Helena as there is for the other stations, nor is there the same likeness between the numbers for St Helena and those of Toronto or Kew as there is between the numbers of Toronto and those of Kew. 72. The fact that the disturbance-diurnal variations of the two force elements at Kew are very like each other while neither of them is very like the corresponding declination variation ( 54) receives continuation from a visual inspection of the Kew curves. In the Philosophical Transactions for 1862 Stewart thus describes the result of an inspection of the disturbances of these curves for the years 1858, 1859, 1860 (disturbance years) : &quot; There are twenty-two cases in which the declination is raised or lowered along with the horizontal force, and only seven cases of an opposite description. Also there are twenty-two cases in which the declination is raised or lowered along with the vertical force, and only eleven cases of an opposite description. Finally, there are thirty-one cases in which both forces are raised or lowered together, and only two cases of an opposite description. There is therefore a decided tendency in the curves of all the elements to be raised or lowered simul taneously, but this tendency is stronger between the horizontal and vertical force curves than between either of these and the declination. It may at the same time be affirmed that with the exception of the disturtmrce of August to Septem ber 1SS9 there is no very prominent case in which the three element &amp;gt; do not rise or fall together.&quot; 73. Peaks and Hollows. These are certain small but abrupt mag netic changes which from the fact that they generally fall within the separating value are not usually regarded as disturbances. These changes can only be brought to light where there is a continuous record of magnetic phenomena such as that derived from self- recording magnetographs. They were first studied at the Kew Observatory by Stewart (Phil. Trans., 1862). We have seen that more than one type of force must be concerned in producing magnetic disturbances. This is confirmed by the appearance of the Kew records, from which it may be seen that no disturbance of any magnitude is due to the action of a single force varying merely in amount but not in direction. For if there were only one type oi force the distance at any moment of a point in the curve of one of the elements from its normal position should bear throughout a disturbance an invariable proportion to the distance of a correspond ing point in the curve of another of the elements from its normal but this is by no means the case. But even if several independent forces are at work it may be thought unlikely that at the same moment a sudden change shoult take place in all ; there is thus a probability that sudden changes of force, as exhibited in peaks and hollows, are changes in one o: the elementary forces concerned. Even if the change is not a very abrupt one, provided that we confine ourselves to such peaks an hollows as present a similar appearance for all the curves, we maj suppose that we are observing changes in one only of the elementary disturbing forces ; for it is unlikely that two or more independen^ forces, changing independently, should produce similar appearance in all of the three curves. Assuming it as probable that similarity of appearance in th curve variations of the three elements denotes a simplicity in the disturbing force, Stewart has discussed all such peaks and hollows at Kew extending over the first two years of their production, ant has obtained a result which is embodied in the following table: TABLE XVIII. Hourly Ratios and Frequency of the Kew Peaks and Hollows, the Vertical Force Disturbance being taken a. Unity. l Hour. Decli nation. Hor. Force. Number of Observations. Hour. Decli nation. Hor. Force. Number of Observations. 0- 1 2-14 2-06 7 12-13 1-76 2-6S 3 1- 2 1-97 2-16 7 13-14 2-00 2-04 3 2- 3 1-86 1-99 11 14-15 2-10 2-14 5 3- 4 1-81 2-05 7 15-16 2-65 2-11 10 4- 5 1-38 1-73 4 16-17 3-48 2-16 15 5- 6 1-57 1-71 1 17-18 3-80 2-14 22 6- 7

18-19 3-94 2-18 28 7- 8 1-82 1-91 2 19-20 3-97 2-25 21 8- 9 1-60 2-20 1 20-21 3-41 2-21 23 9-10

21-22 3-26 2-30 16 10-11 1-33 3-16 1 22-23 2-79 2-00 10 11-12 1-30 2-32 3 23-24 2-30 2-04 13 74. It will be seen from this table that the ratio between simul taneous peaks and hollows of the two components of the force is very icarly constant, the horizontal force disturbance being about double that of the vertical force, so far as size on the curve is concerned. It will also be seen that there is a very marked diurnal range in the ratio which the declination peak or hollow bears to that of the vertical force, this ratio being greatest about 7 A.M. About this hour we have also most peaks and hollows, while in the evening and early morning hours there is so great an absence of these phenomena that the ratios are doubtful. 75. A preliminary comparison between the peaks and hollows at Lisbon and at Kew has been made by Capello and Stewart (Proc. Roy. Soc., January 28, 1864) with the following conclusions. (1) The Kew peaks and hollows are simultaneously produced at Lisbon in all the elements, but to a smaller extent than at Kew. (2) The direction is the same at both stations for the declination and horizontal force peaks and hollows, but it is reversed in the case of the vertical force, so that a sudden small increase of vertical force at Kew corresponds to a diminution of the same at Lisbon. It would be manifestly impossible to discuss with any advantage the nature and origin of these peculiar changes until more exten sive observations of them have been made. As the peak and hollow force is probably of a simple nature, a further knowledge of its character may be of much importance to the theory of terrestrial magnetism. It is interesting to remark that we have in peaks and hollows the- same close relation between the variations of the two force elements that we find in the larger disturbances. It is believed too that during violent disturbances a certain change of type is produced in the peak and hollow force, and more especially is this remarkable in the great disturbance in August and September 1859, where the declination would seem to march in the opposite direction from the two components of the force. We have seen that the same peculiarity characterized on this occasion the larger and more apparent magnetic changes. We shall afterwards refer to a circumstance which may perhaps throw light upon this peculiarity ( 93), meanwhile we conclude by again remarking that during comparative magnetic calms the peak and hollow force shows signs of remaining constant in type, and that it is therefore of great importance that the directors of observatories possessing self- recording magnetographs should take united action to observe tins- force. 76. Other Inequalities of the Disturbance-Diurnal Variation of the Force Components. Sabine has shown that disturbances of the force components present a distribution over the various months of the year very similar on the whole to that which is exhibited by dis turbances of declination. He has likewise shown that disturbances of the force components present a distribution over various years similar to that exhibited by disturbances of declination. Finally, we may probably conclude that disturbances of the force components are smallest at those portions of the earth s surface where disturb ances of the declination are smallest, and largest at those portions where such disturbances are largest. 77. Annual and Semiannual Variation of Horizontal Force and Dip Broun (Trans. Roy. Soc. Edin. for 1861) has discussed the results obtained by Sabine at his magnetical stations, and has shown that differential and absolute observations agree in telling us that the horizontal force is smallest at the equinoxes and greatest at the solstices. Whipple has recently obtained the same result from the Kew observations. We have deduced the following table (XIX.) from the various absolute determinations that have been made at sundry places. In it the annual and semiannual variations of declination, horizontal force, and dip are exhibited, &quot;increase&quot; denoting a push to the west, and &quot; decrease&quot; a push to the east. The method of obtaining these has already been indicated in 66, 67. l We ought to mention that, with the exception of the one occasion already alluded to, a peak of one element always corresponds to a peak of another ele ment, and a hollow of one element to a hollow of another.