178 [TERRESTRIAL MAGNETISM. nearly at right angles, in the polar regions five such arches have been seen at once ; (3) slender luminous beams or columns well- defined and often of a bright light; (4) the corona, the centre of which is invariably near the magnetic zenith, but not always exactly coincident with it; and (5) waves or flashes of light. 90. Auroras exhibit the same annual variation as magnetic dis turbances, and are most frequent about the equinoxes a fact first observed by Maizan. Kaemtz in his Meteorology gives the follow ing table, which is applicable to European auroras. TABLE XXVI. Monthly Frequency of European Auroras. Jan. Feb. Mar. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 229 307 440 312 184 65 87 217 405 497 285 225 Loomis again in the memoir already quoted gives the distribu tion of American auroras over the various months derived from one hundred and thirteen years observations at New Haven and Boston, twenty-five years observations at New York, and two years obser vations in Canada. His results are represented in the following table : TABLE XXVII. Monthly Frequency of American Auroras. Boston and Now Haven. Xew York. Canada. Sum. SI 7(> 16 173 93 86 31 210 no 106 24 240 104 125 38 267 86 83 22 191 83 79 17 179 July 123 100 21 244 102 122 14 238 September 143 131 19 293 99 110 27 236 115 74 26 215 83 60 16 109 It appears from this table that American like European auroras exhibit a maximum of frequency about the equinoxes. 91. Since auroras and magnetic disturbances go together, it is natural to imagine that we should have great auroral displays in years of maximum sun-spots. This is found to be the case, and in the following table (XXVIII.) Wolfs proportional numbers denot ing sun-spot frequency are compared with the number of auroras witnessed in Europe and America as compiled by Loomis. It will be seen from this table that years of maximum auroras coincide very well with years of maximum sun-spots. Sun-Spot Auroral Number. Number. Sun-Spot Auroral Number. Number. Sun-Spot Number. Auroral Number. 1750 83-1 81 1794 38-0 14 1837 136-9 42 1751 52-1 30 1795 23-8 7 ls:;s 104-1 60 1752 45-9 17 1796 15-6 8 j 1839 83-4 65 1753 28-9 15 1797 6-5 5 1840 61-8 80 17-VI 13-5 11 1798 4-6 6 IS 11 38-5 67 1755 9-3 10 1799 7-1 4 IS 12 23-0 53 1756 12-2 9 1800 15-6 6 1843 13-1 37 1757 31-9 7 1801 33-9 6 1844 19-3 39 1 7:.S 47-1 14 ISIfJ 54-7 6 1845 38-3 48 1759 54-6 16 1803 70-7 6 1846 59-6 54 1760 64-7 23 1804 71-4 14 1847 97-4 76 1761 80-2 22 1805 48-0 14 1848 124-9 72 1762 60-0 19 1806 28-4 13 1849 95-4 69 1763 48-4 16 1807 11-1 4 I 1850 69-8 49 1764 36-7 11 1808 7-2 2 1851 63-2 65 1765 21-4 8 1809 3-1 1 1852 52-7 67 1766 14-1 5 1810 o-o 1 1853 38-5 63 1767 35-9 9 1811 re
1854 21-0 35 1768 66-8 30 1812 4-9 1 1855 7-7 20 1769 103-4 40 1813 12-6 4 1856 5-1 11 1770 98 !) 41 1814 16-2 5 1857 22-9 21 1771 86-6 24 1815 35-2 5 < 1858 56-2 37 1772 60 -7 26 1816 46-9 5 i 1859 90-3 50 1773 39-7 33 1817 39-9 7 1860 94-8 48 1774 27-4 88 1818 29-7 11 1861 77-7 36 1775 8-8 22 1S19 23-5 10 1862 61-0 28 1776 21-7 26 1820 16-2 8 1863 45-4 29 1777 92-0 38 1821 6-1 4 1864 45-2 33 1778 151-7 69 1822 3-9 2
- 1865
31-4 34 1779 123-4 70 1823 2-6 1 1866 14-7 31 1780 89-2 67 1824 8-1 1 1867 8-8 22 1781 66-5 57 1825 16-2 4 1868 36-8 32 1782 38-7 57 1826 35-0 12 1869 78-6 1783 22-5 47 1827 51-2 17 1870 131-8 1784 10-3 39 1828 62-1 21 1871 113-8 1785 26-7 56 1829 67-2 25 1872 99-7 1786 81-2 84 1830 67-0 25 1873 67-7 1787 128-2 108 1831 50-4 20 1874 43-1 1788 133-8 105 IS.T. 26-3 18 1875 18-9 1789 116-9 84 1833 9-4 11 1876 11-7 1790 90-6 68 1834 13-3 12 1877 11-1 1791 67-6 46 1835 59-0 15 1878 3-8 1792 59-9 37 1836 119-3 32 1879 7-7 1793 47-3 23 Iii fig. 39 a graphical representation is given of the likeness which subsists between the progress of auroral frequency, spot frequency, and declination ranges. 92. While the results now given leave little doubt as to the fact of a connexion of some sort subsisting between sun-spots on the one hand and mag netic disturbances and auroras on the other, yet it is desirable to obtain evidence as to the closeness of the connexion between auroras and sun spots similar to that which was exhibited in 82, and which showed the close connexion in point of time between sun -spots and disturbances, i Loomis has with this view treated auroras in precisely the same way in which he treated disturbances, and has obtained the following table : TABLE XXIX. Extent of Spotted Solar Surface. Mean of 2ol Days. Days before Aurora. 65 4321 - . 3 52 . 7 51 . 51 . 2 ^ 63 . Days after Aurora. 123456 54-8 52-5 53-3 51-4 53-3 50 7 From which he concludes that "auroral observations in the middle latitudes of America are generally accompanied by a maximum disturbance of the sun s surface on the very day of the aurora." 93. Earth Currents. These are electrical currents which take- place in the moist crust of the earth, and were first detected by W. H. Barlow (Phil. Trans., 1849). At a later period they were systematically observed and studied by C. V. Walker (Phil. Trans. , 1862). They are now continuously recorded by photography at the Royal Observatory, Greenwich. Earth currents are particu larly strong during magnetic disturbances. Sir George Airy has- graphically compared together certain magnetic disturbances as recorded by the Greenwich self-recording magnetographs and the simultaneous earth currents recorded by appropriate galvanometers (Phil. Trans., 1868), and finds it almost impossible to avoid the con clusion that the magnetic disturbances are produced by terrestrial galvanic currents below the magnets. The likeness between the two systems of graphical representations is unquestionably very strik ing. But, while there is no doubt an intimate connexion between earth currents and magnetic disturbances, there is one circum stance which should make us pause before assigning the former as the complete and efficient cause of the latter. It is thus indicated by Lloyd: 1 " When we examine the curves in which Mr Barlow has represented the course of the galvanometric deflexions caused by the earth currents, we observe that the regularity of that course is continually interrupted by rapid reciprocating move ments in which the needle oscillates from one side to the other of the zero- alternately. These movements are similar to those of the magnetometers with which we are familiar; but they are much more rapid, and bear a larger propor tion to the regular changes. ... I have selected for calculation the obser vations made during the six hours commencing at 3 A.M. on May 29, 1848, that being a period of comparative disturbance. The sum of the changes of the galvanometer needle during that period, on the Derby and Rugby line, was equivalent to 571 divisions of the instrument the mean daily range for the entire week being 1T4 divisions and the ratio=50. . . . The sum of the changes of the Greenwich declinometer during the same period was only 57 minutes, the mean daily range being 12-4 minutes. In like manner the sum of the changes of the horizontal force was -0158 and the mean daily range -0034. The ratio is accordingly the sam:- for the two magnetic elements, and its amount is 4 6, or less than one-tenth of the corresponding ratio in the case of the galvano- metrical changes. We learn therefore that the rapid changes of the earth currents are much greater in proportion to the regular daily changes than the corresponding movements of the magnetometers." We shall return to this subject in a subsequent part of this article. 94. Inequalities in Terrestrial Magnetism caused by the Moon. Kreil in 1841 was the first to point out that the moon lias a small influence on the position of the declination needle, and shortly afterwards the same fact was independently discovered by Joint Allan Broun. The more recent observations of Sabine and of Broun, but especially those of the latter, have thrown much light upon the nature of this action. As the lunar influence is not generally large, it is necessary to free the observations from the results of other inequalities, and this has becndone by the two- observers above mentioned. The results given in Table XXX. have been obtained by Sabine (see Walker s Magnetism). 95. Thus (1) the mean effect of the moon upon the declina tion needle is to cause in each lunar day a double oscillation, and Sabine has shown that the lunar influence upon the other magnetic elements is of a similar type. (2) The turning points for both hemispheres are in all cases not far removed from the lunar hours
Trans. Roy. Irish Acad., xxiv. 115.