Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/182

Rh 172 METEOROLOGY [TERRESTRIAL MAGNETISM. TABLE VIII. Local Astro nomical Hours. St Helena. Cape of Good Hope. Hobart Town. Easterly Ratios. Westerly Ratios. Easterly Ratios. Westerly Ratios. Easterly Ratios. Westerly Ratios.

3-24 2-46 2-1 1-6 1-14 0-65 1 3-17 2-39 2-1 1-2 1-26 0-64 2 2-79 1-88 1-6 1-0 1-32 0-71 3 2-00 1-44 1-0 0-8 1-40 0-56 4 0-89 1-29 0-8 0-7 1-39 0-56 5 0-34 0-76 0-4 06 1-32 0-52 6 0-14 0-45 0-4 0-8 1-16 0-72 7 0-05 0-50 0-1 1-2 0-62 1-04 8 0-03 0-44 0-1 1*3 0-40 1-31 9 0-03 0-37 0-2 1-2 0-32 1-79 10 0-07 0-43 0-1 1-1 0-28 1-96 11 o-oo 0-42 0-2 0-8 0-74 2-31 12 o-oo 0-31 0-3 0-7 0-62 2-05 13 o-oo 0-32 0-4 0-6 0-55 1-72 14 o-oi 0-24 0-2 0-6 0-63 1-52 15 o-oo 0-29 0-4 0-5 0-85 1-26 16 o-oo 0-28 0-4 0-4 1-07 0-84 17 0-08 0-24 0-5 0-4 0-87 0-47 18 0-39 0-42 1-0 0-8 1-02 0-44 19 0-87 0-89 1-8 1-2 1-53 0-53 20 1-52 1-52 2-3 1-4 1-58 0-70 21 2-51 1-72 2-3 1-7 1-41 0-55 22 3-08 2-21 2-5 1-8 1-27 0-55 23 2-78 2-60 2-7 1-7 1-24 0-G2 Finally, the westerly disturbances at the three southern stations bear greater marks of a double progression and of irregularity just as they did in the northern hemisphere, and moreover like their northern analogues they are regulated by local rather than by absolute time. 57. Distribution of Declination Disturbance over the Various Months of the Year. Broun was probably the first to remark in reducing the Makerstoun observations that the disturbances were greatest at the equinoxes and least at the solstices. His method was to find for each month the mean diurnal inequality, and then to consider the difference of each individual observation from the monthly mean for that hour as a disturbance, the summation of all such differences for the month denoting the monthly disturbance value. The following table embodies the results at various stations those at Toronto, Hobart Town, and the Cape being given by Sabine, and that at Bombay by C. Chambers, who has pursued Sabine s method of separating disturbances : TABLE IX. Monthly Distribution of Declination Disturbances. Toronto. Bombay. Cape of Good Hope. Ilobart Town. Easterly. Westerly. jj

H Westerly. Easterly. j&amp;gt;i 1^ ~ 73 H jj&amp;gt; 0-55 0-81 0-97 1-23 0-91 0-83 1-35 1-37 1-63 1-12 0-70 0-50 0-57 0-85 0-89 1-24 0-93 0-55 1-13 1-17 1-66 1-17 0-88 0-08 0-84 0-89 1-29 1-04 0-57 0-73 1-18 1-G4 1-20 1-52 0-40 0-68 0-88 0-67 0-93 1-29 1-00 0-82 1-83 1-29 1-04 1-31 0-41 0-53 2-1 1-7 0-7 1-3 0-3 0-3 0-6 0-4 0-8 1-2 1-2 1-2 1-4 1-3 1-1 1-6 0-9 0-4 0-6 0-4 0-9 1-0 1-0 1-2 1-62 1-16 I ll 1-26 0-65 0-30 0-51 0-84 1-29 1-22 0-73 1-29 1-54 1-05 1-11 1-18 0-51 0-32 0-54 0-73 1-50 1-27 0-95 1-29 April May July October November 58. A careful inspection of this table, without attempting a more complete analysis, will, it is thought, lead to the following con clusions : (1) Although for any station the distribution of the easterly dis turbances over the various hours of the day is generally different from that of the westerly, yet the same law of distribution over the various months of the year is followed by the easterly and by the westerly disturbances at any station the law at one station being, however, different from that at another. (2) In all stations there is first an annual inequality exhibiting a maximum generally a short time after the summer solstice with a corresponding minimum for the winter solstice, and secondly a semi annual inequality exhibiting a maximum generally a little after each equinox. (3) The equinox maximum is very conspicuous at Toronto ; but the summer maximum is most conspicuous at the other stations. 59. In 38 it was observed that the observations selected as dis turbed at any station may nevertheless be a mixture of what may be termed true disturbances and of the more prominent specimens of magnetic weather. The truth of this statement would appear to be borne out by the laws now given. In one of these we find that dis turbances, at all stations, have a maximum about the time of the summer solstice and a corresponding minimum about the time of the winter solstice. But the absolute time of the summer solstice for stations north of the equator corresponds with that of the winter solstice for stations south of the line. It would therefore appear that in so far as this law is concerned such disturbances lack the element of simultaneity. On the other hand, a law of this nature would naturally hold for magnetic weather. For at any station the diurnal range of declination is greatest at the summer solstice, and hence any considerable proportional variation of this would, if repre sented by a fixed scale, present the appearance of being greatest likewise at this time. The question thus arises whether this law does not rather apply to magnetic weather than to real disturbance. Again the semiannual inequality of disturbance exhibits through out the globe a maximum at the equinoxes, and thus presents the element of simultaneity which was wanting in the annual. This law may therefore refer to true disturbance, and this view is sup ported by the fact that the aurora which may be regarded as the universal accompaniment of great and simultaneous disturbances obeys, as we shall afterwards see, in those stations where it has been well observed, this very same law, that is to say, it has like wise maxima at the equinoxes. 60. Distribution of Declination Disturbances over Various Years. In 1852 Sabine discovered (Phil. Trans., 1852, p. 103) that dis turbances have a long-period inequality allied to that of sun-spots in such a way that a maximum and a minimum of disturbance coincide with a maximum and a minimum of sun-spot frequency. This will be seen from the following table (X.), in which we have the relative values of declination disturbance at Toronto and Hobart Town compared with the number of groups of spots observed on the sun s disk : Values of Declination Disturbance. Groups of Sun-Spots. Toronto. Hobart Town. 1843 0-55 0-48 34 1844 073 0-82 52 1845 0-62 0-67 114 1846 1-26 1-03 157 1847 1-40 1-44 257 1848 1-43 1-60 330 61. The following table (XL) exhibits the same thing for Bombay. The first column of this table is derived from the magnetic results of C. Chambers, while the sun-spot areas are those of Messrs De la Paie, Stewart, and Loewy. Aggregate Values (in Minutes) of Declination Disturbances. Sun-Spot Areas. 1859 15321 1352 1860 1421-6 1313 1861 951-8 1297 1862 1240-5 1211 1863 691-1 676 We may conclude from these tables that declination disturbances march with sun-spots, but that the alliance between these two phenomena is probably not so intimate as that between declina tion ranges and sun-spots. 62. Distribution of Declination Disturbances over the Surface of the Globe. It is well known that disturbances are comparatively small near the equator, while they are great near the magnetic poles, and greatest of all perhaps near the position of maximum auroras. If we adopt Sabine s system of separating disturbed from undisturbed observations, it is thus clear that the same separating value cannot be adopted at all stations. At first sight this would seem to introduce an element of uncertainty in the estimation of disturbances, but it was soon found by Sabine that no very great nicety is required in this matter. Not only do the laws which regu late disturbances at a given station remain comparatively unaffected by the magnitude of the separating value, but it is likewise easy to tell whether the aggregate disturbance value at one station is de cidedly greater or loss than at another. Probably at present it would be impossible to obtain more definite information than this. 63. The following table (XII.) exhibits the proportion between the aggregate amount of easterly and that of westerly disturbances of the declination at various stations in both hemispheres : Name of Station. Easterly. Toronto 1 40 Point Barrow 1 63 Port Kennedy 1 85 Carlton Fort 1 74 Kew M9 Peking 1 Bombay 1&quot;6 St Helena 1 Cape of Good Hope 1 Hobart Town 1 Falkland I.-les 1 66 Westerly. 1 1 1 1 1 1-21 1 1-30 1-51 1-40 1 64. Annual Variation of Declination. The declination fluctua-