Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/178

Rh 168 METEOBOLOGY [TERRESTRIAL MAGNETISM. declination is concerned. When the diurnal oscillation of a freely- suspended magnet was first observed, the subject of magnetic disturbances was not understood, and the early individual deter minations which have been handed down to us are not such as to justify the expenditure of any very great labour upon them for the purpose of separating the disturbed from the undisturbed obser vations. Inasmuch, however, as the total diurnal inequality of declination (which is in reality the element given by these early observations) does not greatly differ from the solar-diurnal varia tion, we may with much justice and little risk of error give the history of these early observations in connexion with that of the solar-diurnal variation of declination, which is by far the best known, and perhaps the most important, of all the various magnetic changes produced by solar influence. 40. Solar-Diurnal Variation of Declination. Graham, an instru ment maker of London, discovered in 1722 that a freely-suspended magnetic needle is subject to a diurnal oscillation of definite char acter. 1 The next observer was Canton, who in 1756 began a series of nearly four thousand observations, which he communicated to the Royal Society on December 13, 1759, and from which he concludes that the range of the diurnal variation is greater in summer than in winter. Macdonald s observations at Fort Marlborough in Sumatra in 1795 (Phil. Trans., 1796), and Duperrey s in the tropics in 1825, were perhaps the first that might lead us to conclude that the amplitude of the diurnal oscillations of the needle is less in the tropics than in middle latitudes, and that the motion of the needle in the southern hemisphere is in the opposite direction to that in which it moves in the northern .hemisphere at the same hour. 41. Semiannual Inequality.^ The existence of these early observations had led some magneticians prematurely to conjecture that there must be a line somewhere near the equator at which there is no horary variation in declination. In 1847 Sabine com municated to the Royal Society the results of five years observations at St Helena, showing that at that station for the half of the year beginning at the vernal and ending at the autimnal equinox the motion of the needle corresponds nearly to that in the northern hemisphere, whilst for the other half it corresponds nearly to that in the southern hemisphere. Sabine afterwards confirmed and extended his conclusions regarding the semiannual inequality by discussing the results obtained at the various colonial magnetic observatories. More recently, as the result of twelve years observations at Trevandrum, at an observatory established by the rajah of Travancore, John Allan Broun gave in a very complete form the laws of change of the solar-diurnal variation of magnetic declination near the equator, showing the extinction of the mean movement near the equinox. 42. Perhaps the best way of exhibiting what really takes place is the following, which is that adopted by Sabine. The mean annual value of the solar-diurnal variation is of what may be called the northerly type in places of middle latitude in the northern hemisphere, and of what may be called the southerly type in places of middle latitude in the southern hemisphere. Now let us take a northern station, and consider the mean form of its solar- diurnal variation for the six months beginning with the vernal equinox. Here we shall have an oscillation of the northerly type with a range greater than the annual range. For these six months, therefore, we may imagine that the annual range has been supple mented by the superposition on it of a variation with a type similar to its own. At the same station, during the other six months, the solar-diurnal variation is less than the mean of the year, as if the annual variation had been depressed by the superposition on it of a variation with a type the opposite of its own, that is to say, with a southerly type. At a station in the southern hemisphere, again, the mean annual form of the solar-diurnal oscillation is of the .southerly type, reduced during the six months beginning with the vernal equinox by the superposition on it of a variation of northerly type, and increased during the other six months as if by the super position of a variation of southerly type. Thus when the sun is north of the equator we may superpose a variation of the northerly type upon both hemispheres, with the effect of increasing the range in the northern hemisphere and diminishing it in the southern ; and while the sun is south of the equator we may superpose a varia tion of the southern type upon both hemispheres, with the effect of diminishing the range in the northern and increasing it in the southern hemisphere. Near the equator, as at Trevandrum, where Broun made Ins observations, we find the mean annual value of the solar-diurnal variation to be extremely small, if not altogether evanescent. During the six months beginning with the vernal equinox the type is entirely northerly, while for the remaining six months of the year it is entirely soxitherly in character. In fine, at this station the solar-diurnal variation changes its character at the equinoxes, at which time we have, as already observed, an extinction of the mean movement, not indeed an absence of all variation, but rather a 1 See Walker, Terrestrial and Cosmical Magnetism. 2 Thin is the name used by Sabine. but its appropriateness may perhaps be questioned. variation having an undecided character, which for a few days may be of one type and then of the very opposite. There is movement, but no mean movement. 43. In the following table (V.) the solar-diurnal variation is given for Kew, Trevandrum, and Ilobart Town. Of these places the first denotes a station in middle latitude (northern hemisphere), the second an equatorial station, and the third a station in middle latitude (southern hemisphere). &quot;3 Kew. Trevandrum. Ilobart Town. g X gg IB April to Oct. to Whole April to Oct. to Whole April to Oct. to Whole &amp;lt; Sept. March. Year. Sept. March. Year. Sept. March. Year.

-6-15 -4-12 -5-13 -1-30 +0-07 -0-C1 +0-35 + 2-35 + 1-35 i -7-42 4-96 -6-19 1-25 +0-35 -0-45 + 2-15 +4-85 +3-50 2 -6-94 4-C7 -5-81 0-85 + 0-50 0-15 + 3-15 + 5-95 +4-55 3 -5-21 3 35 4-28 -0-35 + 0-61 + 0-13 +3-30 +5-50 +4-40 4 -3-25 -1-95 2-60 +0-03 +0-53 + 0-28 + 2-40 +4-30 + 3-35 5 -1-47 -1-05 -1-26 + 0-15 +0-33 +0-24 + 1-30 +2-70 + 2-00 6 -0-32 0-4C -0-39 + 0-05 + 0-22 +0-13 + 0-75 + 1-55 + 1-15 7 + 0-22 +0-21 + 0-22 -0-15 +0-23 +0-04 + 0-20 + 080 + 0-50 8 +0-44 +0-92 +0-68 0-30 + 0-19 -0-05 -0-30 + 0-30 +0-00 9 +0-52 +1-45 + 0-99 0-28 +0-13 0-08 -0-8. 1 ) 0-25 -0-55 10 +0-70 +1-77 +1-24 -0-20 + 0-09 -0-06 1-10 -0-70 -0-90 11 + 0-90 + 1-84 + V37 -0-07 +0-10 +0-01 -1-15 -085 -1-00 12 +1-19 + 1-67 + 1-43 + 0-07 + 011 + 0-09 1-10 0-80 0-95 13 +1-23 + 1-34 + 1-29 + 0-18 +0-08 + 0-13 -0-75 -0-75 -0-75 14 + 1-56 + 1-22 + 1-39 +0-27 + 0-02 + 0-15 -0-40 0-70 0-55 15 + 1-93 + 1-0!) +1-51 +0-29 -0-11 + 0-09 -0-15 -0-G5 -0-40 16 +2-58 +1-17 + 1-88 + 0-31 -0-28 +0-02 -0-02 -0-78 0-40 17 + 3-60 + 1-43 +2-51 + 0-48 -0-45 +0-01 o-io -1-40 -0-75 18 +459 +1-54 + 3-07 + 1-02 -0-66 + 0-18 0-23 -2-37 -1-30 19 +5-31 + 1-85 +3-58 + 1-48 -0-81 +0-32 -0-50 -3-80 -2-15 20 + 5-20 +2-40 4- 3 SO + 1-20 0-72 +0-24 -1-25 -5-25 -3-25 21 +3-57 + 2-32 +2-95 + 0-47 -0-36 +0-OC -2-10 5-30 3-70 22 +0-38 +0-54 + 0-46 -0-32 -0-13 -0-22 2-20 -3-80 -3-00 23 3-18 2-18 2-68 0-93 -0-07 0-50 -1-40 -0-80 1-15 In this table deflexions towards magnetic east are reckoned positive, deflexions towards magnetic west negative. The scale is in minutes of arc. Also in fig. 33 we have a graphical representation of the solar- diurnal variation for the whole year at these three stations, from which it will be seen that the range at Trevandrum is extremely small, and that the curve for Hobart Town is opposite in appearance to that at Kew. Fig. 33. Finally, in fig. 34 we have a graphical representation of the semi annual inequality or difference from the whole year s mean of the two half-yearly means of Table V., the one half-year (that with thick lines) commencing at the vernal and the other at the autumnal equinox. It will be seen from this figure that the semiannual inequality is of the same character in both hemispheres, the likeness extending even to its minor peculiarities. 44. Change from Month to Month. Charles Chambers, director of the Kolaba Observatory, Bombay, remarks ( Trans. Roy. Soc., December 10, 1868) that &quot; the regular progression from month to month in the diurnal variation is so distinctly shown in the Bombay observations as to lead, on a first inspection, to the supposition that the law of variation is identical throughout the year, the extent only (including a reversal of direction) varying from month to month. But in this respect a different exposition of the character of the variation in different months shows that the first thought would be inaccurate.&quot; He theii proceeds to discuss