Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/179

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 169 at length the monthly values of the solar-diurnal variation at Bombay. Broun has likewise (Trevandrum observations) discussed at length the solar-diurnal variation at the Trevandrum Observa tory. It would hardly be of service to reproduce here the results of these discussions ; but when such analyses become sufficiently extensive they may &quot;be expected to throw light upon the cause of the solar-diurnal variation. In the following table we have mean monthly values of the declin ation range at the Kew Observatory corresponding to forty-eight points in the year derived from sixteen years observations : TABLE I. Containing Monthly Means (unit ^ 22 -Oi) for Forty- eight Points in the Year of the Kcw Solar-Diurnal Declination Ra.ii.gcs Thus January (0) gives the monthly mean of which the middle date is the very commencement of the year, January (1) tliatjor one week after the commencement, and so on. Mean Value. Mean Value. Mean Value. Jan. (0) 312 May (d) 599 Sept. (0) 5!&amp;gt;4 (1) 323 (1) 581 (D 577 (2) 340 (2) 573 (2) 554 (3) 362 (3) 586 ( 532 Feb. (0) 385 June (0) 596 Oct. (0) 513 (1) 401 (1) 605 (1) 496 (2) 418 (2) 610 (2) 478 (3) 438 (3) 604 (3) 463 March (0) 4(i7 July (0) 601 Nov. (0) 445 (l) 508 d) 597 (1) 418 (2) 548 (2) 591 (2) 389 (3) -.587 (3) 593 (3) 360 April (0) 015 Aug. (0) 594 Dec. (0) 340 (1) C32 (1) 601 0) 322 (2) fi3!) (-&amp;gt;) 611 (2) 308 (3) G-&amp;gt;0 (3) 606 (3) 308 It will be seen from this table that, while we have a maximum about the summer and a minimum about the winter solstice, we have unmistakable indications of maxima at or about the equinoxes. This does not take place at a tropical station such as Trevandrum. 45. Behaviour near the Magnetic Pole. Figs. 33 and 34 exhibit the most prominent features of the solar-diurnal variation of declina tion in the extra-tropical regions of the northern hemisphere. If an observer stand over the centre of the needle and look towards the marked cud, or that which points to the north, he will perceive a deflexion towards his right hand which will reach its extreme about 8 A.M. and a deflexion towards his left hand which will reach its extreme about 2 P.M. But are these deflexions to the right and left hand of geographical or of magnetical north ? This question has been answered by Sabine in his discussion of the results of hourly observations of the magnetic declination at Port Kennedy (Phil. Trans., 1863, p. 660). This station is 72 49&quot; N. lat. and 94 19 W. long., and here the marked end of the needle, while it points towards the magnetic pole, points in reality about 35 to the west of south. Now the marked end of the needle when viewed at 8 A.M. is seen at Port Kennedy to have moved to the geographical west but to the magnetical east. It would thus seem that throughout the extra-tropical regions of the northern hemisphere the 8 A.M deflexion of the needle is always towards the magnetic east but not always towards the geographical east, while the deflexion at 2 P.M. will always tend towards the magnetical west but not always towards the geographical west. In fine the oscillations have reference to the north magnetic pole of the earth and not to the north geographical pole. No observations of this nature have been made in the southern hemisphere. 46. Long- Period Inequalities of Declination Range. It was first observed by Lament that the yearly values of the diurnal range of magnetic declination at Munich presented signs of a long-period variation. In 1852 Sabine (Phil, Trans., 1852, p. 103) showed that this inequality corresponded in its progress with that of the frequency of black spots on the surface of the sun. The existence of black spots on the disk of the sun was long ago known to the Chinese. In Europe they were first scientifically observed after the invention of the telescope, and it was deduced from their behaviour that the sun revolves about his axis in about twenty-six days. Hofrath Schwabe of Dessau, from a long series of forty years observations of the sun, was the first to show that the state of the sun s surface as regards spots was not uniform, but that their frequency was subject to an inequality the average period of which was about eleven years. Other inequalities both of longer and shorter periods have been supposed to exist, but the eleven- yearly period is the most prominent and is best assured. Although the sun-spot catalogue of Schwabe is the first with pretensions to rompleteness, yet Professor Rudolf &quot;Wolf has endeavoured to render observations of sun-spots made at different times and by different observers comparable with each other, and has formed a list exhibit ing approximately the relative number of sun-spots for each year. This list extends back into the 17th century, and is of great value in confirming past all doubt the existence of the eleven-yearly period. It will appear below that the sun is probably to be regarded as giving out most light and heat at those times when sun-spots are most frequent. The most accurate and now universally adopted method of estimating sun-spots is to take the spotted area expressed in millionths of the sun s visible hemisphere. To return from this digression, the correspondence between sun- spots and declination ranges detected by Sabine was of such a nature that years of large declination range agreed with those of many sun- spots, and vice versa. In the same year with Sabine (1852) Dr Rudolf Wolf and M. Gautier independently remarked the same coin cidence. Subsequent discussions have entirely confirmed the fact of this connexion, and in May 1879 William Ellis (Phil. Trans., 1880, p. 541) showed that the observations made at the Greenwich Observatory during the years 1841-77 indicated a relation of this nature between the diurnal ranges of horizontal force as well as those of magnetic declination on the one hand and the amount of sun-spot frequency on the other. The general character of this coincidence between sun-spot frequency and declination range is exhibited graphically in fig. 39 below. 47. Ratios of Ranges in Years of Maximum and Years of Mini mum Sun- Spot Frequency. Broun (Trans. Roy. Soc. of Edin., vol. xxvii. ) has shown that the ratios of the diurnal ranges of declination in years of maximum to those in years of minimum sun-spot frequency for places widely apart on the surface of the earth are very nearly alike. This will be seen from the following table : TABLE VII. Ratios of Declination Ranges in Years of Maximum and of Minimum Sun-Spot Frequency. Place Mean /rnax.1 Ratio min.y Observer. Paris 1-71 Cassini and Arago. Gottingen 174 Gauss. Munich 1-66 Lament. Dublin 1-52 Llovd. Hobart Town 1-57 Kay. Toronto 1-51 Younghusband and Lefroy. Trevandrum 1-56 Broun. 48. Closeness of Correspondence Lagging behind of Ranges. Stewart has shown from a discussion of the declination ranges at Kew, Trevandrum, and Prague (Proc. Roy. Soc., March 22, 1877, February 8, 1878, May 16, 1878) that this correspondence between the state of the sun s surface and the diurnal range of declination extends to inequalities of short period as well as to that of which the period is approximately eleven years, but that a particular state of the sun s surface precedes in point of time that of the declination range to which it corresponds, in fine, that the solar cause precedes the terrestrial effect, which latter lags behind to an extent that is sometimes considerable. These conclusions have been confirmed by Ellis (ut sujrra), and have likewise been extended by him to the horizontal force. The close nature of this correspondence, as well as the lagging behind of the terrestrial magnetic effect, will be seen from fig. 35. There are indications that this lagging behind of the magnetic effect is greater for sun-spot inequalities of long than for those of short period, a method of behaviour quite similar to what we find in meteorological phenomena. 49. A nalysis of Long-Period Inequalities. We possess no sun-spot XVI. 22