Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/183

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 173 tions of short period hitherto discussed are not necessarily accom panied by a permanent change of mean position of the needle. We have now to inquire whether there be any fluctuations of long period (besides the secular change discussed in 30-33) tending to alter perceptibly the position of the magnetic needle. This leads us at once to the annual variation, for our knowledge of which we must look to the later-made and more accurate observations, in which all possible sources of error have been carefully eliminated. Broun has made an exhaustive experimental inquiry into the various sources of error which could possibly influence his declination needle at Trcvandrum. His conclusion was that the variations of torsion of a well-made thread are not sufficient to produce a sensible effect upon the position of a powerful magnet. In fact Grubb s magnet, weighing 6000 grains, and Adie s, weighing 1100 grains, give almost identical results. We may extend these con clusions to other observatories where well-devised instruments have been established, and look with much confidence to such instru ments registering correctly the secular as well as the annual change of declination that may be taking place at each locality. 65. The following table (XIII.), borrowed, with the exception of the Trevandrum and Bombay results, from E. Walker s Terrestrial Magnetism, shows the annual variation at seven stations: .Mean Declination. Mean Annual Secular Change. Observation Years. Kew 21 39 W. 7 39-00 E 1858 62 Hobart Town 9 56 E. 1 23-20 W 1844 48 St Helena 23 27 W. 7 57-00 W. 1841-49 The Cape 29 7 W 29-40 W 1841 46 Toronto 1 35 W. 1 57-12 W 1845 51 35 E 1 35-4 E 1854 69 Bombay 31 E. 3 1-0 E 1859 C5 TABLE XIV. Showing the Mean Annual Variation for each Month of the Year at Seven Stations. fc M Toronto. St Helena. Cape of Good Hope. Hobart Town Trevandrum. Bombay. Grubb. Adie. April + 1-5 -41-8 -50-6 -70-3 -20-7 + 9-8 +49-6 +34-8 + 39-6 + 2-6 +34-2 +29-8 - 0-6 9-6 -17-4 -42-6 - 4&quot;2 +47-4 +61-0 +24-6 + 19-8 + 4-2 + 0-6 - 7-8 - 2-4 1-2 -13-8 + 8-4 3-6 -19-8 + 1-8 + 7-2 + 7-2 3-0 + 18-0 + 10-2 +64-2 -10-8 -62-4 58-2 -61-8 43-2 - 4-8 + 25-2 +42-6 + 29-4 +43-2 +49-8 -22-2 28-7 24-1 20-6 12-2 - 4-5 + 13-6 + 27-6 +38-5 + 32-9 + 7-9 + 16-7 + 1-2 + 3-4 + 5-6 + 1-6 2-8 - 8-7 8-0 + 1-2 + 3-3 + 7-1 + 2-3 - 4-9 + 6-3 + 8-7 + 8-7 + 2-2 3-2 -10-5 11-1 3-0 1-3 + 3-2 + 2-6 - 1-2 + 11-0 + 16-3 + 9-8 + 1-7 + 1-6 + 0-4 8-9 + 1-2 18-2 -10-9 + 3-1 - 7-0 May June July August September... October... . November. December. January... . February. . March.. Here + indicates that the marked pole of the needle is to the west and - that it is to the east of its mean position for the year. 66. To cancel the irregularities of this table let us take the means from April to September and from October to March, the former embracing the months around the June solstice and the latter those around the December solstice (Table XV.): Means from April to September. Means from October to March. Kew 2S&quot;7 Toronto .... 4 5 4-17-1 St Helena... 5 4 + 6-3 Cape of Good Hope 28 7 -t-30 9 Hobart Town 18 7 + 17-3 Trevandrum + 2 1 16 Bombay + 6-8 6-8 It will be seen from the above that the means for Trevandrum and Bombay present opposite signs to those for the other stations. The whole amount for Trevandrum is no doubt very small, and Chambers does not regard the evidence for Bombay as conclusive ; but on the whole it would appear that two observatories near one another present evidence of a similar behaviour in declination, and we are therefore disposed to regard it as a reality. 67. Semiannual Variation of Declination. If we look at the numbers of Table XIV., we shall see that there are traces of turning points at the equinoxes. Let us, in order to exhibit this, compare together the sums for the six months grouped around the two equinoxes with those for the six months grouped around the two solstices that is to say, compare the sums for February, March, April, August, September, October, with those for November, December, January, Mav, June, Julv and we thus obtain the following table (XVI.): Sums around Equinoctial Months. Sums around Solstitial Months. Kew + 104-2 85 7 + 96 4 St Helena + 4 2 + 4 8 Cape + 47-4 34-2 Hobart Town 0-7 _l_25~6 Trevandrum 19 -t-20 3 liombay + 0-2 O l 68. Solar-Diurnal Variations of the Horizontal and Vertical Com ponents of Magnetic Force. Although self-recording magnetographs have been established in many observatories throughout the globe, yet, owing to the peculiar difficulties of the task, and the labour of the process of reduction, very little has been done towards determin ing the solar-diurnal variation of the horizontal and vertical compon ents of the earth s magnetic force. Senhor Capello of the Lisbon Observatory has, however, made progress with his reductions, and has already published valuable information regarding the solar- diurnal fluctuation of the two force elements at his observatory. In his attempts to eliminate the disturbances of horizontal and vertical force by the method of Sir E. Sabine, Senhor Capello has experienced considerable difficulty, more particiilarly with the records of the vertical force magnetograph. This instrument and the bifilar have very often been found by him to change their position of equilibrium after strong perturbations. Again there is generally, for any hour, a variation at the beginning and end of the month from the monthly normal value for that hour owing to change of temperature, and this cannot be completely corrected inasmuch as the coefficient of temperature is not exactly known. These two causes combined tend to falsify the results when the plan adopted is the method of comparison between the individual values of any hour and the normal monthly average of that hour. Senhor Capello has found it necessary to select and extract the disturbances, not directly from the hourly values, but by comparing the variation of an individual day with the average diurnal variation derived from the month. To illustrate this method by means of an example, let us imagine that the sum of the twenty-four hourly values for a particular day is 24,000, and that the average monthly diurnal variation would indicate that a particular hour of this day should have a value 990, then, if the value for this hour should prove to be greater or less than 990 by more than a certain amount, it would be set aside as a disturbed observation. Senhor Capello rather thinks it will be desirable somewhat to modify this method, and he concludes his remarks by observing that for this and other similar questions it is most necessary that directors of establishments possessing magnetographs should agree together to employ the same method in their reductions in order that their results may be compar able with each other. With the view of adding weight to these remarks, we may quote the observation of Sir William Thomson, that our ability to analyse mathematically that influence which produces the diurnal variation will depend upon our knowing at a certain number of stations the exact nature of this diur nal variation for each of the three magnetic elements. A complete theory of this diurnal influence must therefore wait upon the concerted action of the directors of the various establishments possessing magnetographs. 69. Change in Horizontal Force Range from Month to Month. Although we do not possess finally accurate determinations of the solar-diurnal variations of either element of the force, yet we are in possession of information regarding the change in the diurnal range of the horizontal force from month to month at the Greenwich Observatory. William Ellis has given us the following table (Phil. Trans., 1880) representing the monthly mean diurnal range of horizontal force at that observatory expressed in ten -thousandths of the whole horizontal force. In the formation of these means, days of great magnetic disturbance were rejected, and also certain other days on which there prevailed a smaller but considerable amount of disturbance estimated according to a general standard formed in the examination of many thousands of photographs. TABLE XVII. Monthly Mean Diurnal Range of Horizontal Force at Royal Observatory, Greenwich. Jan. 13-5 Feb. 14-8 Mar. 20-1 April. 27-4 May. 20-9 June. 27-3 July. 27-2 Aug. 25-2 Sept. 23-2 Oct. 19-8 Xov. 14-3 Dec. 11-6 Thus, like the declination range ( 43), the horizontal force range has a maximum in summer and a minimum in winter, and exhibits a tendency towards maxima at the equinoxes. 70. Long -Period Inequalities of Horizontal Force Range. Lagging Behind. 1 Ellis lias compared the diurnal range of the horizontal force as well as that of the declination at Greenwich with the period of sun-spot frequency, his comparisons extending from 1841 to 1877, and he has deduced the following conclusions: 1 Secchi (Wot ft Attronnmitche ifittheilungen, Xo. 21) seems to have been the first to indicate a relation between the state of the sun s surface and the diurnal variation in the horizontal force.
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