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Page:EB1911 - Volume 17.djvu/389

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374
MAGNETISM, TERRESTRIAL

TABLE XXIX.-All or Ordinary, less Quiet Day Hourly Values (+ to the) West). results for that station, and Th. Moureauxi' has published similar data for Parc St Maur. Tables XXX. to XXXII. are based on a selection of these Forenoom Afternoon' data. Tables XXX., and XXXI. show the annual Hour' Kew Greenwich Parc St Maur Kew Greenwich Parc St Maur Vaflatlon in Sal-11119 S d15tU1'b3~11C@S, the f110I1§ l1ly 1890-1900. 1890—1894. 1883-1897. 1890-1900. 1890-1894. 1893-1897. ValL{@5 being €XDf€SS€d as D€fC€I1ta§ €S Of the aflfh?e% meand xéalue fo; the 12 months. The Parc

I 1 1 I I 1 t aur an atavia ata, owing to the long periods included, are especially noteworthy. Table XXX. 3 ggi gig gig ;l+ g il5'g dpag wnthtthe east (l€.)l andrrwpjt (DVQ/ggcgstuxgarfces 3 O 5I o 3I 0 32 +O 57 +o 52 +O 59 o the<é n+a En separa§ y al e d, I ea mg 4 o 4I 0 23 0 16 +060 +0 5I +055 wi b is ances:ln cprizorga an vertica force, 5 O 28 O IO o OI +046 +084 +068 COH1 lnesxt 1;-P ar; R/Iistur laznceis, treated numeri-6 O O8 +042 +048 +0 2I +004 +007 ca +{.f f dare bt aur to imits required to 7 +O'I3 +o~3o +0-34 -o-06 -0-24 -0-25 qugl Y < 9r /'ftuii ance were d32 '° li” Df 207 1" H 8 +029 +048 +O 47 0 27 O 50 0 54 an I2');.1I'l, t edc0rresponT1}1;1g f m1ts for Batavia 9 +040 +056 +053 O 47 o 68 O 74 were 5 3% Ilg an dgq/B A e imits forlD, at I0 +0.44 +0.53 +0.51 0.6I 0.73 0.70 I, <?Al;0;1n§ ,2, .;m ay an o art were respective y 3 -6, H °+0'48 +050 +844 -062 '°'77 '°'79 At Parc St Maur the disturbance data from I2 +0 45 +0 44 +0 38 0 54 0 61 0 67 all three elements give distinct maxima near the § 31 A satisfactory definition of magnetic disturbance is about as difficult to lay down as one of heterodoxy. The idea in its Ma new generality seems to present no difficulty, but it is a very mf b different matter when one comes to details. Amongst ur the chief disturbances recorded since 1890 are those of Wes' February 13-14 and August 12, 1892; July 20 and August 20, 1894; March 15-16, and September 9, 1898; October 31, 1903; February 9-10, 1907; September IH2, 1908 and September 25, 1909. On such days as these the oscillations shown by the magnetic curves are large and rapid, aurora is nearly always visible in temperate latitudes, earth currents are prominent, and there is interruption-sometimes very serious-in the transmission of telegraph messages bothin overhead and underground wires. At the other end of the scale are days on which the magnetic curves show practically no movement beyond the slow regular progression of the regular diurnal inequality. But between these two extremes there are an infinite variety of intermediate cases. The first serious attempt at a precise definition of disturbance seems due to General Sabine "5“. His method had once an extensive vogue, and still continues to be applied at some important observatories. Sabine regarded a particular observation as disturbed when it differed from the mean of the observations at that hour for the whole month by not less than a certain limiting value. His definition takes account only of the extent of the departure from the mean, whether the curve is smooth at the time or violently oscillating makes no difference. In dealing with a particular station Sabine laid down separate limiting values for each element. These limits were the same, irrespective of the season of the year or of the sun-spot frequency. A departure, for example, of 3'-3 at Kew from the mean value of declination for the hour constituted a disturbance, whether it occurred in December in a year of sun-spot minimum, or in June in a year of sun-spot maximum, though the regular diurnal inequality range might be four times as large in the second case as in the first. The limiting values varied from station to station, the size depending apparently on several considerations not very clearly defined. Sabine subdivided the disturbances in each element into two classes: the one tending to increase the element, the other tending to diminish it. He investigated how the numbers of the two classes varied throughout the day and from month to month. He also took account of the ag regate value of the disturbances of one sign, and traced the diurnai and annual variations in these aggregate values. He thus got two sets of diurnal variations and two sets of annual variations of disturbance, the one set depending only on the number of the disturbed hours, the other set considering only the aggregate value of the disturbances. Generally the two species of disturbance variations were on the whole fairly similar. The aggregates of the -{- and - disturbances for a particular hour of the day were seldom equal, and thus after the removal of the disturbed values the mean value of the element for that hour was generally altered. Sabine's complete scheme supposed that after the criterion was first applied, the hourly means would be recalculated from the undisturbed values and the criterion applied again, and that this process would be repeated until the disturbed observations all difiered by not less than the accepted limiting value from the final mean based on undisturbed values alone. If the disturbance limit were so small that the disturbed readings formed a considerable fraction of the whole number, the complete execution of Sabine's scheme would be exceedingly laborious. As a matter of fact, his disturbed readings were usually of the order of 5% of the total number, and unless in the case of exceptionally large magnetic storms it is of little consequence whether the First choice of disturbed readings is accepted as final or is reconsidered in the light of the recalculated hourly means.

Sabine applied his method to the data obtained during the decade 1840 to 1850 at Toronto, St Helena, Cape of Good Hope and Hobart, also to data for Pekin, Nertchinsk, Point Barrow, Port Kennedy and Kew. C. Chambers 35 applied it to data from Bombay. The ~yearly publication of the Batavia observatory gives corresponding equinoxes; a minimum at midwinter is clearly

Shown, and also one at midsummer, at least in D and H. A decline in disturbance at midwinter is visible at all the stations, but at Batavia the equinoctial values for D and V are inferior to those at midsummer.

Table XXXII. shows in some cases a most conspicuous diurnal variation in Sabine's disturbances. The data are percentages of TABLE XXX.-Annual Variation of Disturbances

(Sabine's numbers).

Parc St Maur Toronto Bombay BatavTa Hobart K

1883-97. 1841-48. 1859-65. 1883-99. 1843-48.

Month. E. W. TE. W. E. W. E. W. Sis. W.

January. 78 60 55 66 89 89 180 223 165 182

February. 116 92 75 86 94 67 138 144 121 116

March 126 107 92 94 129 97 IO2 87 114 104

April. 105 113 115 II4 106 129 67 73 110 102 May. 101 118 IOI 101 63 99 72 71 62 53

June- - 77 39 95 72 73 81 45 27 32 37

July 82 104 140 126 121 173 62 46 0 49

August . 88 II3 137 133 154 131 69 69 £6 78

5€Dt@mb€1' 134 157 163 139 111 103 135 144 135 114 October 119 II5 IOI 111- 140 128 95 88 124 123 November 99 94 73 85 43 43 106 91 79 III

December 75 58 SI 72 72 55,124 137 123 130

the totals for the whole 24 hours. But whilst at Batavia the easterly and westerly disturbances in D vary similarly, at Parc St Maur they follow opposite laws, the easterly showing a prominent maximum near noon, the westerly a still more prominent maximum near midnight. The figures in the second last line of the table, if divided by 0-24, W1ll g1V€ the percentage of hours which show the species of disturbance indicated. For instance, at Parc St Maur, out of 100 hours, 3 show disturbances to the west and 3-7 to the east; or in all 6 -7 show disturbances of declination. The last line gives the average size of a disturbance of each type, the unit being 1' in D and Iyin H and V.

At Batavia disturbances increasing and decreasing the element are about equally nu merous, but this is exceptional. Easterly disturbances of declination predominated at Toronto, Point Barrow, Fort Kennedy, Kew, Parc St Maur, Bombay and the Falkland Islands whilst the reverse was true of St Helena, Cape of Good Hope, Pekin and Hobart. At Kew and Parc St Maur the ratios borne by the TABLE XXXI -Annual Variation of Disturbances. Parc St Maur. i Toronto. i Batavia.

Month. Numbers, Aggregates. Numbers. Aggregates. H. V. H. V. H. V. H. V.

ilranuary 81 5 1 58 56 96 1 5 1 89 1 54

ebruary 1 96 133 94 74 IOS 123 1 IO 125

March . 126 118 94 IOS 116 105 117 103

April . 94 1 1 1 1 50 149 104 76 105 73

May 108 133 Q0 112 IOI 92 105 95

June 90 85 36 50 82 69 79 66

]u1y . . 99 128 61 71 Q0 83 95 81

August 1 13 92 75 108 QI QI 98 QI

September 119 122 171 160 113 111 114 115

October . IOI Q4 148 129 1 14 89 104 86

November . 104 81 98 75 99 102 100 IOI

J December 70 f 51 128 100 89 IO; 847 1 I0