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375
MAGNETISM, TERRESTRIAL

TABLE XXXII.-Diurnal Variation of Disturbances (Sabine's numbers). variations, however, in Tables XXXII.

and XXXIV. are dissimilar. Thus in

Parc St Maur. Batavia. the case of H the largest disturbance - numbers at Parc St Maur occurred be-Hour. D. H. V. D. H. V. tween 6 a.m. and 6 p.m., whereas in Table XXXIV. they occur between

E. W. -|- - -|- - E- W- 'l' '* °'l' - 4 p.m. and midnight. Considering the - - comparative proximity of Parc St

o-3 10-1 20-3 9-o 8-3 5-7 9-2 1-1 5-8 13- 1 6-6 4-0 7-4 Maur and Potsdam, one must conclude 3-6 12-3 8-2 8-4 8-0 6-4 I0'4 7-6 7-3 14-2 4-8 6-3 10-0 that the apparent differences between 6-9 15-7 3-8 14-1 12-5 7-2 9-o 24-9 16-8 12-1 9-9 21-2 21-7 the results for these two stations are 9-noon 16-2 5-1 18-0 15-6 12-9 15-4 38-5 33-0 8-6 15-8 19-8 16-4 due almost entirely to the difference noon-3 19-3 6-7 15-3 16-5 18-2 18-3 18-8 24-7 16-8 21-1 23-5 22-1 in the definition of disturbance. 3-6 14-8 9-7 12-5 15-4 22-9 21-8 6-4 5-4 13-3 16-9 12-6 12-7 One d1ff1culty 1n the Potsdam pro-6-9 5'7 21-2 1 1-4 13-2 18-9 1 1-2 2-3 3-4 9-9 13-6 7-1 4-1 cedure is the maintenance of a uniform 9-12 5-9 25-o 11-2 10-5 7-8 4-7 0-4 3-8 I2°O II'I 5-6 5-4 standard. Unless very frequent refer- encedis dmade tohthe curve? of some

ly ' ....... 6 0. 1.52 1.61 1.1 1.1 stan ar year t ere must e a ten-I;;ildla1yl;nDei O 88 O 72 I 15 I 56 I 04 0 96 0 4 44, 9 3 dency to enter under “ 3 " in quiet mlean Size 1.72 1.69 13.0 19.5 16.7 15.5 ears a number of hours which would -' be entered under “2 " in a highly

eastern to the western disturbances were I'I9 and 1-23 respectively, uncertainty is

disturbed year. Still, such a source of

and so not much in excess of unity; but the preponderance of easterly disturbances at the North American 37 stations was considerably larger than this.

§ 32. From the point of view of the surveyor there is a good deal to be said for Sabine's definition of disturbance, but it is less satisfactory from other standpoints. One objection has been already indicated, viz. the arbitrariness of applying the same limiting value at a station irrespective of the size of the normal diurnal range at the time. Similarly it is arbitrary to apply the same limit between IO a.m. and noon, when the regular diurnal variation is most rapid, as between IO p.m. and midnight, when it is hardly appreciable. There seems a distinct difference of phase between the diurnal inequalities on different types of days at the same season; also the phase angles in the Fourier terms vary continuously throughout the year, and much more rapidly at some stations and at some seasons than at others. Thus there may be a variety of phenomena which one would hesitate to regard as disturbances which contribute to the annual and diurnal variations in Tables XXX. to XXXII. Sabine, as we have seen, confined his attention to the departure of the hourly reading from the mean for that hour, Another and equally natural criterion is the apparent character of the magneto graph curve. At Potsdam curves are regarded as “ 1 ” quiet, “ 2 " moderately disturbed, or “ 3 " highly disturbed. Any hourly value to which the numeral 3 is attached is treated as disturbed, and the annual Potsdam publication contains tables giving the annual and diurnal variations in the number of such disturbed hours for D, H and V. According to this point of view, the extent to which the hourly value departs from the mean for that hour is immaterial to the results. It is the greater or less sinuosity and irregularity of the curve that counts. Tables XXXIII. and XXXIV. give an abstract of the mean Potsdam results from 1892 to 1901. The data are percentages: in Table XXXIII. of the mean monthly total, in Table XXXIV. of the total for the day. So far as the annual variation is concerned, the results in Table XXXIII. are fairly similar to those in Table XXX. for Parc St Maur. There are pronounced maxima near the equinoxes, especially the spring equinox. The diurnal unlikely to have much influence on the diurnal, or even on the annual, variation.

§ 33. A third method of investigating a diurnal period in disturbances is to forrn a diurnal inequality from disturbed days alone, and compare it with the corresponding inequalities from ordinary or from quiet days. Table XXXV. gives some declination data for Kew, the quantity tabulated being the algebraic excess of the disturbed day hourly value over that for the ordinary day in the mean diurnal inequality for the year, as based on the II years 1890 to 1900. The disturbed day inequality was corrected for non-cyclic change in the usual way. Fig. 1 1 shows the results of Table XXXV. graphically. The irregularities are presumably

due to the limited number,

209, of disturbed days employed; to

get a smooth curve would require

probably a considerably longer period

of years. The differences between

disturbed and ordinary days at Kew

are of the same general character as

those between ordinary and quiet

days in Table XXIX.; they are,

however, very much larger, the range - 2°

in Table XXXV. being fully 5% times "2

that in Table XXIX. If quiet days

had replaced ordinary days in Table

XXXV., the algebraic excess of the

disturbed day would have varied

from +2'°7 at 2 p.m. to -4'-I at

II p. m., or a range of 6'~8.

§ 34. When the mean diurnal inequality in declination for the year at Kew is analysed into Fourier waves, the chief difference, it will be remembered, between ordinary and uiet days was that the amplitude of the 24-hour term was enhanced in the ordinary days, whilst its phase angle indicated an earlier occurrence of the maximum. Similarly, the chief difference between the Fourier waves for the disturbed and ordinary day inequalities at Kew is the increase in the amplitude of the 24-hour term in the former by over 70 %', and +2' ' 2

0 0

Midi. 5 Noon 5 Mldt.

Fic.. 11.

occurrence of its maximum by

the earlier

TABLE XXXIII.-Annual Variation of Potsdam Disturbances. about 1 hour 50 minutes. It is clear from

- these results for Kew, and it is also a neces-Element. jan. Feb. Mar. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 531-y inference from the differences ob- tained b Sabine's method between east

D 129 170 149 Q0 86 57 62 64 Q9 118 94 82 and westygr + and - disturbances, that H 109 133 131 102 109 82 94 QI 89 101 75 84 there is present during disturbances some V 106 17 1 1 70 108 121 56 64 74 93 87 78 70 influence which affects the diurnal inequality - '- - in a regular systematic way, tending to Mean 115 158 1 50 IOO 105 65 73 76 Q4 102 82 79 make the value of the element higher during - ' I some hours and lower during others than it TABLE XXXIV.-Diurnal Variation of Potsdam Disturbances. iflitollkgvaifyilieflééigriigael-Zutgiigehisma diiiéiginirli- crease in the range of the regularidiurnal HOUTS- l'3- 4“5- 7”9- I0““00n- I“3- 4“6- 7'9- I0'I2- inequality on disturbed days; but whether - - this is the general rule or merely a local D I4'9 I I ' I 8'° 5'2 5'7 13' I 22'5 19'5 peculiarity is a subject for further research. H 10'5 3'4 8'0 3'5. I V3 17'6 I9'2 16'5 § 35. There are still other ways of attack- l3'5 9'7 5'7 4'7 ' 8'5 I7'2 2I'5 19'2 ing the problem of disturbances. W. Ellis 27 made a complete list of disturbed days at

Mean 13'0 9'7 7'2 6'1 8'5 l6'° 21'I 13'4 Greenwich from 1848 onwards, arranging TABLE XXXV.-Disturbed Day less ordinary Day Inequality (Unit 1', +to West). them in classes according to the amplitude of the disturbance shown on the curves.

Of the 18,000 days which he considered,

Ellis regarded 2,119, or only about 12 %,

Hour

as undisturbed. On 11,898 days, of

66 %, the disturbance movement in dem

22 21

1 2 3 4 5 6 I 7 8 9 10 II I2

a.m. -3-4 -2-6 -2-0 -o-3 -T-1-6 +1-9 Y+2'3 +2~o +2-1 -'l-2~0 +1-6 +1-8 Lp. +1-8 + - + - +1-7 +I'4 o-o -1-3 -2-8 -3-5 -2-6 -3-5 -2-4 clination was under 1o'; on 3614, or

20 %, the disturbance, though exceeding

10', was under 3o'; on 294 days it lay