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XXXV

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Originally appearing in Volume V17, Page 376 of the 1911 Encyclopedia Britannica.
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XXXV., the algebraic excess of the disturbed day would have varied Midi. 6 Noon s from +2'1 at 2 p.m. to -4'•I at FIG. II. 11 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 quiet 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 the earlier occurrence of its maximum by about i hour 50 minutes. It is clear from these results for Kew, and it is also a necessary inference from the differences obtained by Sabine's method between east and west or + and - disturbances, that there is present during disturbances some influence which affects the diurnal inequality in a regular systematic way, tending to make the value of the element higher during some hours and lower during others than it is on days relatively free from disturbance. At Kew the consequence is a notable in-crease in the range of the regular diurnal inequality on disturbed days; but whether this is the general rule or merely a local peculiarity is a subject for further research. § 35. There are still other ways of attacking the problem of disturbances. W. Ellis 27 made a complete list of disturbed days at Greenwich from 1848 onwards, arranging 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 %, as undisturbed. On 11,898 days, of 66%, the disturbance movement in declination was under 10'; on 3614, or 20%, the disturbance, though exceeding to', was under 30'; on 294 days it lay 0 -2r Midt. Parc St Maur. Batavia. Hour. D. H. V. D. H. V. E. W. + - + - E. W. + ' - + - 0-3 I0.1 20.3 9.0 8'3 5.7 9.2 I'I 5.8 13.1 6.6 4.0 7.4 3-6 12.3 8.2 8.4 8•o 6.4 10.4 7.6 7.3 14.2 4.8 6'3 10.0 6-9 15.7 3.8 14.1 12.5 7.2 9.0 24.9 16.8 12.1 9.9 21.2 21.7 9 -noon 16.2 5.1 18•o 15.6 12.9 15.4 38.5 33.0 8.6 15.8 19.8 16.4 noon-3 19.3 6.7 15.3 16.5 18.2 18.3 18.8 24.7 16.8 2I•I 23.5 22.1 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 6-9 5.7 21.2 11.4 13.2 18.9 11.2 2.3 3.4 9.9 13.6 7.1 4.1 9-12 5.9 25.0 11.2 10.5 7.8 4'7 0.4 3.8 12.0 11.1 5.6 5.4 Mean number ( 0.88 0.72 1.15 1.56 1.04 0.96 0.46 0.44 1.62 1.61 1.19 1.13 per day Mean size .. .. .. 1.72 1.69 18•o 19.5 16.7 15.5 eastern to the western disturbances were 1.19 and 1.23 respectively, and so not much in excess of unity; but the preponderance of easterly disturbances at the North American S7 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 to a.m. and noon, when the regular diurnal variation is most rapid, as between to 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 Element. Jan. Feb. Mar. April. May. June. July. Aug. Sept. Oct. Nov. Dec. D 129 170 149 90 86 57 62 64 99 I18 94 82 H 109 133 131 102 109 82 94 91 89 101 75 84 V io6 171 170 to8 121 56 64 74 93 87 78 70 Mean 115 158 150 too 105 65 73 76 94 102 82 79 Hours. 1-3. 4-6. 7-9. 10-noon. 1-3. 4-6. 7-9• 10-12. D 14.9 11.1 8•o 5.2 5.7 13.1 22.5 19.5 H Io.5 8.4 8•o 8.5 11.3 17.6 19.2 16.5 V 13.5 9'7 5.7 4'7 8.5 17.2 21.5 19.2 Mean 13.0 9.7 7.2 6.1 8.5 16.0 21•I 18.4 Hour. I 2 3 4 5 6 7 8 9 10 II 12 a.m. -3.4 -2.6 -2.0 -0.3 +i•6 +P9 +2.3 +2.0 +2•I +2.O +1.6 +I.8 p.m. +1.8 +2.2 +2.1 +1.7 +1.4 0.0 -1.3 -2.8 -3'5 -2.6 -3.5 -2.4 between 30' and 6o'; while on 75 days it exceeded 6o'. Taking each class of disturbances separately, Ellis found, except in the case of his " minor " disturbances—those under io'—a distinct double annual period, with maxima towards the equinoxes. Subsequently C. W. Maunder," making use of these same data, and of subsequent data up to 1902, put at his disposal by Ellis, came to similar conclusions. Taking all the days with disturbances of declination over to', and dealing with 15-day periods, he found the maxima of frequency to occur the one a little before the spring equinox, the other apparently after the autumnal equinox; the two minima were found to occur early in June and in January. When the year is divided into three seasons—winter (November to February), summer (May to August), and equinox—Maunder's figures lead to the results assigned to Greenwich disturbed days in Table XXXVI. The frequency in winter, it will be noticed, though less than at equinox, is considerably greater than in summer. This greater frequency in winter is only slightly apparent in the disturbances over 6o', but their number is so small that this may be accidental. The next figures in Table XXXVI. relate to highly disturbed days at Kew. The larger relative frequency at Kew in winter as compared to summer probably indicates no real difference from Greenwich, but is simply a matter of definition. The chief criterion at Kew for classifying the days was not so much the mere amplitude of the largest movement, as the general character of the day's curve and its departure from the normal form. The data in Table XXXVI. as to magnetic storms at Greenwich are based on the lists given by Maunder" in the Monthly Notices, R.A.S. A storm may last for any time from a few hours to several days, and during part of its duration the disturbance may not be very large; thus it does not necessarily follow that the frequencies of magnetic storms and of disturbed days will follow the same laws. The table shows, however, that so far as Greenwich is concerned the annual variations in the two cases are closely alike. In addition to mean data for the whole 56 years, 1848 to 1903, Table
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