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XXXV ., the algebraic excess of the disturbed See also:day would have varied Midi. 6 See also: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 See also:declination for the See also:year at See also:Kew is analysed into See also:Fourier waves, the See also:chief difference, it will be remembered, between See also:ordinary and quiet days was that the See also:amplitude of the 24-See also:hour See also:term was enhanced in the ordinary days, whilst its phase See also: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 See also:differences obtained by See also:Sabine's method between See also:east and See also:west or + and - disturbances, that there is See also:present during disturbances some See also:influence which affects the diurnal inequality in a See also:regular systematic way, tending to make the value of the See also:element higher during some See also:hours and See also:lower during others than it is on days relatively See also: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 See also:general See also:rule or merely a See also:local peculiarity is a subject for further See also:research. § 35. There are still other ways of attacking the problem of disturbances. W. See also:Ellis 27 made a See also:complete See also:list of disturbed days at See also: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 See also:movement in declination was under 10'; on 3614, or 20%, the disturbance, though exceeding to', was under 30'; on 294 days it See also:lay 0 -2r Midt. Parc St Maur. See also: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 See also: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 See also:North See also:American S7 stations was considerably larger than this. § 32. From the point of view of the surveyor there is a See also:good See also:deal to be said for Sabine's See also: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 See also: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 See also: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 See also:annual and diurnal See also:variations in Tables See also:XXX. to XXXII. Sabine, as we have seen, confined his See also:attention to the departure of the hourly See also:reading from the mean for that hour. Another and equally natural criterion is the apparent See also:character of the magneto-graph See also:curve. At See also:Potsdam curves are regarded as " 1 " quiet, " 2 " moderately disturbed, or " 3 " highly disturbed. Any hourly value to which the See also: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 See also: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 See also: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 See also:maxima near the equinoxes, especially the See also:spring See also:equinox. The diurnal Element. See also:Jan. Feb. See also:Mar. See also:April. May. See also:June. See also:July. Aug. See also: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 See also:case of his " See also:minor " disturbances—those under io'—a distinct See also:double annual See also: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 See also:early in June and in See also:January. When the year is divided into three seasons—See also:winter (See also:November to See also:February), summer (May to See also:August), and equinox—Maunder's figures See also:lead to the results assigned to Greenwich disturbed days in Table See also: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 See also:matter of definition. The chief criterion at Kew for classifying the days was not so much the See also:mere amplitude of the largest movement, as the general character of the day's curve and its departure from the normal See also: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 See also:storm may last for any time from a few hours to several days, and during See also: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 See also:laws. The table shows, however, that so far as Greenwich is concerned the annual variations in the two cases are closely alike. 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