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373
MAGNETISM, TERRESTRIAL


singill zzinid migllzt be accidental. In the case of D at Greenw'ch 1891 1 ere rom the other years, and of two more recent years e 1

xamined

by Ellis” one, 1904, agreed with 1891. At Kew, on the average of the 1 1 years 1890 to 1900, the quiet-day mean annual value of declina-

tion exceeded the ordinary day value, but the apparent excess O'~O2 IS too small to possess much si niticance 3 ~ . .

Another property more recently discovered 1n quiet days IS the non-cyclic change. The nature 0 this phenomenon will be readily understood from the following data from the N°""-"°”" riod 1890 to 1900 at Kew32. The mean daily c""3”' fi; all days is calculated from the observed change.

1 1-year

Change

annual

V.

I I

Mean annual change -5-79 -2-38 +25'9“/ Mean dail chan e all da s 0 O16 0 oo 0 o D. I 1. | H.

y g . Y - ' - ~ 7' + ° 7'Y

Meandailychange, qu1et days +0~044 -0-245 +3-347 -22-67

-0-067

0.84»y

those of the average day. Before accepting such a phen as natural instrumental peculiarities must be c r f Thus the changes during the representative quiet day differed from 0I'l'1€IlOI1

nsidered.

p a e u ly co

The secular change is really based on the absolute instruments, the diurnal changes on the magneto graphs, and the first idea likely to occur to a critical mind is that the apparent abnormal change on quiet days represents in reality change of zero in the magneto graphs. If, however, the phenomenon were instrumental, it should appear

s 1 0 zero amounting in a year to over 1,2007 in H and t equally on days other than quiet days, and we should thus have a h'ft f ' ' .

n

90, in I. Under such circumstances the curve would be co 0 about

tinually

drifting off the sheet. In the case of the Kew magneto graphs, a careful investigation showed that if any instrumental change occurred

in the declination magneto graph during the 1 1 years it did not exceed a few tenths of a minute. In the case of the H and V magne at Kew there is a slight drift, of instrumental origin, due to ing of the magnets, but it is exceedingly small, and in the H is in the opposite direction to the non-cyclic change tographs

weaken-

case of

on quiet

days. It only remains to add that the hypothesis of instrumental origin was positively disproved by measurement of the curves on ordinary days.

It must not be supposed that every quiet day agrees with age quiet day in the order of magnitude, or even in the S1 the aver-

n, of the

non-cyclic change. In fact, in not a few rnonths the sign of the non cyclic change on the mean of the quiet days differs from that obtained for the average quiet day of a period of y ea

Kew, between 1890 and 1900, the number of months duriigl the mean non-cyclic change for the five quiet days selecte rs. At

g which

by the

astronomer royal (Sir W. H. M. Christie) was plus, zero, or minus. was as follows:-

Element. I I

D. I. H. V.

I3 112 47

16 II 9

9 74

Number -}- 63

H 0 I4

55 101

vv "

l

The sign denotes westerly movement in the declination, and increasing dip of the north end of the needle. In the case of I and

H the excess in the number of months showing the normal sign is overwhelming. The following mean non-cyclic changes days are from other sources 1on quiet

Greenwich Falmouth I Kolaba

Element. (1890-1895). (1898-1902). (1894-1901). / V I I

D + o-o3 -5- o»o5 + 0-07

g Y H 9+ 4'37 + 3'0v 7 -l- 3-91 The results are in the same direction as at Kew, + meaning in the case of D movement to the west. At Falmouthw, as at Kew, the non-cyclic change showed a tendency to be small in years of few sun-spots.

§ 30. In calculating diurnal inequalities from quiet days the non cyclic effect must be eliminated, otherwise the result would on the hour at which the “ day " IS. supposed to commence. value recorded at the second midnight of the average da depend

If the

exceeds

that at the first midnight by N, the elimination is effected by applymg to each hourly value the correction N(12-n)/24, where hour counted from the first midnight (0 hours). This the change to progress uniformly throu hout the 2 h n is the

ZSSU 11165

Unless

g 4 ours.

this is practically the case-a matter difficult either to prove or dis rov th

p e- e correction may not secure exactly what IS ai This method has been employed in the previous tables. that differences do exist between diurnal inequalities deriv quiet days and all ordinary days was stated explicitly in § obvious in Tables VIII. to XI. An extreme case is represe med at.

The fact

e

d from

4, and is

nted by

i

the data for Jan Mayen in these tables. Figs. 9 and IO are vector diagrams for this station, for all and for quiet days during May, June and July 1883, according to data got out by Lllideling. As shown by the arrows, fig. IO (quiet days) is in the main described in the normal or clockwise direction, but fig. 9 (all days) is described in the opposite direction. Liideling found this peculiar difference ll/I I

15 QA 'El

in 11  -s:=E
'.t¥'§  W;

M' .

-is 14 13 Q22 A11 82 gf;

Days 15 20L'1; Y

12 . -

L.1Q -ses .... 14 219 — iss! 0 7 ' 2 S (4

20 .2 s a,

5 12 24

21, 24 1 0

  • in-

9 3 M

FIG. 9. Fig. 10.

between all and quiet days at all the north polar stations occupied in 1882-1883 except Kingua F 3ord, where both diagrams were described clockwise.-In

temperate latitudes the differences of type are much less, but still they exist. A good idea of their ordinary size and character in the case of declination may be derived from Table XXIX., containing data for Kew, Greenwich and Parc St Maur. The data for Greenwich are due to W. E1lis3°, those for Parc St Maur to T. Moureaux 33. The quantity tabulated is the algebraic excess of the all or ordinary day mean hourly value over the corresponding quiet day value in the mean diurnal inequality for the year. At Greenwich and Kew days of extreme disturbance have been excluded from the ordinary days, but apparently not at Parc St Maur. The number of highly disturbed days at the three stations is, however, small, and their influence is not great. The differences disclosed by Table XXIX. are obviously of a systematic character, which would not tend to disappear however long a period was utilized. In short, while the diurnal inequality from quiet days may be that most truly representative of undisturbed conditions, it does not represent the average state of conditions at the station. To go into full details respecting the differences between all and quiet days would occupy undue space, so the following brief summary of the differences observed in declination at Kew must suffice. While the inequality range is but little different for the two types of days, the mean of the hourly differences from the mean for the day is considerably reduced in the quiet days. The 24-hour term in the Fourier analysis is of smaller amplitude in the quiet days, and its phase angle is on the average about 6°-75 smaller than on ordinary days, implying a retardation of about 27 minutes in the time of maximum. The diurnal inequality range is more variable throughout the year in quiet days than on ordinary days, and the same is true of the absolute ranges. The tendency to a secondary minimum in the range at midsummer is considerably more decided on ordinary than on quiet days. When the variation throughout the year in the diurnal inequality range is expressed in Fourier series, whose periods are the year and its sub multiples, the 6-month term is notably larger for ordinary than for quiet days. Also the date of the maximum in the 12-month term is about three days earlier for ordinary than for quiet days. The exact size of the differences between ordinary and quiet day phenomena must depend to some extent on the criteria employed in selecting quiet days and in excluding disturbed days. This raises difficulties when it comes to comparing results at different stations. For stations near together the difficulty is trifling. The astronomer royal's quiet days have been used for instance at Parc St. Maur, Val Ioyeux, Falmouth and Kew, as well as at Greenwich. But when stations are wide apart there are two obvious difficulties: first, the difference of local time; secondly, the fact that a day may behtypically quiet at one station but appreciably disturbed at the ot er.

If the typical quiet day were simpl the antithesis of a disturbed day, it would be natural to regard the non-cyclic change on quiet days as a species of recoil from some effect of disturbance. This view derives support from the fact, pointed out long ago by Sabine 34, that the horizontal force usually, though by no means always, is lowered by magnetic disturbances. Dr van Bemmelen 35 who has examined non-cyclic phenomena at a number of stations, seems disposed to regard this as a sufficient explanation. There are, however, difficulties in accepting this view. Thus, whilst the non cyclic effect in horizontal force and inclination at Kew and Falmouth appeared on the whole enhanced in years of Sun-Spot maximum, the difference between years such as 1892 and 1894 on the one hand, and 1890 and 1900 on the other, was by no means proportional to the excess of disturbance in the former years. Again, when the average non-cyclic change of declination was calculated at Kew for 207 days, selected as those of most marked irregular disturbance between 1890 and 1900, the sign actually proved to be the same as for the average quiet day of the period.