5IO 



NATURE 



[August 17, 19 16 



within the sun, or in his atmosphere, the agent must 

 have worked at something like i6o million million 

 million million horse-power. . . . This result, it seems 

 to me, is absolutely conclusive against the supposition 

 that terrestrial magnetic storms are due to magnetic 

 action of the sun ; or to any kind of dynamical action 

 taking place within the sun, or in connection with 

 hurricanes in his atmosphere, or anywhere near the 

 sun outside. It seems as if we may also be forced 

 to conclude that the supposed connection between 

 magnetic storms and sun-spots is unreal, and that the 

 seeming agreement between the periods has been a 

 mere coincidence. We are certainly far from having 

 any reasonable explanation of any of the magnetic 

 phenomena of the earth ; whether the fact that the 

 earth is a magnet; that its magnetism changes vastly, 

 as it does from century to century ; that it has some- 

 what regular and periodic . . . solar diurnal . . . 

 variations ; and (as marvellous as the secular variation) 

 that it is subject to magnetic storms." 



To-night I shall confine myself to three of the out- 

 standing problems enumerated by Lord Kelvin : the 

 secular change, the solar diurnal variation, and the 

 phenomena of magnetic disturbances. 



Secular Change. 



Our knowledge of secular change prior to the nine- 

 teenth century is confined to declination and dip. For 

 these elements we have in some districts data covering 

 more than three centuries. 



The total range of D (declination) observed in Lon- 

 don has exceeded 35°. The only actual turning point 

 observed, 24-6° W., presented itself about 1818, the 

 direction of secular change then altering from westerly 

 to easterly. We have no idea how the value, ii^° E., 

 observed in 1580 stood to the previous turning point. 

 The declination was approximately the same as at 

 present in 1730. When, if ever, it will have the same 

 value again, we have not the ghost of an idea. The 

 change in each of the centuries 1600 to 1700 and 1700 

 to 1800 was about 16°, whereas during the last 

 hundred years the change has been only about 9°. The 

 rate of change has, however, markedly increased of 

 late years, as may be recognised on consulting Fig. i, 

 which shows the change at Kew during the last fifty 

 years. 



The turning point in the dip, when it attained its 

 highest value, presented itself about 1723, or nearly a 

 century before the turning point in D. _ The dip in 

 London is now lower than it has been since observa- 

 tions began. Of late "years the rate of change has 

 been very small, but whether this heralds the near 

 approach of a minimum, or is merely a temporary 

 slackening, we do not know. 



The intensity of magnetic force changes as well as 

 the direction. Thus at Kew between 1890 and 1900 

 H (horizontal force) increased from o- 18 169 to o- 18428 

 c.g.s. When dealing with such small changes as 

 ordinarily present themselves in terrestrial magnetism, 

 it is convenient to employ as unit ly, or oooooi c.g.s. 

 Thus the mean annual rise of H from 1890 to 1900 

 was 26 7. After 1900 the rate of increase of H rapidly 

 fell off, and the element seems to have attained a 

 maximum and begun to diminish. V (vertical force) 

 has been diminishing for some time. 



Diurnal Variation. 

 To give a full account of the diurnal variation as It 

 presents itself at different parts of the earth would 

 require a large treatise. Here I shall confine myself 

 to data from two stations, and to certain aspects 

 only of these data. The one station, Kew, is fairly 

 representative of the British Isles. The other station 

 is that used in 1911-12 as the base station of the 

 National Antarctic Expedition under the late Captain 

 NO. 2442, VOL. 97] 



Robert Falcon Scott, R.N. The reduction of the Ant- 

 arctic observations has been prosecuted at Kew Ob- 

 servatory for the last two years under my supervision. 

 For permission to make a free use of existing data 

 I am indebted to the committee of the Captain Scott 

 Antarctic Fund. 



The tragic fate of Captain Scott is still no doubt 

 fresh in your memories. It produced a great impres- 

 sion on his countrymen, who saw in it evidence that 

 the characteristics on which the nation prided itself 

 in more warlike times still survived. The appreciation 

 of courage is practically universal, but even a scientific 

 audience may have to be reminded that the prosecu- 

 tion of pure science under the arduous conditions pre- 

 vailing in the Antarctic calls for no small measure 

 of pluck and endurance. It also calls, if success is to 

 be attained, for other qualities, which though making 

 less appeal to the public imagination, are perhaps 

 of equal value for the welfare of a nation, viz., scien- 

 tific knowledge and forethought. If I am able to- 

 night to mention Important deductions from the Ant- 

 arctic observations, it is to the physical observers, 

 Dr. Simpson, F.R.S., and Mr. C. S. Wright, that 

 recognition is in the first place due. In spite of the 



zr 



20° 



19° 



18° 



17° 



16° 15" 



Fig. I. — Changes of westerly declination at Kew since 1865. Change in the 

 last fifty years 5 J°. Present annual change 9'. 



great diftlcultles arising from the low temperature anc 

 the extraordinarily disturbed magnetic conditions, thej 

 secured an almost unbroken record for a period ol 

 nearly twenty-two months. 



In Fig. 2 the vector diagrams refer to mean results 

 from the whole year. The full-line diagram repre- 

 sents at either station results based on all, or all bu' 

 highly disturbed days, the dotted-line diagram resultil 

 from quiet days only, the origin, the centre of tl 

 cross, being the same for the two. The Antarc^ 

 quiet davs (selected by myself) were ten a month, 

 against five at Kew (international quiet days). Th 

 a priori we should have expected less difference h 

 tween the two Antarctic diagrams than between t 

 two Kew ones. As regards tj^pe, there Is, In fact. If 

 difference in the Antarctic, but as regards amplitu< 

 the difference at Kew is slight, and not always 

 favour of the all-day vector, whereas in the Antarct 

 the excess of the all-day vector is conspicuous 

 every hour. 



