410 



NATURE 



[June 3, 1909 



■ third satellite obseived at the Johannesburg Observatory on 

 April 3. 



Before and after the transit both the north polar cap 

 and the dark marking along the north torrid zone of the 

 satellite were noticed. When the satellite was about three- 

 quarters of its journey across the planet a double dark 

 spot was seen in its position, and re-focussing failed to 

 alter the apparition. Approaching the limb of the planet 

 the n.p. part of the double spot was replaced by a bright 

 spot, smaller than the satellite, but s.p. the remaining grey 

 mark. A few minutes before internal contact took place 

 the dark grey spot disappeared, whilst the bright spot 

 increased in size. 



Immediately after last contact J iii. was seen against the 

 sky nearly round, but perhaps shaded off a little towards 

 Jupiter, and with a small north polar bright spot with a 

 darkish band below it. When the dark double spot was 

 visible it looked like a close double star, darlc instead of 

 bright, having a separation of o'g" and an angle estimated 

 nt 300°. 



The Spectrum of Magnesium in Hydrogen. — The signi- 

 ficance of the " magnesium hydride " bands in the spectrum 

 of sun-spots lends great importance to anv investigation of 

 their nature, and a paper, by Mr. E. E. Brooks, which 

 <ippears in the April number of the Asirophysical Journal 

 <vol. x.xix., No. 3, p. j-7), is therefore of astronomical 

 interest. 



Experimenting at the Leicester Technical School with 

 magnesium in hydrogen, Mr. Brooks employed a unidirec- 

 tional, but pulsating, current, which is intermediate 

 between arc and spark, and arrived at the following con- 

 •clusions regarding its spectrum :— (i) The spectrum repre- 

 sents some transitional unstable stale ; (2) although 

 hydrogen is essential, the production of the " hydride " 

 spectrum appears to depend far more upon the nature of 

 the discharge than upon the quantity of the gas present ; 

 (3) a trace of water vapour appears to be more effective 

 than hydrogen, yet its presence cannot be regarded as 

 ■essential ; (4) if due to a hydride the substance is probablv 

 ■decomposed as fast as it is formed. 



The Pertukbatio.ns of Brooks's Comet (18S9 V) by 

 Jupiter in 1S86. — From his investigations of the perturba- 

 tions, and the resulting path, of Brooks's comet; Prof. 

 Poor concluded that this object could not be identified with 

 ^the lost comet of Le.xell. 



In this regard an interesting paper, bv Herr G. Deutsch- 

 land, appears in No. 4321 of the Astrono'inischc Nacbrichtcn. 

 :j,'ivmg the results of a re-investigation of the planetary 

 perturbations, taking into account the oblateness of Jupiter. 

 Ihese results exhibit variations from those previouslv 

 obtained by Prof. Poor, especially in the time of the comet's 

 nearest approach to the planet. 



Recent Observation of Daniel's Comet, 1907 d.— 

 Among the photographic observations recorded by Prof. 

 Uolf in No. 4321 of the Astronomische Nachrichten is one 

 of an object which is, possibly, Daniel's comet of 1907. 

 Owmg to the faintness of the object and the poor sky, tne 

 identilication is not quite certain, although the 'image 

 appears on two plates. The middle of the exposure was at 

 i3h. 25.4m. (Konigstuhl M.T.) on April 19, and the 

 position of the object was o = i5h. iS-7m., 8= -7° 37'; 

 magnitude i6'5. 



The Varhisle Star b.1909 Urs^e Majoris.— In a note 

 tippearing in No. 4324 of the Astroiwmische Nachrichten, 

 Prof. Wolf announces that the variable star near the spiral 

 nebula Mioi had decreased in brightness more than half a 

 magnitude by May 9. 



T' 



POLAR MAGNETIC STORMS.' 

 'HE last ten or twenty years have been marked by great 

 activity in Arctic and Antarctic expeditions. The 

 results obtained in the department of terrestrial magnetism 

 form a great contribution to knowledge, and prove that 

 continued effort in this direction will do much to remove 

 the difficulties that enshroud the problem. 



' The Norwegian Aurora Polaris Expedition, 1902-3. Vol. i., "On the 

 Cause of Magnetic Storms and the Origin of Terrestrial Magnetism." By 

 Kr. Birkeland. Pp. viii + ars; 2r plates. (Christiania : H. Ascheboug and 

 ■Co. ; London : Longmans, Green and Co., n.d.) Price 22s. net. 



NO. 2066, VOL. 80] 



I he present expedition was the development of pre- 

 liminary expeditions carried out in the preceding si.x years 

 by Dr. Birkeland, the object being the study of the con- 

 nection between and origin of aurorie and magnetic 

 storms. The funds were provided by the Norwegian 

 Government, by learned societies in Norway, and by Dr. 

 Birkeland himself. 



The preliminary expeditions had indicated the frequent 

 occurrence of magnetic storms having a probable origin 

 vertically above the vicinity of the North Cape, and the 

 plan of the 1902-3 expedition was to make simultaneous 

 observations at four stations in that region. The four 

 stations were on Iceland, Spitsbergen, Nova Zembla, and 

 in Finmark. Each of the stations was provided with a 

 similar set of recording magnetographs of the pattern 

 due to Eschenhagen. These are admirably suited for e.x- 

 peditions on account of their portability and simplicity of 

 adjustment. They can be given a higli sensibility, although 

 some may doubt whether the high value used by Dr. 

 Birkeland was altogether a blessing for the purpose of 

 studying magnetic storms. It was, of course, a distinct 

 advantage in studying the minute and extremely regular 

 periodic movements that were frequently observed. In 

 addition, each station was provided with auxiliary meteor- 

 ological and electrical apparatus, and one of the stations 

 had an instrument for recording earth currents. 



The present volume begins with a description of the 

 equipment and installation of the various stations, and 

 those of us who live in temperate regions may well reflect 

 on the advantage of making physical observations without 

 having to interrupt work for the purpose of suppressing 

 the scientific ardour of a polar bear. 



In analysing the magnetic storms, Dr. Birkeland was 

 able to obtain simultaneous records from twenty-three 

 observatories in various parts of the world, in addition to 

 those from the four special stations. Dr. Birkeland at 

 the outset indicates that the results have been analysed 

 and presented with the view of supporting the theory he 

 holds, viz. that these storms are due to the incidence of 

 (negatively) charged corpuscles projected from the sun. 

 The desirability of such a method of procedure may be 

 open to question, but we think that the author has gained 

 immensely by so doing, and the results are put in such a 

 form that their value is not in the least prejudiced by 

 whether we accept his ultimate conclusions or not. 



The method is in outline as follows. .•\ny disturbance 

 of the magnetic needle may be represented as the effect 

 of a certain electric current. The course of a storm may 

 thus, so far as the horizontal components are concerned, 

 be represented by an arrow of certain length in a certain 

 direction. For each particular storm discussed the records 

 from the various observatories are shown in a plate re- 

 duced to a uniform time scale. In the text a general 

 description of the storm is given, followed by charts show- 

 ing with arrows the direction and magnitude of the 

 assumed disturbing current at different stages and at 

 different places. These are followed by a discussion as 

 to the general character of the horizontal current required 

 to produce the storm. 



The current charts are remarkably simple, and give an 

 extremely clear presentation of the results free from any 

 theory. In this way the existence of certain well-defined 

 types of storm is established. 



The supposition that these arrows represent true electric 

 currents of corpuscles is almost a natural consequence. 

 .•\rguments are given to show that they cannot be earth 

 currents, but that they are probably due to streams in 

 the upper regions of the air, the general height being 

 some 400 kilometres. 



It is remarkable that the stream so frequently sets 

 between the four stations, and thus confirmatory evidence 

 is obtained from the different signs of the vertical-force 

 variations on opposite sides of the stream. 



It is found that the horizontal stream is not always 

 sufficient to account for the facts, but that the horizontal 

 portion must be regarded as a bend in a stream descend- 

 ing vertically, and then with greater or less rapidity re- 

 turned into space. 



Dr. Birkeland supports, and we think very ably, his 



J arguments by reference to experiments on a highly 



magnetised sphere (a terrella) placed in the path of kathode 



