S66 Notices respecting New Boohs. 



The Norwegian Aurora Polaris Expedition 1902-1903. Volume I. 



On the Cause of Magnetic Storms and the Origin of Terrestrial 



Magnetism. By Kn. Bikkelakd. First Section. Quarto. 



315 pp. & 21 PJates. Christiania, 1 908. London : Longmans, 



Green & Co. (Price 22s. net.) 

 The volume discusses records derived daring 1902-3 from mag- 

 netographs at four temporary stations situated respectively in 

 the extreme north of Norway, in Nova Zembla, in Spitsbergen, 

 and in Iceland, and simultaneous records from a number of 

 magnetic observatories in various parts of the world. The 21 

 large plates at the end of the volume reproduce the photo- 

 graphic curves taken during times of special magnetic disturb- 

 ance from October 6, 1902, to March 31, 1903. In the body of 

 the work are numerous charts illustrating the disturbing forces 

 at the several arctic and cooperating stations at various stages of 

 each disturbance. The Expedition was due to Prof. Birkeland, 

 who personally defrayed nearly half the total expense, the 

 remainder of the funds coming about equally from the Norwegian 

 Government and from three private individuals. The main object 

 was the further development of the theory, which Prof. Birkeland 

 j:>ropounded a good many years ago, that magnetic disturbances 

 are due to electrical discharges in the upper atmosphere, which 

 appeal to our visual organs as aurora. To elucidate the nature of 

 the phenomena, Prof. Birkeland has made many experiments with 

 a terella, or minature earth, of iron, highly magnetized by currents 

 in a surrounding coil, which is exposed to discharges from a 

 cathode in a highly exhausted glass bulb. With this apparatus 

 he has produced a number of visual effects resembling aurora, 

 photographs of some of which appear in the work. The cathode 

 is supposed to represent the sun, the cathode rays from which 

 reach the earth's atmosphere, where their further course is 

 dependent on the earth's magnetic field. The mathematics 

 answering to this hypothesis has been developed by Prof. Stormer, 

 to whose calculations there are various references. Prof. Birkeland 

 practically defines disturbance as the difference between the value 

 of a magnetic element at the instant considered and the value at 

 the same hour on the representative undisturbed day. This dis- 

 turbance he regards as measuring the magnetic force at the time 

 and place due to the hypothetical auroral current, and from the 

 way in which the amplitude and direction of the resultant dis- 

 turbance vary from station to station he draws conclusions as to 

 the nature, locus, and intensity of the current. The simplest 

 type of magnetic disturbance he believes to be accounted for by 

 a current which approaches to and recedes from the earth 

 along radii, the intermediate — or, as the author terms it, the 

 "horizontal" — portion being a straight line of length 21 whose 

 extremities are at the same height, and whose centre is at a 

 distance h above the ground. 



Formulae for the magnetic force due to such a current are 

 developed on p. 102, and numerical results for special values of h 

 and I are given on p. 103, the current being taken to be one 



f 



