﻿Notices 
  respecting 
  New 
  Books. 
  253 
  

  

  and 
  two 
  nearly 
  equal 
  minima 
  in 
  May 
  and 
  August. 
  The 
  existence 
  

   of 
  maxima 
  near 
  the 
  equinoxes 
  has 
  been 
  generally 
  observed 
  in 
  the 
  

   northern 
  hemisphere. 
  

  

  During 
  a 
  single 
  appearance 
  of 
  an 
  auroral 
  arc 
  there 
  was 
  usually 
  

   a 
  movement 
  from 
  the 
  horizon 
  towards 
  the 
  zenith 
  and 
  back 
  again 
  

   — 
  sometimes 
  several 
  such 
  alternations 
  — 
  the 
  maximum 
  altitude 
  

   attained 
  varying 
  considerably 
  even 
  at 
  the 
  same 
  season. 
  As 
  be- 
  

   tween 
  one 
  month 
  and 
  another 
  Mr. 
  Arctowski 
  remarks, 
  p. 
  63, 
  on 
  

   a 
  distinct 
  tendency 
  for 
  the 
  altitude 
  to 
  be 
  greater 
  at 
  the 
  equinoxes 
  

   and 
  less 
  at 
  midwinter. 
  The 
  azimuth 
  of 
  the 
  crown 
  of 
  the 
  arc, 
  

   though 
  usually 
  not 
  very 
  remote 
  from 
  the 
  magnetic 
  meridian, 
  

   varied 
  considerably. 
  On 
  p. 
  35, 
  fig. 
  25 
  illustrates 
  an 
  interesting 
  

   case 
  in 
  which 
  the 
  lower 
  of 
  two 
  simultaneously 
  existing 
  arcs 
  

   appeared 
  distinctly 
  elliptical, 
  whilst 
  the 
  crowns 
  of 
  the 
  two 
  arcs 
  

   differed 
  appreciably 
  in 
  azimuth. 
  

  

  A 
  number 
  of 
  other 
  points 
  are 
  dealt 
  with, 
  on 
  which 
  the 
  obser- 
  

   vations 
  w 
  T 
  ere 
  somewhat 
  few 
  to 
  throw 
  much 
  fresh 
  light, 
  and 
  

   references 
  are 
  given 
  to 
  many 
  recent 
  authorities. 
  In 
  a 
  few 
  cases 
  — 
  

   e.g., 
  in 
  that 
  of 
  the 
  supposed 
  26-day 
  period 
  dealt 
  with 
  on 
  p. 
  bO 
  — 
  

   Mr. 
  Arctowski 
  is 
  perhaps 
  a 
  little 
  too 
  ready 
  to 
  treat 
  as 
  facts 
  what 
  

   are 
  more 
  safely 
  regarded 
  as 
  hypotheses; 
  but 
  this 
  is 
  a 
  matter 
  of 
  

   opinion. 
  

  

  Mr. 
  Arctow 
  r 
  ski's 
  volume 
  is 
  the 
  work 
  of 
  an 
  observer 
  who 
  has 
  

   mainly 
  in 
  view 
  the 
  description 
  of 
  phenomena, 
  and 
  not 
  the 
  attain- 
  

   ment 
  of 
  a 
  complete 
  physical 
  explanation. 
  In 
  many 
  respects 
  Prof. 
  

   Birkeland's 
  volume 
  stands 
  in 
  marked 
  contrast. 
  The 
  occasion 
  for 
  

   its 
  appearance 
  was 
  an 
  expedition, 
  partly 
  at 
  the 
  expense 
  of 
  the 
  

   Norwegian 
  Government, 
  which 
  occupied 
  a 
  station 
  on 
  the 
  mountain 
  

   of 
  Haldde, 
  near 
  Bossekop, 
  in 
  the 
  north 
  of 
  Norway, 
  during 
  the 
  

   winter 
  of 
  1899-1900. 
  In 
  addition 
  to 
  Prof. 
  Birkeland, 
  the 
  

   expedition 
  comprised 
  two 
  physical 
  assistants 
  — 
  one 
  killed 
  by 
  an 
  

   avalanche 
  — 
  and 
  a 
  telegraph 
  engineer. 
  It 
  had 
  some 
  of 
  the 
  newest 
  

   patterns 
  of 
  instruments, 
  including 
  self-recording 
  magnetographs 
  

   of 
  the 
  Eschenhagen 
  pattern, 
  with 
  arrangements 
  for 
  rotating 
  the 
  

   drum 
  carrying 
  the 
  photographic 
  paper 
  in 
  24 
  hours, 
  as 
  usual, 
  or 
  in 
  

   2 
  hours. 
  It 
  had 
  also 
  an 
  electrograph, 
  with 
  similar 
  slow 
  or 
  rapid 
  

   drum 
  rotation, 
  in 
  which 
  radium 
  took 
  the 
  place 
  of 
  the 
  ordinary 
  

   water-dropper 
  as 
  " 
  collector." 
  

  

  The 
  book 
  gives 
  only 
  a 
  general 
  preliminary 
  summary 
  of 
  the 
  

   observations 
  — 
  more 
  especially 
  of 
  the 
  magnetic 
  ones 
  — 
  and 
  a 
  good 
  

   deal 
  of 
  it 
  is 
  devoted 
  to 
  Prof. 
  Birkeland's 
  theoretical 
  deductions 
  

   and 
  to 
  laboratory 
  experiments 
  supporting 
  his 
  views. 
  From 
  the 
  

   observational 
  standpoint, 
  the 
  most 
  interesting 
  part 
  is 
  that 
  dealing 
  

   with 
  the 
  magnetograph 
  records. 
  By 
  pre-arrangement, 
  quick 
  

   rotation 
  of 
  the 
  drum 
  carrying 
  the 
  photographic 
  paper 
  was 
  simul- 
  

   taneously 
  adopted 
  on 
  a 
  number 
  of 
  days 
  at 
  Haldde 
  and 
  Potsdam, 
  

   and 
  Prof. 
  Birkeland 
  was 
  subsequently 
  allowed 
  free 
  access 
  to 
  all 
  

   the 
  Potsdam 
  records. 
  Time 
  telegraph-signals 
  were 
  received 
  at 
  

   Haldde 
  from 
  Christiania 
  twice 
  a 
  week, 
  and 
  special 
  attention 
  was 
  

  

  