﻿140 
  Intelligence 
  and 
  Miscellaneous 
  Articles. 
  

  

  the 
  Nimrud 
  crater 
  where 
  it 
  separated 
  off 
  part 
  of 
  the 
  large 
  lake 
  to 
  

   form 
  the 
  shallow, 
  so-called 
  ' 
  hot 
  lake,' 
  and 
  also 
  to 
  the 
  north 
  of 
  

   Nimrud, 
  where 
  it 
  rose 
  up 
  fissures 
  and 
  in 
  a 
  small 
  crater. 
  

  

  6. 
  A 
  violent 
  earthquake 
  in 
  1881 
  which 
  destroyed 
  the 
  village 
  of 
  

   Teghurt, 
  at 
  the 
  eastern 
  base 
  of 
  the 
  crater- 
  wall, 
  was 
  the 
  last 
  sign 
  of 
  

   activity; 
  but 
  earthquakes 
  are 
  still 
  frequent 
  in 
  the 
  Plain 
  of 
  Mush 
  at 
  

   the 
  western 
  foot 
  of 
  the 
  Nimrud 
  Dagh, 
  and 
  recent 
  fault-scarps 
  are 
  

   clearly 
  visible 
  along 
  the 
  borders 
  of 
  this 
  faulted 
  depression. 
  

  

  The 
  speaker 
  mentioned 
  that 
  he 
  had 
  presented 
  his 
  model 
  of 
  the 
  

   crater 
  to 
  the 
  Museum 
  of 
  Practical 
  Geology 
  ( 
  Jermyn 
  St.) 
  and 
  the 
  

   rocks 
  and 
  slides 
  to 
  the 
  British 
  Museum 
  (Natural 
  History), 
  where 
  

   his 
  fossils 
  from 
  Armenia 
  are 
  already 
  preserved. 
  

  

  XV. 
  Intelligence 
  and 
  Miscellaneous 
  Articles. 
  

  

  COUPLED 
  CIRCUITS 
  AND 
  MECHANICAL 
  ANALOGIES, 
  

   Phil. 
  Mag. 
  Dec. 
  1917. 
  

  

  To 
  the 
  Editors 
  of 
  the 
  Philosophical 
  Magazine. 
  

   Gentlemen, 
  — 
  

  

  pENTENAEIAN 
  PEEIGAL 
  is 
  clean 
  forgotten 
  today, 
  and 
  bis 
  

   ^ 
  valuable 
  kinematic 
  work 
  on 
  his 
  lathe. 
  His 
  method 
  should 
  

   be 
  revived 
  of 
  drawiug 
  the 
  ellipse 
  or 
  other 
  Lissajous 
  figures 
  of 
  

   combined 
  vibration, 
  as 
  on 
  p. 
  515, 
  fig. 
  2. 
  

  

  The 
  enveloping 
  rectangle 
  is 
  divided 
  up 
  into 
  elementary 
  rect- 
  

   angles 
  by 
  lines 
  spaced, 
  not 
  equidistant, 
  but 
  in 
  equal 
  time 
  of 
  

   simple 
  vibration. 
  

  

  Perigal 
  does 
  this 
  by 
  describing 
  a 
  semicircle 
  on 
  each 
  side 
  of 
  the 
  

   rectangle, 
  and 
  then 
  produces 
  the 
  ordinates 
  of 
  points 
  at 
  equal 
  

   angular 
  interval 
  round 
  the 
  circumference. 
  

  

  Starting 
  at 
  any 
  point 
  of 
  crossing 
  and 
  tacking 
  across 
  the 
  

   diagonal 
  of 
  an 
  elementary 
  rectangle, 
  a 
  succession 
  of 
  points 
  is 
  

   made 
  on 
  an 
  ellipse 
  inscribed 
  in 
  the 
  rectangle, 
  and 
  the 
  points 
  

   are 
  close 
  enough 
  to 
  be 
  joined 
  up 
  in 
  a 
  continuous 
  curve, 
  such 
  

   as 
  Perigal 
  could 
  cut 
  in 
  his 
  lathe. 
  

  

  If 
  m 
  and 
  n 
  steps 
  are 
  taken 
  for 
  a 
  diagonal, 
  the 
  Lissajous 
  curve 
  

   appears 
  for 
  a 
  combination 
  of 
  two 
  vibrations 
  of 
  m 
  and 
  n 
  fold 
  

   frequency, 
  and 
  the 
  phase 
  difference 
  of 
  lead 
  or 
  lag 
  is 
  settled 
  by 
  

   the 
  position 
  of 
  the 
  starting 
  point. 
  

  

  Tours 
  sincerely, 
  

   Dec. 
  13, 
  1917. 
  &. 
  Greenhill. 
  

  

  1 
  Staple 
  Inn, 
  W.C. 
  1, 
  

  

  