﻿188 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  7 
  

  

  is 
  then 
  brought 
  back 
  to 
  the 
  original 
  position 
  by 
  the 
  driving 
  train 
  of 
  wheels. 
  

   This 
  arrangement 
  is 
  equivalent 
  to 
  a 
  push 
  over 
  the 
  distance 
  between 
  the 
  two 
  

   points, 
  those 
  of 
  meeting 
  and 
  leaving 
  the 
  pallet. 
  Here 
  friction 
  is 
  less, 
  but 
  is 
  

   not 
  completely 
  absent, 
  since 
  there 
  is 
  friction 
  in 
  unlocking 
  the 
  train 
  each 
  time 
  

   it 
  moves. 
  

  

  If 
  electric 
  working 
  is 
  introduced 
  friction 
  can 
  be 
  entirely 
  avoided. 
  An 
  

   impulse 
  can 
  be 
  given 
  magnetically 
  at 
  the 
  middle 
  point 
  of 
  the 
  swing, 
  but 
  the 
  

   difticulty 
  of 
  keeping 
  this 
  impulse 
  constant 
  and 
  applying 
  it 
  at 
  exactly 
  the 
  right 
  

   time 
  seems 
  to 
  make 
  this 
  method 
  less 
  satisfactory 
  than 
  an 
  electrically-operated 
  

   form 
  of 
  the 
  second 
  type 
  of 
  drive, 
  which 
  is 
  now 
  exceedingly 
  simple. 
  The 
  

   pendulum 
  merely 
  lifts 
  a 
  small 
  pallet, 
  contact 
  with 
  which 
  causes 
  a 
  current 
  to 
  

   flow, 
  which 
  by 
  means 
  of 
  a 
  magnet 
  shifts 
  the 
  stop 
  of 
  the 
  pallet. 
  Contrary 
  

   to 
  some 
  rather 
  positive 
  statements, 
  it 
  has 
  been 
  found 
  by 
  several 
  experimenters 
  

   that 
  the 
  contact 
  in 
  this 
  form 
  of 
  drive 
  can 
  be 
  operated 
  without 
  any 
  friction 
  and 
  

   with 
  pressures 
  less 
  than 
  1 
  gram. 
  

  

  Some 
  very 
  simple 
  equations 
  were 
  developed 
  for 
  determining 
  the 
  magnitude 
  

   of 
  the 
  errors 
  with 
  this 
  arrangement, 
  and 
  hence 
  the 
  best 
  practical 
  dimensions 
  to 
  

   give 
  it 
  in 
  any 
  particular 
  case. 
  These 
  equations 
  are 
  applicable 
  to 
  the 
  other 
  

   forms 
  of 
  drive, 
  and 
  show: 
  (1) 
  A 
  light 
  and 
  long 
  pallet 
  is 
  preferable 
  as 
  long 
  as 
  

   the 
  pressure 
  is 
  sufficient 
  for 
  proper 
  contact. 
  This 
  is 
  because 
  the 
  errors 
  due 
  to 
  

   friction, 
  or 
  to 
  displacement, 
  or 
  wear 
  of 
  the 
  stops, 
  become 
  less 
  as 
  the 
  length 
  

   increases. 
  (2) 
  The 
  error 
  from 
  displacement 
  of 
  the 
  stops, 
  that 
  is, 
  from 
  im- 
  

   proper 
  timing 
  of 
  the 
  driving 
  pressure, 
  is 
  a 
  minimum 
  when 
  this 
  pressure 
  ex- 
  

   tends 
  over 
  half 
  the 
  swing. 
  Contrary 
  to 
  much 
  received 
  opinion, 
  therefore, 
  an 
  

   instantaneous 
  impulse 
  at 
  the 
  middle 
  of 
  the 
  swing 
  may 
  be 
  a 
  relatively 
  disad- 
  

   vantageous 
  method 
  of 
  driving. 
  (3) 
  It 
  is 
  possible, 
  and 
  sometimes 
  probably 
  

   advantageous, 
  with 
  the 
  second 
  form 
  of 
  drive, 
  to 
  arrange 
  to 
  compensate 
  for 
  the 
  

   circular 
  error 
  of 
  the 
  pendulum, 
  that 
  is, 
  the 
  error 
  caused 
  by 
  variation 
  in 
  arc 
  

   of 
  swing. 
  (4) 
  In 
  the 
  Riefler 
  mechanism, 
  which 
  belongs 
  to 
  the 
  second 
  type 
  of 
  

   drive, 
  the 
  driving 
  pressure 
  acts 
  over 
  an 
  arc 
  which 
  is 
  dependent 
  on 
  the 
  speed 
  

   with 
  which 
  the 
  escapement 
  wheel 
  revolves 
  when 
  unlocked. 
  This 
  is 
  really 
  a 
  

   disadvantageous 
  element 
  in 
  the 
  design, 
  against 
  which, 
  however, 
  are 
  to 
  be 
  

   set 
  the 
  efficiency 
  of 
  the 
  unlocking 
  arrangement 
  and 
  the 
  general 
  good 
  workman- 
  

   ship 
  of 
  this 
  make 
  of 
  pendulum. 
  

  

  Adjournment 
  at 
  10 
  p.m. 
  was 
  followed 
  by 
  a 
  social 
  hour. 
  

  

  H. 
  H. 
  Kimball, 
  Recordin'g 
  Secretary. 
  

  

  BIOLOGICAL 
  SOCIETY 
  

  

  627th 
  meeting 
  

  

  The 
  627th 
  meeting 
  of 
  the 
  Biological 
  Society 
  of 
  Washington 
  was 
  held 
  in 
  the 
  

   lecture 
  hall 
  of 
  the 
  Cosmos 
  Club, 
  May 
  14, 
  1921, 
  at 
  8.00 
  o'clock. 
  President 
  

   HoLLisTER 
  was 
  in 
  the 
  chair, 
  and 
  28 
  persons 
  were 
  present. 
  The 
  minutes 
  for 
  

   the 
  62()th 
  meeting 
  of 
  April 
  80 
  were 
  read 
  and 
  approved, 
  and 
  the 
  following 
  were 
  

   elected 
  to 
  membership: 
  Dr. 
  Rudolph 
  Kuraz, 
  Mr. 
  E. 
  C. 
  Leonard 
  and 
  

   Robert 
  F. 
  Griggs. 
  It 
  was 
  announced 
  that 
  the 
  present 
  meeting 
  was 
  the 
  

   last 
  before 
  the 
  summer 
  recess. 
  

  

  Informal 
  Communications: 
  Dr. 
  T. 
  S. 
  Palmer 
  stated 
  that 
  doubt 
  rests 
  upon 
  

   the 
  native 
  origin 
  of 
  oppossums 
  in 
  California. 
  There 
  is 
  a 
  record 
  ninety 
  years 
  

   old 
  of 
  oppossums 
  on 
  the 
  California-Mexico 
  border. 
  Dr. 
  Grinnell 
  shows, 
  

   however, 
  that 
  oppossums 
  were 
  introduced 
  in 
  the 
  San 
  Jose 
  neighborhood 
  in 
  

  

  