﻿86 
  

  

  Messrs. 
  Michelson 
  and 
  Stratton 
  on 
  

  

  and 
  fairly 
  accurate 
  machine 
  which 
  would 
  save 
  the 
  consider- 
  

   able 
  time 
  and 
  labour 
  involved 
  in 
  such 
  computations. 
  

  

  The 
  principal 
  difficulty 
  in 
  the 
  realization 
  of 
  such 
  a 
  machine 
  

   lies 
  in 
  the 
  accumulation 
  of 
  errors 
  involved 
  in 
  the 
  process 
  of 
  

   addition. 
  The 
  only 
  practical 
  instrument 
  which 
  has 
  yet 
  been 
  

   devised 
  for 
  effecting 
  this 
  addition 
  is 
  that 
  of 
  Lord 
  Kelvin. 
  

  

  In 
  this 
  instrument 
  a 
  flexible 
  cord 
  passes 
  over 
  a 
  number 
  of 
  

   fixed 
  and 
  movable 
  pulleys. 
  If 
  one 
  end 
  of 
  the 
  cord 
  is 
  fixed 
  

   the 
  motion 
  of 
  the 
  other 
  end 
  is 
  equal 
  to 
  twice 
  the 
  sum 
  of 
  the 
  

   motions 
  of 
  the 
  movable 
  pulleys. 
  The 
  range 
  of 
  the 
  machine 
  

  

  however, 
  limited 
  to 
  a 
  small 
  number 
  of 
  elements 
  on 
  account 
  

  

  is 
  

  

  