320 
Mi*.  S.  A.  Shorter  on  the 
term.  In  the  accompanying  figure  are  drawn  a  number  o£ 
curves  representing  various  types  of  motion  according  to  this 
equation.  Curve  I.  represents  the  type  naturally  suggested 
to  the  mind  by  a  casual  examination  of  the  equation.  It 
may  happen,  however,  that  the  steady  decrease  of  the  term 
Ae~ut  is  so  rapid  as  to  disguise  the  oscillatory  nature  of  the 
motion.     Curves  II.,  III.,  and  IY.  illustrate  this  point,  and 
Fig.  1. 
show  the  continuity  of  this  motion  with  the  aperiodic  motion 
represented  in  Curve  V. 
A  very  dilute  solution  of  saponine  affords  on  standing  an 
example  of  continuous  change  from  oscillatory  motion  of  the 
first  type  to  that  of  the  second  type.  When  the  surface  is 
fresh  the  disk  may  be  set  into  oscillation  by  twisting  the 
torsion-head.     The  surface  viscosity  gradually  increases  till 
