Fix.  750.    Path  of  a  thnndar-ttonn  from  Hudson  river  to  Cape  Cod.    The  curved  Unes  show  sue-
eessive  positions  of  its  axis  at  intenrals  of  fifteen  minutes,
by  a  dotted  line.    (After  Davis.)

the  moisture.  The  possible  beginnings  of  a  thunder-
storm may  be  ol^rved  on  a  summer  day  when
white  cumulus  thunder-heads  are  beginning  to
form.  These  clouds  often  show  in  a  most  convinc-
ing manner  the  presence  of  ascending  currents  of
air  in  their  midst,  for  they  seem  to  be  boiling  over
and  puf&ng  up  at  the  top  under  the  compelling
action  of  hot  air  rising  from  below.  The  origin  of
the  electric  phenomena  which  accompany  thunder-
storms has  not  been  satisfactorily  explained.  Th3
frequent  occurrence  of  hail  with  summer  thunder-
storms is  taken  as  a  further  indication  of  the  action
of  the  currents  ascending  and  descending  within
the  sphere  of  disturbance.
Thunder-storms  are  common  in  the  doldrums
or  about  the  equator,  where  they  generally  occur
in  the  afternoon  or  early  evening,  and  are  accom-
panied by  very  heavy  rains.  The  most  violent
equatorial  thunder-storms  are  met  with  near  the
lands  under  the  equator,  especially  in  equatorial
Africa  and  over  the  Atlantic  just  west  of  that
continent.  They  are  of  short  duration,  although  of
extreme  violence,  and  their  passage  is  succeeded
by  a  return  of  the  characteristic  calms  of  the  tor-
rid zone.  In  South  America  violent  thunder-storms
characterize  the  summer  season  in  the  Argentine
Republic  and  Uruguay.  Sometimes  the  valley
breezes  ascending  the  slopes  of  mountainous
regions  become  strong  enough  during  the  season  to


Tornadoes.
