June  i,  1893,] 
THE  TROPICAL  AGRICULTURIST. 
753 
THUNDERSTORMS  -.—THEIR  CAUSE 
AND  EEEECT: 
DURATION  OF  A LIGHTNING  FLASH  ‘ONE- 
MILLIONTH  OF  A SECOND”! 
With  reference  to  recent  experiences  in  Colombo, 
the  following  account  of  the  development  of 
Thunderstorms  from  the  latest  Encyclopedia 
Britannica  affords  a good  many  points  of  interest.  It 
will  be  observed  that  our  experience  on  Galle 
Face  the  other  day  of  a special  and  sudden  inoreaee 
in  the  rainfall— a perfect  torrent  in  faot  it  became — 
following  on  the  flash  end  crash — accords  with  the 
scientific  explanation.  Very  interesting  is  it  to 
learn  that  the  duration  of  a lightning  flash  is  only 
about  “one-millionth  of  a second”  ! If  it  could 
be  made  to  last  one  teuth  of  a second,  it  “would 
give  near  objects  an  illumination  100,000  times 
more  brilliant  than  that  of  moonlight"  1 And  onoe 
more  that  the  so-ca'led  “ flash”  is  really  “ a column 
of  intensely  heated  air  driven  outwards  from  (he 
track  of  the  discharge  at  a rate  initially  far 
greater  than  that  of  sound.”  The  rate  at  which 
Bound  travels,  it  will  be  remembered  is  roughly 
speaking,  about  a mile  in  five  seconds.  We 
quote  as  follows  : — 
There  can  be  little  doubt  that  atmospheric  electri- 
city, at  least  in  the  great  developments  which 
characterize  a thunderstorm,  is  due  in  some  way  to 
water.  Before  a great  thunderstorm  the  lower  air 
is  usually  at  an  abnormally  high  temperature,  and 
fully  saturated  with  water  vapour,  so  that  it  is  in  a 
thoroughly  unstable  condition.  Immense  cloud  masses, 
often  miles  in  vertical  thickness,  which  produce  almost 
midnight  darkness  by  day  in  the  region  of  the  storm, 
and  which  appear,  when  seen  from  a distance,  as  if 
boiling  upwards,  are  always  a notable  feature  of  great 
thunderstorms.  These  are  usually  accompanied  by 
torrents  of  rain,  or  by  violent  hail-showers.  And  it 
is  commonly  observed  that  each  flash  of  lightning  is 
followed,  after  a brief  interval,  by  a sudden  but 
temporary  increase  in  the  rate  of  rainfall.  At  what 
stage  of  its  transformations  the  electrification  is  deve- 
loped by  water-substance  is,  as  yet,  only  guessed  at, 
— thougn  it  seems  most  reasonable  to  conclude  that  it 
is  anterior  to  the  formation  of  cloud,  i.e.,  to  the 
condensation  of  vapour.  And,  though  the  idea  was 
at  one  time  very  generally  held  and  still  has  many 
upholders,  it  seems  unlikely  to  be  the  direct  result 
of  evaporation.  For,  were  it  due  directly  either  to 
evaporation  or  to  condensation,  it  is  almost  im- 
possible to  doubt  that  proof  would  long  since  have 
been  furnished  by  careful  experiment,  even  if  made 
on  a scale  so  limited  as  that  afforded  bv  our  labora- 
tories. No  trace  of  electrical  effect  has’ been  found 
to  attend  the  precipitation  of  moisture ; and  the  elec- 
trical effects,  sometimes  considerable,  which  have 
been  found  associated  with  evaporation  have  always 
been  accompanied  by  relatively  violent  physical  and 
mechanical  actions  which  are  not  observed  in  con- 
junction with  atmospheric  electricity.  It  has  been 
suggested  by  some  authorities  that  the  electricity  of 
a thunderstorm  is  developed  during  the  formation 
of  hail,  by  others  that  it  is  due  to  the  molecular 
actions  which  accompany  the  diminution  of  total 
surface  when  two  or  more  drops  of  water  coalesce 
into  a single  one.  It  has  been  ascribed  to  the  fric- 
tion of  moist  against  dry  air,  and  to  the  dust- 
particles  which  appear  to  be  necessary  for  the  con- 
densation of  vapour.  Agarn,  it  has  been  suggested 
that  it  may  be  a mere  phenomenon  of  contact  electri- 
city due  to  the  impact  of  uncondensed  vapour  particles 
•95 
! on  particles  of  air.  It  is  almost  unnecessary  to 
i observe  that,  whatever  hypothesis  we  adopt,  some 
explanation  must  be  given  of  two  important  points:  — ( 1) 
; What  becomes  of  the  electricity  equal  and  opposite 
j to  that  in  each  drop,  which  must  be  produced 
simultaneously  with  it  ? (2)  By  what  means  is  the 
attraction  between  the  drops  and  the  recipient  of  the 
opposite  charge  of  electricity  overcome  so  that  the 
drops  may  be  enabled  to  part  with  their  charge  ? 
It  is  to  be  presumed  that  gravity  satisfies  the  second 
of  these  questions.  As  to  the  first,  it  seems  to 
necessitate  the  presence  of  something  besides 
water,  in  order  that  the  electric  separation  may 
be  commenced,  and  thus  appears  to  be  fatal 
to  the  capillary  theory  indicated  above.  What- 
ever be  the  true  source  of  the  charge,  it  is 
easy  to  see,  by  known  properties  of  electricity,  that 
even  an  exceedingly  small  charge  on  each  vapour 
particle  would  lead  to  a very  high  potential  as  soon 
as  a visible  drop  is  formed,  and  that  as  a drop  in- 
creases in  size  its  potential  is  proportional  to  its 
surface.  That  drops  of  rain  are  often  individually 
electrified  to  a very  high  potential  is  proved  by  the 
frequent  occurrence  of  “ lumiuous  rain,”  when  the 
ground  is  feebly  lit  up  by  tbe  multitude  of  tiny 
sparks  given  out  by  the  drops  as  they  come  near  it. 
The  flakes  of  falling  snow,  also,  ere  often  strongly 
eleotrified,  so  that  smart  sparks  have  been  drawn 
from  an  umbrella  on  which  the  snow  was  falling. 
But  the  law  of  eleotrio  repulsion  shows  us  at  once 
that,  as  soon  as  the  drops  in  a cloud  are  sufficiently 
electrified,  at  least  the  greater  part  of  their  charge 
must  pass  to  the  boundary  of  the  clond.  When  this 
occurs,  the  nature  of  the  further  behaviour  of  the 
charge  presents  no  difficulty.  The  reason  for  our 
singularly  complete  ignorance  i f the  source  of  at- 
mospheric electricity  seems  to  lie  in  the  fact  that 
it  cau  only  be  discovered  by  means  of  experiments 
made  on  a scale  very  much  larger  than  is  attain- 
able with  the  ordinary  resources  of  a laboratory. 
The  difficulties  will  probably  be  easily  overcome  by 
the  first  nation  which  will  go  to  the  expense  of  pro- 
viding the  necessary  means. 
Numberless  other  explanations  of  the  origin  of 
thunderstorms  have  been  suggested ; but  the  more 
reasonable  of  these  do  little  more  than  shift  tbe 
difficulty,  for  they  begin  by  assuming  (without  any 
bint  as  to  its  source)  an  electrification  of  the  earth 
as  a whole,  or  of  the  lower  (sometimes  the  upper) 
layers  of  the  atmosphere.  Induction,  convection,  &c., 
are  then  supposed  to  effeot  the  rest.  Another  and 
much  less  reasonable  class  of  explanations  depends 
upon  magneto. electricity.  Some  of  these  introduce 
the  so-called  “ unipolar  ” induction  supposed  to  be 
due  to  the  rotation  of  the  earth,  which  behaves  like 
a gigantic  magnet.  Of  this  nature  is  the  suggestion 
of  Edlund,  which  was  reoently  crowned  by  the 
Aoademy  of  Sciences  of  Paris.  That  rapid  varia- 
tions in  the  earth’s  magnetio  elements,  such  as  often 
oocur  on  a large  scale  as  in  a “ magnetic  storm,” 
have  at  least  a share  in  the  production  of  the  aurora  is 
a perfectly  reasonable  and  even  plausible  hypothesis, 
long  ago  brought  forward  by  Balfour  Stewart.  But 
we  h<ve  yet  to  seek  the  source  of  these  variations. 
The  brightness  of  a flash  of  lightning  is  nsually 
much  underrated.  It  is  true  that  it  rarely  gives  even 
at  night  an  illuminatiou  greater  tbau  that  due  to 
moonlight.  But  it  must  be  remembered  that  Swan 
has  proved  that  tbe  impression  of  a flash  on  the  eye 
depends  upon  the  duration,  being  nearly  proportional 
to  it,  aud  steadily  increasing  for- about  a tenth  of  a 
second.  Now  the  duration  of  a light  ning-flash  (roughly 
speaking)  is  only  about  one-millionth  of  a sec  md.  This 
is  proved  by  the  fact  that  the  most  rapidly  rotating 
bodies  appear  (o  be  absolutely  steady  when  illumi- 
nated by  it.  Hence,  if  it  could  be  made  to  last  for 
a tenth  of  a second,  it  would  give  near  objeots  an 
illumination  one  hundred  thousand  times  more  brilliant 
than  that  of  moonlight.  It  must  be  remembered 
that  the  flash  is  not  a mere  line,  but  a column,  of 
intensely  heated  air,  driven  outwards  from  the  traok 
■ f the  discharge  at  a rats  initially  far  greater  than 
that  of  sound. 
