The  Dielectric  Strength  of  Air.  237 
much    accelerated  by  notable   variations  in    the   amount  of 
atmospheric  carbonic  acid. 
As  regards  the  rate  of  denudation,  it  may  be  added  that 
this  may  have  varied  to  a  very  considerable  extent  in  different 
epochs  on  account  o£  geological  changes  o£  various  kinds. 
Changes  may  have  taken  place  in  the  relative  area  of  sea  and 
land,  or  in  the  elevation  o£  the  land,  or  in  the  distribution  of 
sea  and  land,  that  would  have  a  very  marked  influence  on 
the  amount  of  rainfall,  and  therefore  also  on  the  rate  of 
denudation.  Reference  was  made  in  one  of  the  former 
articles  of  this  series  to  the  large  quantity  of  water  that  is 
contained  in  stratified  and  metamorphic  rocks  ;  and  the 
inference  that  may  reasonably  be  drawn  from  its  presence 
there,  viz.,  that  the  ocean  in  very  early  times  contained 
a  considerably  larger  quantity  of  water  than  it  does  now, 
from  which  again  it  may  be  inferred  that  the  area  of  the 
ocean  was  more  extensive  and  that  of  the  land  less  extensive 
than  at  the  present  day.  On  the  other  hand,  it  is  quite 
uossible  that  the  basins  of  the  great  oceans  were  much 
deeper  in  very  early  times  than  they  are  now,  and  there- 
fore also  that  the  relative  areas  of  sea  and  land  may  not 
have  changed  to  a  very  great  extent.  In  any  case,  it 
is  obvious  that  the  rate  of  denudation  may  have-  been 
influenced  by  quite  a  variety  of  circumstances,  and  it  is 
possible  that  a  careful  study  of  the  whole  subject,  and  also 
a  careful  study  of  all  the  circumstances  affecting  the  secular 
cooling  of  the  earth,  may  show  the  need  of  revising  the 
estimates  of  the  age  of  the  earth,  derived  from  both  lines  of 
enquiry. 
XX.    The  Dielectric  Strength  of  Air. 
By  Alexander  Russell,  M.A.,  M.I.E.E* 
Table  of  Contents. 
1.  Introduction. 
2.  Historical. 
3.  The  electric  intensity  between  two  concentric  spheres. 
4.  The  electric  intensity  inside  a  concentric  main. 
5.  The  corona  round  a  cylinder. 
6.  The  stress  in  the  dielectric  round  two  particles  having  equal  and 
opposite  charges  of  electricity. 
7.  Proof   of  the   series-formula   for   the   maximum   electric   intensity 
between  two  equal  spheres. 
8.  Approximate  formulae  for  the  maximum  electric  intensity  between 
two  equal  spheres. 
9.  The  disruptive  discharge  between  two  spherical  electrodes. 
10.  The  maximum  electric  intensity  between  a  sphere  and  a  plane. 
*  Communicated  by  the  Physical  Society:  read  November  24,  1903. 
