154  Geological  Society . 
bility  in  picking  up  and  recognizing  faint  sounds  than  upon  mere 
habit  of  making  contacts.  When  the  observers  were  interchanged, 
the  observed  interval  of  time  appeared  still  too  large,  but  in  this  case 
by  0S'02.  It  is  clear  that  such  personal  equations  are  not  elimi- 
nated by  an  interchange  of  observers,  nor  by  return  signals. 
In  the  reduction  of  the  equations,  the  coefficient  of  elasticity  of 
air  under  a  constant  volume  (that  is  to  say,  the  ratio  of  the  in- 
crement of  pressure  for  an  increment  1°  F.  of  temperature  to  the 
pressure  at  32°  F.)  was  regarded  as  an  unknown  quantity,  as  well  as 
V,  the  velocity  of  sound  at  32°  F.  The  reduction  of  the  equations 
furnished  by  the  observations,  which  were  38  in  number,  gave 
V=  1090*6  feet  per  second, 
a=0'0019, 
Regnault's  value  of  a  being  0*0020. 
There  appeared  to  be  but  little  difference  between  the  residual 
errors  as  dependent  on  the  motion  of  the  air.  The  author  grouped 
the  residuals  into  two  classes,  according  to  the  dampness  of  the  air ; 
but  there  appeared  to  be  no  appreciable  difference  in  the  velocity  as 
dependent  upon  dampness. 
GEOLOGICAL  SOCIETY. 
[Continued  from  p.  76-] 
June  21,  1871. — Joseph  Prestwich,  Esq.,  F.li.S.,  President, 
in  the  Chair. 
The  following  communications  were  read : — 
1.  "On  some  supposed  Yegetable  Fossils."  By  William  Carru- 
thers,  Esq.,  F.R.S.,  F.G.S. 
In  this  paper  the  author  desired  to  record  certain  examples  of 
objects  which  had  been  regarded,  erroneously,  as  vegetable  fossils. 
The  specimens  to  which  he  specially  alluded  were  as  follows: — 
Supposed  fruits  on  which  Geinitz  founded  the  genus  Guilielmites, 
namely  Carpolites  umbonatus,  Sternb.,  and  Guilielmites  jpermianus, 
Gein.,  which  the  author  regarded  as  the  result  of  the  presence  of 
fluid  or  gaseous  matter  in  the  rock  when  in  a  plastic  state ;  some 
roundish  bodies,  which,  when  occurring  in  the  Stonesfield  slate, 
have  been  regarded  as  fossil  fruits,  but  which  the  auth©r  considered 
to  be  the  ova  of  reptiles,  and  of  which  he  described  two  new  forms ; 
and  the  flat,  horny  pen  of  a  Cuttlefish  from  the  Purbeck  of  Dorset- 
shire, described  by  the  author  as  Teudopsis  Brodiei,  sp.  n. 
2.  "  Notes  on  the  Geology  of  part  of  the  County  of  Donegal." 
By  A.  H.  Green,  Esq.,  F.G.S. 
In  this  paper  the  author  described  the  geological  structure  of  the 
country  in  the  neighbourhood  of  the  Errigal  Mountain,  with  the 
view  of  demonstrating  the  occurrence  in  this  district  of  an  inter- 
stratification  with  mica-schist  of  beds  of  rock  which  can  hardly  be 
distinguished  from  granite,  the  very  gradual  passage  from  alterna-' 
