30 
On  the  Climate  of  the  British  Islands 
on  the  soil,  sinks  below  the  freezing-point.  The  air  on  the  sur- 
face is  often  6°  or  10°  * colder  than  that  4 feet  above  the  ground; 
the  greater  weight  of  the  cold  stratum  causes  it  to  sink  into  the 
hollows  of  the  fields  and  the  valleys ; and  it  is  in  these  situations 
in  calm  clear  nights  that  hoar-frost  is  most  destructive. 
In  the  south-west  of  England  hoar-frost  is  most  injurious 
in  the  spring  to  the  early  vegetables,  the  grass,  the  barley, 
and  the  fruit-blossoms.  On  the  north-east  of  Scotland  the 
potatoes  suffer  in  the  latter  part  of  the  summer  before  they 
are  ripe. 
The  early  vegetable  productions  near  Penzance  need  to  be 
guarded  from  the  effects  of  hoar-frost,  which  is  there  found  more 
injurious  than  ordinary  frost.  The  most  usual  preventive  is  a 
light  covering  of  straw.  It  is  also  common  to  kindle  a fire  to 
windward,  that  the  smoke  may  pass  over  the  crop ; the  current  of 
air  thus  produced  has  more  influence  than  the  heat  of  the  fire  or 
smoke,  for  a free  and  open  circulation  of  air  is  the  best  preven- 
tive against  hoar-frost. 
During  the  past  December  ( 1 84 9)  hoar-frost  for  several  nights 
in  succession  was  most  abundant  on  the  grass-plot  before  my 
house.  The  constant  change  from  frost  to  thaw  injured  and  cut 
the  tender  garden-plants,  and  the  grass  drooped  under  the  effects 
produced.  On  holding  a blade  of  grass  between  my  eye  and  the 
sun,  I perceived  that  the  sap-vessels  were  in  many  places  rup- 
tured, which  was  shown  by  a dark-green  spot ; in  a few  days  these 
spots  turned  yellow,  and  a dry  wind  setting  in,  the  top  of  the 
blades  withered  and  died. 
Thus  in  order  to  get  an  early  piece  of  pasture  in  the 
spring  the  grass  should  be  protected  during  the  continuance  of 
cold  nights.  A light  covering  of  straw,  or  fibrous  dung,  would 
prevent  the  radiation  of  heat  into  space,  and  protect  the  tender 
shoots  from  hoar-frost,  so  as  to  fit  them  for  rapid  growth  under 
the  influence  of  a clear  spring  sky.  There  can  be  no  question 
that  this  is  the  mode  in  which  the  much  lauded  “ fibrous  cover- 
ing ” acts  in  promoting  vegetable  growth,  rather  than  by  exciting 
electrical  action.  The  results  of  Dr.  Fyfe’s  experiments  on 
electro-culture  show,  that  the  application  of  electricity  to  vegeta- 
tion was  followed  by  no  benefit  whatever. f 
70.  Cattle  lying  on  low  lands  on  a clear  night,  are  exposed  to  a 
severe  and  injurious  amount  of  cold  : it  is  probably  for  this  reason 
that  sheep  prefer  to  lie  on  the  most  elevated  ground,  or  on  a bank. 
Cattle  hemels  in  this  point  of  view,  however  rough,  are  highly 
favourable  to  health  and  growth. 
* On  the  19th  August  Dr.  YY'ells  found  the  air  on  the  ground  at  6h.  45m.  7^c 
older  than  that  4 feet  above  the  surface  ; at  8h.  45m.  the  difference  was  12°. 
f Kdin.  Phil.  Jour.,  Oct.  to  Dec.  1845,  p.  153. 
