in  its  Effect  on  Cultivation. 
19 
greatly  modified,  or  rather  overcome,  by  secondary  causes.  On 
the  American  coast,  the  isothermal  line  of  41°  is  found  in  lati- 
tude 44°.  Tracing  this  line  eastward,  it  appears  thrown  up  in  a 
loop  far  to  the  north  of  Scotland,  and  cuts  the  coast  of  Norway  in 
latitude  64° — the  same  mean  temperature  occurring  on  the  Eu- 
ropean coast,  20°  further  north  than  on  the  American. 
46.  The  isothermal  zone,  between  the  lines  of  41°  and  50°  of 
mean  temperature,  on  the  coast  of  the  New  World,  is  about  300 
miles  wide ; on  the  shores  of  the  Old  World  its  width  is  extended 
to  1100  miles,  and  within  the  warmest  part  of  this  unexampled 
extent  of  nearly  similar  temperature  lie  our  favoured  isles.  It  is 
a problem  more  important  than  curious  to  solve  the  cause  of 
this  peculiarity  of  climate,  for  the  nature  of  the  effects  produced 
can  only  be  clearly  understood  when  their  origin  is  ascertained. 
This  inquiry  will  not  therefore  be  based  upon  any  ingenious 
theory,  but  upon  the  existence  of  well-established  facts. 
47.  After  a careful  consideration  of  the  facts  which  bear  on 
this  intricate  subject,  the  only  consistent  solution  appears  to  be, 
that  the  peculiarities  of  our  climate  are  caused  mainly  by  the  two 
great  currents  of  the  North  Atlantic  Ocean — the  Arctic  current 
sweeping  away  the  enormous  quantities  of  loose  ice  of  the  frigid 
zone  to  the  American  coast,  so  screening  us  from  the  rigour  of  the 
frozen  north ; and  that  mighty  ocean  river,  the  Gulf-stream, 
bringing  to  our  shores  the  heated  waters  of  the  torrid  zone. 
48.  I have  made  an  attempt  to  arrange  and  combine  the  lead- 
ing facts  which  bear  on  this  subject  in  the  chart  which  accom- 
panies this  essay.  The  temperature  of  the  sea  is  deduced  from 
the  numerous  thermometrical  observations  of  Humboldt,  Fitz- 
roy,  King,  Sabine,  Lawson,  and  other  sources.  The  position  of 
the  isothermal  lines  is  fixed  by  Dove’s  Tables  of  Temperature. 
49.  The  Arctic  current  originates  in  the  sea  on  the  north  of 
Europe,  and  runs  from  Spitzbergen  across  the  upper  part  of  the 
Atlantic  to  the  coast  of  Greenland ; passing  between  that  country 
and  Iceland,  it  rounds  Cape  Farewell,  sweeps  the  western  coast  of 
Greenland  to  the  67°  of  latitude;  from  thence  it  turns  west  and 
south  along  the  shores  of  Labrador  and  Newfoundland,  where  it  is 
lost  in  the  waters  of  the  Atlantic.  The  breadth  of  this  current  is 
in  some  places  from  250  to  300  miles.  Its  velocity  varies,  in  some 
parts  of  its  course,  from  8 to  16  miles  per  day.  The  icy  masses 
it  bears  along  are  supposed  to  be  about  two  months  in  making 
the  circuit  from  Cape  Farewell  to  the  coast  of  Labrador.  Im- 
mense fields  and  bergs  of  ice  are  annually  borne  southward  by 
this  current.  Along  the  coast  of  Greenland,  they  are  found  to 
extend  from  200  to  300  miles  from  the  shore,  marking  the  width 
of  the  stream.  A continuous  line  of  polar  ice  extends  throughout 
the  course  of  this  current  from  Iceland  to  Newfoundland.  In 
c 2 
