Amsipt.?i887arm"}     8°me  Remarks  on  Grape  Culture.  435 
those  which  cause  the  heavier  deposition  of  dews  in  these  localities. 
During  clear  nights  currents  of  cool  air  run  downwards  on  the  in- 
clined lands  to  the  bottom  of  the  valleys.  These  currents  are  the  re- 
sult of  the  sudden  depression  of  temperatures  sustained  by  the  surface 
of  the  earth  in  consequence  of  rapid  radiation ,  by  which  the  stratum 
of  air  in  immediate  contact  with  that  surface  becomes  specifically 
heavier  by  condensation,  and  descends  into  the  valley,  which  then 
rapidly  cools,  while  the  warm  air  of  the  valley  is  lifted  up,  and  im- 
pinges on  the  sides  of  the  hills,  and  so  far  as  this  warm  stratum  ex- 
tends there  is  no  condensation  of  moisture  such  as  occurs  in  the  low 
grounds  in  the  form  of  heavy  dews  in  summer,  and  which  in  cool 
weather  freezes  and  becomes  hoar-frost.  The  effects  of  this  stratum 
of  warm  air  upon  vegetation  on  hillsides  is  very  well  defined  where 
early  autumn  frosts  have  destroyed  the  foliage  of  the  trees  below  a 
certain  line,  which  is  sometimes  called  the  vernal  line,  or  line  of  no 
frost ;  above  this  line,  and  within  the  limits  of  the  extent  of  the  warm 
stratum  or  zone,  vegetation  is  unharmed.  The  altitude  to  which  this 
line  reaches  above  the  bottom  of  the  valley  is  dependeut  upon  the 
mean  temperature  of  the  day  and  night,  or  rather  upon  their  compara- 
tive difference  at  the  time  of  its  occurrence  ;  when  the  temperatures  of 
both  are  high,  the  lower  places  only  are  affected  by  the  frost,  but  when 
low,  the  frost  will  extend  higher  np  on  the  hills.  If  we  consider  the 
climate  conditions  of  localities  where  grapes  do  well,  we  will  find  that 
they  are  those  which  are  nearly  exempt  from  dews,  and,  as  a  conse- 
quence, all  varieties  of  grapes  retain  their  foliage  during  summer.  In 
other  words,  the  distinguishing  peculiarity  of  a  good  grape  climate  is 
that  of  the  entire  absence  of  mildew  on  the  foliage  of  the  grape,  and 
this  is  entirely  independent  of  cultural  processes  of  manipulation  or 
training,  or  of  the  quality  of  the  soil  in  which  they  are  planted,  al- 
though the  latter  may  sometimes  exert  an  auxiliary  influence. 
In  illustration  of  the  conditions  which  constitute  a  good  grape  cli- 
mate mention  may  be  made  of  the  high  lands  bordering  Keuka  Lake, 
in  Steuben  County,  New  York.  These  steep  hillsides  are  covered- 
with  vineyards  which  extend  for  several  hundred  feet  above  the  level 
of  the  lake ;  the  soil  is  shaly,  and  in  many  places  the  surface  is  very 
thickly  covered  with  loose  stones.  On  these  hillsides  mildew  is  com- 
paratively unknown,-  the  Catawba,  Iona,  Delaware,  and  indeed  all  va- 
rieties of  native  grapes,  except  those  which  require  a  longer  season 
than  the  climate  affords,  mature  to  a  degree  of  perfection  which  they 
