Jamaica  for  the  most  part  abounds  in  rivers  and  springs,  though 
some  parts  of  the  western  and  midland  districts  are  more  or  less 
destitute  of  water.  St.  Ann  and  Trelawny,  for  example,  have  no 
streams  of  any  importance  over  the  greater  part  of  their  area,  and 
these  parishes  are  among  the  least  malarious.  On  the  other  hand, 
St.  Thomas,  Portland  and  St.  Mary  are  well  watered,  having  numerous 
large  rivers  which  rush  precipitously  down  from  the  mountains  and 
spread  out  into  broad  shallow  streams  with  swampy  margins  at  their 
entrance  into  the  sea.  During  the  dry  season  many  of  these  rivers 
form  comparatively  shallow,  fordable  streams,  but  owing  to  the 
shortness  of  their  courses  and  the  rapidity  with  which  they  descend 
from  the  hills,  they  are  liable  to  sudden  floods,  and  in  a  few  minutes 
a  peaceful  stream  is  transformed  into  a  dangerous  raging  torrent. 
This  is  well  seen  in  the  Yallah’s  district,  where  the  river  spreads  out 
into  a  series  of  shallow  mouths  communicating  with  swampy  grass- 
grown  pools  extending  over  a  large  area. 
The  mean  rainfall  for  the  wTole  Island  from  1880  to  igo6  was 
/6-g  inches,  but  the  rainfall  varies  greatly  in  different  parts, 
being  greatest  in  the  north-eastern  division,  where  the  annual 
rainfall  is  frequently  over  1 00  inches.  The  driest  months  of  the  year 
appear  to  be  January,  February  and  March,  while  the  greatest  rainfall 
occurs  in  May,  June  and  October.  (See  Tables  II  and  III).* 
The  mean  temperature  varies  according  to  elevation.  In  Kingston 
the  mean  temperature  for  eighteen  years  was  while  in  the 
mountains  the  temperature  falls  as  low  as  45°. 
As  the  mountains  are  inhabited  to  a  considerable  height,  it  is  thus 
possible  to  obtain  a  very  large  variety  and  range  of  climate,  and  in 
the  upper  parts  it  may  be  described  as  ideal. 
Table  IV*  shows  the  average  annual  temperature  at  different 
elevations. 
IV.  GENERAL  HISTORY  OF  MALARIA 
For  the  benefit  of  those  who  are  unfamiliar  with  the  history  of 
Malaria,  a  brief  sketch  of  the  principal  facts  may  be  of  advantage. 
Malaria  is  a  disease  which  has  been  recognised  for  many  centuries 
and  was  known  to  the  ancients  as  Marsh  Fever,  owing  to  its  general 
*  Meteorology  of  Jamaica,  by  Marshall  Hall. 
