on  Air>  Sec . 2 1 7 
prevented  by  too  much  moifture,  or  the  effluvia  are  ab- 
forbed  in  paffing  through  the  fuperincumbent  bed  of 
water:  perhaps  the  cold  generated  by  evaporation  may 
have  fome  effedt. 
This  fully  proves  the  propriety  of  Sir  john  Pringle’s 
remark,  where,  in  giving  cautions  for  avoiding  difeafes 
arifing  from  putrid  air,  he  fays,  “ As  for  cantonments  in 
“ marfhy  grounds,  if  the  troops  muft  remain  there  in 
“ the  dangerous  feafon,  it  will  be  better  to  float  the  fields 
“ entirely,  than  to  leave  them  half  dry ; for  the  fhal- 
44  lower  the  water  the  more  it  will  corrupt,  and  the  eva- 
44  poration  will  be  greater  in  proportion.”  How  beauti- 
fully is  this  illuftrated  by  the  twenty-firft,twenty-fecond, 
twenty*third,  and  twenty-fourth  experiments!  An  in- 
ftance  of  the  perfedt  agreement  of  faithful  obfervation 
with  truth  and  nature. 
exp.  xxv.  Two  ounces  of  dirt  fwept  from  the  ftreets 
were  inclofed  in  the  phial  as  before ; after  Handing  toge- 
ther  twenty-four  hours,  the  air  was  found  to  be  dimi- 
niihed  confiderably,  from  62°  to  50°. 
Hence  it  appears  how  well  the  magiftrates  confult 
the  health  of  the  inhabitants,  as  well  as  the  neatnefs 
of  cities  and  large  towns,  by  enforcing  due  attention  to 
the  cleaning  and  paving  of  the  ftreets  in  their  refpec- 
tive  diftridts. 
