^'wiri.  fsn!'"'}       Water  Air-pump  and  its  Uses,  503 
A  greater  distance  between  the  water  air-pump  and  evaporating 
basin  necessarily  increases  the  condensation  of  the  aqueous  vapor  in 
Fig.  2. 
the  connecting-pipe.  To  prevent  their  returning  into  the  evaporating 
basin,  a  metallic  recipient,  forming  a  ring  about  6  cm.  high,  is  in- 
serted between  the  basin  and  head.  This  ring  has  on  its  inside  a  gut- 
ter, connecting  on  one  side  with  a  pipe  of  any  desired  length.  (See 
tig.  2.)  The  pipe  has  two  stop-cocks,  the  inside  one  being  kept  open 
while  the  outer  is  closed.  To  discharge  the  accumulated  water,  the 
inner  cock  is  closed,  and  the  water  run  off  by  emptying  the  outer  one, 
which  is  then  again  closed  and  the  inner  one  gradually  opened,  to 
prevent  the  water  accumulated  in  the  gutter  from  being  forced  back 
into  the  basin  by  the  air  now  contained  in  the  pipe.  It  will  be  ob- 
served that  evaporation  may  thus  be  uninterruptedly  continued  with- 
out interfering  with  the  vacuum. 
The  more  the  pressure  is  reduced  inside  the  apparatus,  the  more 
violent  will  be  the  concussions  produced  by  the  vapors  escaping  from 
the  liquid,  increasing  the  liability  of  the  latter  to  boil  over.  Great 
care  must  therefore  be  exercised  in  the  application  of  heat,  or  the 
violence  of  the  concussions  at  once  lessened  by  admitting  a  little  air 
