60 
Our  Canaries 
an  ideal  condition  of  things  requires  a  certain  amount  of  expert  knowledge  to 
carry  it  into  effect. 
THE  ELEMENTARY  POINTS. 
One  of  the  fundamental  laws  of  the  science  is  to  provide  inlets  for  fresh  pure 
air  at  or  near  the  level  of  the  floor,  and  corresponding  outlets  for  the  discharge 
of  foul  used-up  air  in  the  highest  parts  of  the  interior  space.  Of  course  this  is 
simply  utilising  the  known  natural  law  that  heated  air,  being  lighter  than  cold, 
automatically  rises  to  the  top  in  a  confined  space,  and  thus  the  air  which  becomes 
more  or  less  heated  in  the  process  of  breathing,  and  through  the  presence  of 
animal  bodies,  rises  upwards  and  passes  off  through  the  outlet  provided  for  the 
purpose,  at  the  same  time  that  a  corresponding  supply  of  fresh  air  is  pouring 
SLIDING  VENTILATOR   FOR  BIRD-ROOM. 
in  at  the  lower  apertures.  In  an  existing  wooden  structure  this  system  may 
generally  be  carried  out  by  boring  a  row  of  half-inch  holes  with  an  auger  round 
the  top  of  the  walls,  or  in  the  apex  of  the  gables,  as  the  case  may  be,  in  tjbe 
manner  shown  in  the  inset  to  illustration  of  lean-to  aviary  on  page  6,  and^ 
corresponding  row  around  the  bottom  of  the  walls. 
PROVISIONAL  FORESIGHT. 
In  building  a  new  structure,  however,  it  would  be  far  better  to  dispense  with 
such  makeshifts,  and  insert  a  proper  ventilator  in  the  floor ;  and  instead  of  the 
holes  in  the  gables  of  a  span  roof  building,  leave  an  inch  or  so  of  open  space 
all  across  the  ridge  of  the  roof,  and  cover  this  with  a  small  super-imposed 
span  raised  an  inch  or  so  above  the  proper  roof. 
The  amount  of  space  allowed  in  all  cases  for  ingress  and  egress  of  air  must 
be  regulated  by  the  size  of  the  structure,  and  the  number  of  birds  it  is  intended 
to  accommodate.  Sufficient  should  be  provided  to  ensure  the  atmosphere  in 
the  house  being  free  from  any  stufi"y,  birdy  smell,  and  yet  not  enough  to  make 
it  quite  as  chilly  in  cold  weather  as  it  is  in  the  open  air.  A  simple  yet  practical 
test  is  to  take  a  short  walk  in  the  open  air  in  the  morning,  after  the  birds  have 
been  closed  up  all  night,  and  then  go  into  the  room  and  note  the  efifect  of  the 
air  in  the  house.    If  the  ventilation  is  insufficient,  the  air  will  feel  quite  oppres- 
