566  On  the  Cost  of  Agricultural  Buildings. 
where  the  material  abounds  ; the  mode  of  construction  is  so  well 
understood  by  the  workmen  in  such  districts,  that  it  requires  no 
comment  here,  except  that  the  stone  be  well  bonded,  as  described 
for  brickwork,  set  on  its  natural  bed,  that  is,  in  the  position  in 
which  it  is  found  in  the  quarry,  and  cramped  with  metal  cramps 
where  requisite. 
Carpentry  is  a mechanical  art,  requiring  considerable  skill  on 
the  part  of  the  carpenter ; and  in  consequence  of  the  art  being 
scarcely  understood  by  non-professional  gentlemen,  the  builder'  is 
enabled  to  defraud  his  employer  with  impunity,  should  he  be  so 
inclined. 
The  scientific  framing  of  roofs,  floors,  partitions,  and  all  other 
descriptions  of  framing,  affords  the  most  satisfactory  applications 
which  can  be  made  of  mechanical  science  to  the  arts  of  common 
life  ; but  it  is  a science  rarely  acquired  by  country  artizans.  This 
want  of  skill  leads  to  an  injudicious  disposal  of  materials,  often- 
times wastefully  applied,  and  they  are  frequently  incapable,  from 
position  when  fixed,  to  resist  the  various  strains  to  which  they  are 
subjected. 
Roofs  in  particular  should  be  so  constructed  that  the  tendency 
which  they  have,  from  their  position,  to  overturn  the  walls  they 
rest  upon,  be  counteracted.  This  is  most  effectually  guarded 
against  when  beams  of  timber  or  rods  of  iron  extend  across  the 
building  at  the  top  of  the  walls,  and  secured  to  the  wall-plates  ; 
they  are  then  called  “ ties,”  and  these  ties  should  not  be  placed 
more  than  10  feet  apart.  It  is  from  the  absence  of  them,  and  the 
low  pitches  or  angles  often  given  to  roofs,  that  premature  decay 
and  destruction  are  occasioned  to  buildings  thus  constructed.  The 
annexed  diagram  will  illustrate  this  fact. 
Let  A b,  B c,  c A,  be  three  timbers,  of  which  the  two  last  rest 
on  the  first,  A b ; it  will  at 
once  be  seen  that  B c,  A c,  by 
their  weight  alone,  will  slip  at 
the  points  of  bearing  and 
fall  ; at  these  points,  therefore, 
they  must  be  secured,  which 
may  be  done  by  form  and  by 
auxiliary  means ; by  its  own 
weight  the  beam  A B would 
bend,  tending  to  fall  inwards,  and  to  overthrow  the  walls; 
but  if  b c and  a c are  firmly  secured  at  A,  B,  c,  and  a B is 
tied  up  by  c D,  neither  the  first  nor  second  evil  can  occur, 
and  we  have  a truss ; the  mechanical  action  of  tie-beams,  prin- 
cipals, king-posts,  queen-posts,  princesses,  struts,  collar-beams, 
or  straining  pieces,  is  involved  in  this  principle  ; as  where  the 
tie-beam  is  of  great  length  it  is  upheld  by  a king-post,  two  queen- 
