568  On  the  Cost  of  Agricultural  Buildings. 
be  the  position  of  purlins  ; and  it  must  not  be  forgotten  that  struts, 
as  well  as  all  inclined  timbers,  increase  in  stren?th  with  the  size 
• . . 7 o 
of  the  angle  of  inclination,  until  at  maximum,  when  the  piece  will 
be  vertical,  or  the  sine  equal  to  the  radius. 
Struts  should  never  on  any  account  rest  on  the  tie-beam,  as 
they  then  tend  very  considerably,  according  to  the  weight  of  the 
roof,  to  deflect,  and  to  strain  the  beam,  which  in  its  turn  acts  most 
injuriously  upon  the  walls,  which  it  tends  to  upset. 
All  the  timber  used  in  trusses  should  have  been  felled  at  least 
two  years,  and  should  have  been  at  least  six  months  out  of  water; 
builders,  however,  often  employ  timber  soaked  with  water,  and 
the  result,  after  a few  months,  is  considerable  shrinkage,  and 
consequently  splitting  from  abutting  joints  ; to  this  most  injurious 
evil  struts  and  principals  from  their  position  are  peculiarly  liable, 
the  first  at  the  heads,  and  the  second  at  the  feet ; but  preparations 
against  the  evil  may  be  made  by  leaving  the  abutting  joints  open 
at  the  internal  angle  nearest  the  post. 
The  introduction  of  iron  in  trusses  is  comparatively  an  intro- 
duction of  recent  date,  but  within  the  last  few  years  its  use  has 
rapidly  increased.  Wrought-iron  for  trusses  should  be  of  the 
best  scrap  quality,  and  free  from  all  flaws  ; it  should  be  carefully 
examined  piece  by  piece  before  it  is  allowed  to  be  used,  and  all 
faulty  samples  should  be  thrown  aside.  Spikes  should  be  flat- 
edged,  and  they  should  be  driven  with  the  edge  across  the  fibre 
of  the  wood,  and  not  longitudinally  with  the  timber,  which  would 
tend  to  split  it. 
With  regard  to  bolts,  when  they  are  round,  the  holes  made  to 
receive  them  should  be  exactly  of  the  same  diameter  as  that  of 
the  bolts,  and  should  be  driven  exactly  parallel  to  the  sides  of  the 
wood  that  are  to  be  parallel  to  the  bolt;  for  square  bolts  the  hole 
should  be  somewhat  less  than  the  diagonal  of  the  bolt. 
Bolts  and  nuts  should  be  well  greased  before  they  are  used ; 
when  of  great  length  they  should  be  of  sufficient  diameter  to  resist 
torsion,  and  they  should  never  be  placed  too  near  the  end  of  a 
timber;  care  should  be  taken  that  screw-bolts  and  screws  are  not 
hammered,  as  the  fibres  of  the  wood  are  thereby  torn  ; it  is  a 
gross  error  to  fancy  that  the  elasticity  of  the  wood  will  prevent 
this  laceration.  Whenever  it  is  feared  that  a nut  may  slacken  by 
a retrograde  motion,  a second  nut  may  be  added,  which  will 
effectually  prevent  any  loosening. 
Iron  straps  and  stirrups  should,  before  use,  be  heated  to  a blue 
heat,  and  then  saturated  with  raw  linseed  oil,  as  a preventive 
against  rust.  A strap  one  inch  wide  may  be  At  thick;  li  inch 
wide,  t5t  thick  ; and  2 inches  wide,  £ thick.  Straps  should  be 
secured  by  screws,  the  heads  of  which  should  be  countersunk, 
and  sometimes  by  bolts.  Cast-iron  plates  and  shoes  are  very 
