On  the  Cost  of  Agricultural  Buildings. 
567 
posts,  and  two  princesses,  or  wrought-iron  bolts,  bearing  the  name 
of  bolt  instead  of  post;  the  principals,  from  their  own  as  well  as 
the  superincumbent  weight,  are  liable  also  to  deflection,  which 
is  opposed  by  struts  and  collar-beams.  It  is  by  a considerable 
application  of  these  auxiliaries  that  scientific  trusses  are  con- 
structed with  little  weight  and  great  economy  of  timber,  which 
otherwise  often  could  not  be  put  together  at  all  for  want  of  beams 
of  sufficient  size.  Under  30  feet  span,  a tie-beam  A,  and  prin- 
cipals B B,  with  king-post  c,  and  struts  d d,  are  sufficient ; under 
20  feet  span  we  may  dispense  with  the  struts ; but  we  shall  require 
an  extra  depth  for  the  principals. 
From  30  to  45  feet  span,  a truss  should  be  composed  of  two 
queen-posts,  and  a straining  piece  between  their  heads,  and  struts 
from  the  queen-posts  to  the  principals. 
From  45  to  70  feet  span  a truss  will  require  two  queen-posts, 
and  two  small  queen-posts  (or  princesses),  with  a straining-piece 
between  the  queen-posts  and  struts,  from  the  queen-posts  to  the 
princesses  ; also  struts  from  the  princesses  to  the  principals  ; above 
60  feet  span  a straining-sill  may  be  added  on  the  tie-beam  between 
the  queen-posts. 
The  duty  of  the  king-post  is  to  suspend  the  tie-beam  and 
prevent  its  deflection  ; the  common  practice,  of  morticing  the 
king-post  close  into  the  tie-beam,  is  objectionable,  as  being  by 
no  means  the  most  efficient  way  of  counteracting  the  bending  of 
the  beam  ; for,  on  the  contrary,  from  the  general  contraction  of 
all  the  timbers,  as  u’ell  as  by  any  settlement  which  will  most 
likely  occur  in  the  principals,  the  king-post,  as  well  as  all  timbers 
effecting  a similar  purpose,  will  press  down  into  the  tie-beams, 
and  of  course  deflect  them  ; it  is  a common  practice  to  attempt  a 
counteraction  of  this,  by  cambering  or  arching  the  tie-beam,  but 
this  is  only  in  a measure  submitting  a truss  to  continual  action, 
whilst  perfect  repose  is  what  is  required  ; and  this  repose  will 
be  best  obtained  by  suspending  the  tie-beam  from  the  king-post, 
«Scc.,  by  the  means  of  wrought-iron  stirrups.  After  deflection, 
from  settlement  or  otherwise,  the  tie-beam  should  be  wedged  or 
screwed  up,  when  all  the  members  will  be  restored  to  their 
intended  actions. 
The  only  duty  of  struts  is  to  prevent  the  principals  from  bend- 
ing under  their  own  weight  or  that  laid  upon  them,  and  they  must 
therefore  be  directed  to  the  points  of  deflection,  which  will  also 
