IHE.  FAIEBAIEN  ON  THE  EESISTANCE  OF  TUBES  TO  COLLAPSE. 
411 
fatal  accidents  should  have  occurred  from  the  collapse  of  the  internal  flues,  followed 
immediately  by  the  explosion  and  rupture  of  the  outer  shell.  To  remedy  such  evils, 
and  to  place  the  security  of  vessels  so  important  to  the  community  upon  a more  certain 
basis,  it  is  essential  that  every  part  should  be  of  uniform  strength  to  resist  the  forces 
brought  to  bear  upon  it.  The  equahzation  of  the  powers  of  resistance  is  the  more 
important,  as  the  increased  strength  of  the  outer  shell  is  absolutely  of  no  value,  so  long 
as  the  internal  flues  remain,  as  at  present,  liable  to  be  destroyed  by  collapse,  at  a 
pressure  of  only  one-third  of  that  required  to  burst  the  envelope  which  surrounds  them. 
The  following  Table,  deduced  from  my  own  experiments,  exhibits  the  safe  working 
pressure,  and  the  bursting  pressure  of  boilers  of  different  diameters,  calculated  for  an 
external  shell  of  a thickness  of  fths  of  an  inch. 
Diameter  of  boiler. 
Working  pressure. 
Bursting  pressure. 
ft. 
in. 
lbs. 
lbs. 
3 
0 
118 
708i 
3 
6 
101 
607 
4 
0 
88i 
53] 
4 
6 
78f 
472 
5 
0 
70f 
425 
5 
6 
64J 
386i 
6 
0 
59 
354 
6 
6 
54i 
326f 
7 
0 
50i 
303i 
7 
6 
47 
283i 
8 
0 
44 
265f 
8 
6 
411 
250 
Taking  from  the  above  Table  the  strength  of  a boiler  7 feet  in  diameter,  we  find  its 
bm’sting  pressure  to  be  303  lbs.  per  square  inch.  For  such  a boiler  the  flues  would  be 
ordinarily  3 feet  in  diameter,  and  of  the  same  thickness  of  plates  as  the  shell ; and  by 
the  formula,  log  P=1'5265-|-2T9  log  100/^ — log(L.D.},  we  obtain  for  their  collapsing 
pressure  87  lbs.  per  square  inch.  As,  however,  the  formula  does  not  apply  with  strict- 
ness to  tubes  of  such  length,  the  actual  collapsing  pressure  will  be  somewhat  greater 
than  this.  The  immense  excess  of  strength  in  the  outer  shell  is,  however,  sufficiently 
apparent ; the  extra  thiclmess  of  boiler  plate  which  causes  it  being  so  much  material 
thrown  away,  adding  nothing  to  the  strength  whilst  the  flues  remain  in  so  dangerously 
weak  a condition. 
To  meet  this  disparity  of  strength,  the  experiments  indicate  the  necessity  of  shorter 
flues,  and  one  of  them  shows  how  this  may  be  obtained,  practically  and  efficiently, 
without  interfering  with  the  present  construction  of  boilers.  In  Experiment  6,  Table  I., 
the  tube  F w^as  divided  into  three  parts  by  two  rigid  rings  soldered  upon  its  exterior, 
and  its  powers  of  resistance  were  thus  increased  in  the  ratio  of  three  to  one ; virtually, 
the  length  was  reduced  in  this  ratio,  and  the  strength  was  actually  increased  from  43 
to  140  lbs.  per  square  inch. 
It  is  proposed  to  apply  a similar  construction  to  the  flues  of  boilers,  to  equalize  their 
powers  of  resistance  with  those  of  the  outer  shell,  on  the  supposition  that  the  law  of 
