396 
ME.  FAIEBAIEN  ON  THE  EESISTANCE  OF  TTJBES  TO  COLLAPSE. 
Taking  Experiment  20  as  correct,  we  have  for  the  collapsing  pressm-e  of  a similar 
tube,  5 feet  long,  60 : 30 ; : 22  : ^=11,  or  1-5  lb.  less  than  Experiment  19.  Similarly, 
58^ : 30 : : 22  : ^=11-2,  or  0*2  lb.  more  than  in  Experiment  18.  From  these  results  we 
may  reasonably  conclude  that  the  law  affecting  the  strength  of  tubes  is,  other  things 
being  the  same,  that  the  collapsing  pressure  varies  inversely  as  the  length. 
The  tube  S,  when  compared  with  the  6-inch  tubes  only  one-half  the  length,  required 
a pressure  of  less  than  one-fourth  to  cause  collapse.  This  apparently  low  pressure, 
though  at  first  sight  anomalous,  is  confirmed  by  the  result  of  Experiment  19.  Similarly, 
comparing  tubes  C and  D,  Table  I.,  with  tubes  O and  P,  Table  111.,  we  have, 
Length.  Diameter.  Pressxire. 
CandD  ....  39  ...  4 ...  65 
O and  P . . . . 39  . . . 8 . . . 31-o  ; 
that  is,  whilst  the  diameters  are  to  one  another  as  1:2,  the  pressm-es  of  collapse  are 
as  65 : 31-5,  or  as  2:1  very  nearly.  These  comparisons,  which  might  be  continued, 
evidently  point  to  a law  affecting  the  diameters  similar  to  that  of  the  lengths. 
In  order  to  ascertain  the  different  powers  of  resistance  of  tubes  composed  of  thick 
plates  and  of  different  diameters,  a strong  tube  only  9 inches  in  diameter,  and  formed 
of  a plate  ^-inch  thick,  was  constructed,  to  match  and  compare  with  another  tube,  also 
of  ^-inch  plate,  and  18f -inches  in  diameter.  The  9-inch  tube  was,  however,  found  to  be 
too  strong  for  the  retaining  powers  of  the  cylinder,  which  it  would  not  have  been  safe  to 
have  trusted  above  500  lbs.  per  square  inch.  Finding  the  strength  of  the  small  tube 
too  great  for  the  containing  vessel,  two  new  tubes  were  made,  one  with  a lap-joint  as  at 
A in  the  annexed  sketch,  and  another  with  a butt-joint  as  at  B.  These 
tubes  were  made  of  plates  |^th  of  an  inch  thick,  the  object  of  the  dif- 
ference being  two-fold ; — -first.,  to  ascertain  to  what  extent  the  strength 
of  the  tube  was  reduced  by  the  lap-joint ; and  secondly,  to  compare  with 
the  tube  18f  inches  in  diameter,  and  double  the  thickness  of  plates. 
In  the  construction  of  boilers  the  lap-joint  is  almost  invariably  in  use; 
and  it  must  at  once  appear  obvious  that  any  such  departure  from  the 
true  circle  in  cylindrical  tubes  must  injure  their  powers  of  resistance  to 
external  pressure. 
Fig.  2. 
