The  pass  horses  are  all  of  the  same  length  and  are  made  of  forged
steel  of  the  shape  shown  in  Plate  XVIII.  The  direct  stress  to  which
the  horse  is  subjected  is  very  small,  and  proportions  are  used  which
indicate  in  direct  tension  a  factor  of  safety  of  about  45;  however,  it
is  essential  for  the  horse  to  have  sufficient  strength  to  resist  bending
stresses  and  stresses  due  to  accidental  blows  from  boats,  etc.,  experi¬
ence  of  late  having  indicated  that  the  size  at  present  used  is  none  too
large,  if  large  enough.  Occasionally  a  horse  is  broken  from  one  cause
or  another,  but  if  a  few  extra  ones  are  on  hand  a  diver  can  replace  the
broken  horse  by  a  new  one.  This  brings  up  the  question:  "At  what
time  during  construction  should  the  movable  parts  be  placed?”
Under  the  contract  for  the  250  feet  of  the  pass  first  built  at  Dam  No.  4,
none  of  the  movable  parts  were  placed,  and  they  have  had  to  be  put
in  place  after  the  cofferdam  had  been  removed  and  while  the  sill  was
from  3  to  6  feet  under  water  and  in  a  stiff  current.  A  diver  is  used
to  place  the  journals  of  the  horse  axles  in  the  horse  boxes  and  to
fasten  them  in  place  with  the  quoins  provided  for  that  purpose;  the
rest  of  the  work  of  assembling  the' parts  is  done  from  the  deck  of  the
maneuvering  boat.  The  style  of  horse  box  used  at  Dam  No.  4  is
shown  in  Plate  XIX.  With  this  style  of  box  it  is  necessary  for  the
horse  to  be  raised  vertically  after  its  axle  is  shoved  under  the  boxes
and  before  the  quoins  can  be  placed  underneath  the  axle;  this  gives
considerable  trouble  to  the  diver.  The  cost  of  placing  the  horses
under  water  is  greater  than  the  cost  of  placing  them  while  "in  the


14


dry,”  and  the  result  is  not  subject  to  inspection.  If  possible,  the
movable  parts  should  be  assembled  in  place  before  flooding  the  coffer¬
dam.
