As  it  is  occasionally  necessary  to  make  various  miscellaneous  repairs
to  lock  gates  and  to  paint  the  ironwork,  the  gate  recesses  should  be
so  constructed  as  to  admit  of  being  un watered  with  a  minimum  expendi¬
ture  for  pumping. The  recesses  at  Lock  No. 4  gave  considerable
trouble  when  being  unwatered  for  the  purpose  of  gate  construction,
as  no  special  arrangements  had  been  made  to  facilitate  the  unwater¬
ing  of  a  gate  recess  without  unwatering  the  entire  lock  or  the  entire
length  of  a  gate  track. A  good  arrangement  for  coffering  off  a  gate
recess  consists  of  two  sets  of  vertical  slots  in  the  recess  walls  near
the  entrance  to  the  recess,  and  timbers  which  can  be  placed  horizon¬
tally  with  their  ends  supported  in  the  slots. If  these  wooden  bulk¬
heads  are  not  in  themselves  sufficiently  water-tight,  the  space  between
them  can  be  filled  with  ashes,  sawdust,  or  river-bed  material. A  sump
should  be  left  in  the  bottom  of  the  recess,  and  it  should  be  located
near  the  entrance  so  that  a  floating  pump  can  be  used  if  desired. It
was  originally  believed  to  be  necessary  to  provide  for  coffering  off  the
entire  length  of  a  gate  track,  and  foundation  boxes  for  trestles  of
Poiree  needle  dams  were  placed  across  the  lock  on  both  sides  of  each
gate  track  as  shown  in  Plate  XIII. These  double  sets  of  boxes  are
not  now  considered  essential,  as  it  is  believed  that  if  it  is  necessarv
to  pump  out  the  entire  length  of  a  gate  track  the  lock  itself  can  be  as
easily  coffered  off  and  pumped  out,  both  gates  being  inspected,  painted,
etc.,  at  the  same  time. Of  course,  means  for  doing  this  easily  should
be  provided. The  cofferdams  for  Lock  No. 4  are  of  removable  steel
trestles  which  fit  into  boxes  fastened  to  the  foundation  above  the
upper  gate  track  and  below  the  lower  gate  track,  the  water  seal  being


V


Pla  te


11


formed  of  Poiree  needles  made  of  2  by  12  inch  14-foot  hemlock  planks
supported  by  6  bv  6  inch  waling  pieces. Plate  XIV  shows  a  section
of  one  of  these  cofferdams. These  dams  are  quite  water-tight. When
Lock  No. 