for each line.  The instructor stands at one end of the
             space between; an assistant at the other end.  On the
             instructor’s whistle, the lines advance, guiding carefully
             on their leaders.  When about ten paces apart, they charge,
             each seeking to break and roll up the opposing line. 
             Sticks are carried and used as rifles with bayonets fixed. 
             Any other use disqualifies.  Use of the butt is barred.  One
             thrust on the plastron or mask, or two hits on the
             extremities, disables the recipient, who must promptly
             retire—­or be retired.  The combat continues until the
             second whistle, blown not more than 30 seconds after
             contact; when they cease fighting promptly, separate, and
             form as before.
      c. Criticism:
         After each combat, the instructor will criticize the manner
             of advance and of fighting, especially the alignment kept
             in the advance and the team work in combat, and the
             advantage taken of opponents’ mistakes.  He counts the
             casualties and awards the decision.  He must continually
             urge the men never to lag behind nor advance ahead of the
             line, never to allow large gaps to occur in the line, and
             always to seize the advantage given by opponents who
             disregard these principles.
      d. The terrain for this exercise should be frequently varied. 
             It may also be conducted at night, the opposing sides
             being clearly distinguished.[Q]

[Footnote Q:  The last exercise was devised and perfected by M. Jules Leslabay, Master of Fencing, Harvard R.O.T.C., 1917.  It is more completely described in his “Manual of Bayonet Training.”]

Machine Guns.

1.  Properties of the machine guns are divided into three general classes:  Mode of action, fire, and inconspicuousness.

   (a) THE MODE OF ACTION.—­The machine gun acting only by its fire
      can prepare an attack or repulse an offensive movement, but it
      does not conquer ground.  The latter role is almost exclusively
      that of infantry which is fitted for crossing all obstacles. 
      When it will suffice to act by fire, employ the machine gun in
      preference to infantry, preserving the latter for the combined
      action of movement and fire.  By the employment of the machine
      gun economize infantry, reserving a more considerable portion
      of it for manoeuvre purposes.
   (b) FIRE.—­Machine gun fire produces a sheath, dense, deep but
      narrow.  The increase of the width of the sweeping fire gives to
      the sheath a greater breadth, but when the density becomes
      insufficient,