Cook's glider gun, velocity -20c/77 ($\approx$ -c/4).

  

Figure 2.34: The glider gun glides, and having glided, glides on. The horizontal lines delimit one single period.

The glider gun has a velocity intermediate between the velocities of the streams whixh it launches, giving them ample opportunity to clear the domain of the gun. If A and B gliders annihilate on colliding, there is no obstacle to having a whole phalanx of glider guns, although they won't produce any extra effect on the remainder of the plane (except maybe for spacing) beyond the effect of a single gun.