  In this experimental study, we investigate the interaction of
gravity-capillary solitary waves generated by two surface pressure sources
moving side by side at constant speed. The nonlinear response of a water
surface to a single source moving at a speed just below the minimum phase speed
of linear gravity-capillary waves in deep water ($c_{min}\approx23$ cm
s$^{-1}$) consists of periodic generation of pairs of three-dimensional
solitary waves (or lumps) in a V-shaped pattern downstream of the source. In
the reference frame of the laboratory, these unsteady lumps propagate in a
direction oblique to the motion of the source. In the present experiments, the
strengths of the two sources are adjusted to produce nearly identical responses
and the free-surface deformations are visualized using photography-based
techniques. The first lumps generated by the two sources move in intersecting
directions that make a half angle of approximately 15 degrees and collide in
the centerplane between the sources. A steep depression is formed during the
collision, but this depression quickly decreases in amplitude while radiating
small-amplitude radial waves. After the collision, a quasi-stable pattern is
formed with several rows of localized depressions that are qualitatively
similar to lumps but exhibit periodic amplitude oscillations, similar to a
breather. The shape of the wave pattern and the period of oscillations depend
strongly on the distance between the sources.
