The motion of the lighter makes Decoud "glad" because it gives him the
external reference that his intellect requires. He comments on the troop ship
"trying to make out where they are" (with a deeper meaning that they
too are lost in a world of pure subjectivity) then at once tests the water again
as if to reassure himself as to his own different condition. "Smartly"
also means "intelligently"; the sentence can be read to say, "as
we are moving I have used my mind quite well," underscoring the desire of
his intelligence for an objective reference point.