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.