The two talked for a moment longer, and then Collins left the roof. Later, Ned saw him moving through the street below in the direction of the place where the Nelson had been left.  The boy hardly knew what to make of Collins.  He might be a steam pump salesman, just as he had described himself, and, again, he might be a spy sent out by Lyman's enemies to discover the plans of the Boy Scouts--even to wreck the Nelson if possible.  He decided to, if possible, learn something of the fellow before taking him on board the aeroplane.