After the disappearance of the boy, Ned removed Collins to the deep end of the cut and placed Jackson there as a guard.  He did not want the captive to know what was going on, as a shout to his friends, if they again visited the ledge, might put them in possession of the facts regarding the empty tanks of the Nelson.  Then it would be an easy matter for them to prevent the getting of the gasoline from the wrecked Vixen.