midnight
`` He has about as much curiosity as a turtle , '' Stan said sourly . `` Sure , an ' they may put on a guard with a brain , '' O'Malley said hopefully . They sat down and tried to think up another scheme . At midnight the guard was changed and they tried their trick on the new man . He was less interested than the first one . He turned his back on them and let them whisper . The boys gave it up and sat down to wait .