Beamed Up

Two Corkmen were escaping from a well-protected jail at night. All they had with them was a flashlamp, so when they reached the forty-foot wall surrounding the jail one Corkman said to the other:
'I'll shine the flashlamp up to the top of the wall and you climb up along the beam'.
'Hold on', said the second Corkman, 'how do I know that you won't switch off the lamp when I am halfway up?'