Seven Keys to Baldpate by Earl Derr Biggers

For fully five seconds Mr. Magee and the man with whom he had collided stood facing each other on the balcony. The identical moon of the summer romances now hung in the sky, and in its white glare Baldpate Mountain glittered like a Christmas-card. Suddenly the wind broke a small branch from one of the near-by trees and tossed it lightly on the snow beside the two men -- as though it were a signal for battle.

"A lucky chance," said Mr. Magee. "You're a man I've been longing to meet. Especially since the professor left his window open this afternoon."