nine o'clock
But the mate 'll do that easy . ' E promised me . '' `` Bill , you _ are _ an angel ! '' It was , after all , absurdly easy , as Bill had promised ; and the easier by help of the river-fog , which by nine o'clock -- the hour agreed upon -- had gathered to a thick grey consistency . If the dock were policed at this hour , no police , save by the veriest accident , could have detected the children crouching with ` Dolph behind a breastwork of paraffin-casks , and waiting for Bill 's signal -- the first two or three bars of _ The Blue Bells of Scotland _ whistled thrice over . The signal came . The gang-plank was out , ready for the crew 's return ; and at the head of it Bill met the fugitives , with a caution to tread softly when they reached the deck .