The din was terrific. The sound of the grunting and groaning of the tug's noisy engine was magnified tenfold, echoing and re-echoing along the domed expanse, while clouds of sulphurous smoke permeated everything. Yet, the while, there was the comforting thought that, unless the general direction of the canal had changed, every revolution of the tug's propellers was bearing the fugitives nearer the frontier and freedom.