Now in all these measures, as in the surveillance of {207} shipping and the protection of Anglo-French traffic, the Dover Patrol necessarily played a commanding and indeed vital part. Upon it devolved the guarding of the southern of the two outlets by which alone the German submarines might escape into the Atlantic; and the difficulties were trebled by the enemy's possession of the West Flanders ports. With the geography and defences of these and their strategical significance we shall deal more particularly in the next chapter as with the splendid episode in which the Dover Patrol rendered them largely valueless to the enemy. But it must never be forgotten that, for nearly four years, the Dover Patrol carried on its work with the hostile ports of Zeebrugge and Ostend always within three hours' steaming. Darkness is the friend of the destroyer, daylight the friend of the submarine. Both were stationed at these enemy ports; and the strain upon the Dover command can thus be gauged. Further, it has to be remembered that through no other channel in the world passes so continual a procession of ships, how integral in the life of this country let a single incident suffice to show. In perhaps the darkest hour of the war, a serious proposal was made to the Government completely to seal the Straits of Dover for a certain defensive purpose. The proposal was examined, and it was then ascertained that, as regarded London alone, one of the following alternatives must immediately follow. Either it would have to be arranged, at a time when pressure upon our rolling-stock was at its severest, that no less than seventy-two additional trains should enter London daily, or that more than {208} three and a half millions of London's population should be removed to the Atlantic ports that it was proposed to use. The suggestion was thus found to be wholly impracticable, but its examination at least proved the immense responsibility resting upon the Dover Patrol and the officers in charge of it.