But history has always shown that it is a very difficult matter to impose our will upon a weaker naval adversary, and that, instead of giving us the opportunity of destroying his armed naval forces, he usually keeps the main body of those forces--the Battle Fleet--in positions of safety in fortified harbours, where they are a constant threat to the sea communications of the stronger naval Power, and force upon that Power a watching policy so that the enemy may be engaged, should he put to sea, before he is able to gain any advantage. The watching policy in the great wars of the Napoleonic era was carried out by keeping our squadrons, through fair or foul weather, in the vicinity of those ports of the enemy in which his fleet lay. Occasionally our ships were driven off by stress of weather, but they regained their stations as soon as conditions permitted. During this war, however, the advent of the submarine and destroyer, and, to a lesser extent, the use of the mine rendered such dispositions impossible. No large ship could cruise constantly in the vicinity of enemy bases without the certainty that she would fall an early victim to the attacks of submarines. Destroyers could, it is true, afford some measure of protection, but destroyers have a very limited range of action, and could not keep the sea off the enemy’s distant coast even in good weather for a sufficient length of time. Periodical relief of the destroyers was an impossibility, owing to the great numbers that would be required for this purpose. Moreover, even if the submarine danger could be overcome, the heavy ships would be so open to attack by enemy destroyers at night, if cruising anywhere near enemy bases, that they would certainly be injured, if not sunk, before many days had passed. These facts had been recognised before the War and a watching policy from a distance decided upon, the watch being instituted for the purpose of preventing enemy vessels from gaining the open sea, where they would constitute a danger to our sea communications. Now a watch maintained at a distance from the port under observation is necessarily only partial, except in circumstances where the enemy has to pass through narrow straits before gaining open water. The chances of intercepting enemy ships depend entirely on the number of watching vessels and the distance that those on board them can see. At night this distance is very short--on a dark night not more than a quarter of a mile, and even in daylight, under the average conditions of visibility obtaining in the North Sea, it is not more than six to eight miles. The North Sea, though small in contrast with the Atlantic, is a big water area of about 120,000 square miles in extent. The width across it, between the Shetland Islands and Norway (the narrowest portion), is 160 miles, and an additional 40 miles (the Fair Island Channel) would need to be watched also if a patrol were established along this line.