Nor was the protection for submarine attack left unprovided for. The whole of the under-water surface was armour-plated, not merely by one skin but by two complete layers of steel, the thickest being on the inside. In the double bottoms thus formed, oil, the food for the motors, was stored. A powerful torpedo might fracture the outer armoured skin and release the oil in that particular section, but having the thickest plating inside it was considered almost a matter of impossibility for the latter to be holed and thus admit the burning oil--a source of danger that had long been recognized--into the vitals of the ship. Submarine warfare, in the opinion of many naval experts, had had its day. At the height of five hundred feet a scouting aeroplane could easily detect the presence of a submarine so long as it was daylight. By night a submarine would be fairly safe from observation, but conversely her commander could not with certainty attack a hostile ship that had taken the precaution of manoeuvring with screened lights. In addition to the danger of mistaking friend for foe there was also the possibility, nay probability, of being unable to see the enemy’s ship. It was, however, admitted that the submarine’s chance was to attack either at dawn or sunset, with a fairly choppy sea running, and no aircraft to upset the calculations of the officer at the periscope. Nor had the vast changes occasioned by the development of aircraft been confined to naval affairs. Fortifications, hitherto considered impregnable, were rendered untenable by reason of the danger from attack from above; and in this respect the reorganization of the Portsmouth defences might be taken as an example of what had to be done in other naval and military towns of the British Isles. As is well known Portsmouth, the principal naval arsenal of the British Empire, is defended by a triple line of fortifications; while to prevent subsidized tramp steamers from emulating Togo’s feat at Port Arthur by being sunk at the entrance to the harbour a line of massive concrete blocks were placed from the shore to the east of Southsea Castle, extending seawards as far as to Horse Sand Fort--one of the three built upon the bottom of the sea. This form of defence was severely criticized, for it proved a source of danger to trading and other private ships, while at high tide a torpedo-boat could with impunity pass over the submerged artificial reef. Consequently a permanent breakwater, fashioned after the manner of that superb work protecting Plymouth Sound, took the place of the worse than useless concrete blocks; a similar one was constructed from Ryde Sands to the Noman Fort, and thus, with the exception of the main channel between these two hitherto sea-girt forts, Spithead was rendered almost immune from torpedo-boat attacks.