So far the Russians had not ventured to make any attack upon this stronghold, so strongly was it defended, not only by its submarine squadrons and systems of mines, guarding the entrances to all the harbours, but also by the large force of air-ships which had been stationed there since the new naval warfare had broken out. The warning which Alan had conveyed in his letter to his father was based on the knowledge that a general attack was soon to be made upon it both by air and sea, with the object of crippling the power of the Aerians in the Southern Ocean. No time had been lost in acting upon this warning. The aerial squadron was increased to forty, with the _Ariel_ as flagship, and twenty new submarine vessels, the largest and best possessed by the Aerians, had been despatched from Port Natal to reinforce the fleet of thirty-five already at Kerguelen Island. With these must of course be counted the _Narwhal_, under the command of Alan and Alexis. The strength of the attacking force could only be guessed at, as even Alan did not know it, but it was not expected that, however strong a force the Russians might bring up by sea, they would be able, after the disaster of Antarctica, to muster more than a dozen air-ships. The Aerian headquarters was at Christmas Harbour, on the northern shore of the island. This is an admirably-sheltered inlet running westward into the land between Cape François and Arch Point, and its upper and narrower half forms an oval basin nearly a mile long by a quarter of a mile broad, walled in by high perpendicular basaltic cliffs, and containing a depth of water varying from two to sixteen fathoms, as compared with twenty-five to thirty fathoms in its outer half. North of the harbour, Table Mount rises to a height of thirteen hundred feet, and to the south is a huge mass of basalt over eleven hundred feet high. On both of these elevations were mounted batteries of guns capable of throwing projectiles of great size and enormous explosive energy to a distance of several miles. There were altogether twelve of these batteries placed on various heights about the island, and the guns composing them were mounted on swivels, which enabled them to be trained so as to throw the projectile either into the sea or high up into the air. Soon after daybreak on the fourth day after Alan’s letter had been received the outlook on Cape François, a bold mass of basalt to the north of the outer bay, telephoned “_Narwhal_ in sight” to the settlement at the head of the harbour. Immediately on this message being received the commander of the station, named Max Ernstein, a man of about thirty-four, and the most daring and skilful submarine navigator and engineer in the service of the Council, went on board his own vessel, the _Cachalot_, and set out to welcome the long-lost son of the President and convey to him the commission which had been sent out by air-ship from Aeria.