One of these was a highly improved form of an apparatus which had just come into use on board battleships and cruisers when the War of the Terror broke out. This was an electrical contrivance which gave warning, more or less reliable, of the approach of torpedoes, by translating the aqueous vibrations set up by them into sound-waves, which increased in intensity as the hidden destroyer came nearer. This invention had been lost sight of when all the warships of the world were sunk in the South Atlantic after the proclamation of the Universal Peace. Ernstein’s was therefore a new discovery, or rediscovery, but the advantages of his position, far ahead of the scientific skill of the nineteenth century, had enabled him to produce a much more perfect instrument, and his apparatus, which was attached to all the battleships of the Federation, not only gave warning of the approach of an enemy, but indicated his direction, the number of revolutions at which his propellers were working, and his distance at any given moment. This not only enabled the commander of a Federation battleship to detect the presence of an enemy, but it enabled him to distinguish between friend and foe. As soon as the phonetic indicator showed that another ship was approaching he stopped his own propellers, started them, and stopped them again. The vibrations thus set up and interrupted would be conveyed to the indicator of the approaching ship, if she had one, and she would at once return the signal. If the signal was not returned it was safe to conclude that the coming vessel was an enemy and could be rammed accordingly. When this invention replaced the tell-tale needle that had been in use a year before, an alteration in tactics became necessary, and the fighting order became more extended. A mile instead of a thousand yards was now the limit within which the Federation battleships were not permitted to approach each other, save under special circumstances. Every vessel acted as an independent unit, subject only to the general instructions. Ernstein’s second invention was of a simpler but none the less effective character. Knowing that the Moslem and Russian squadrons would be forced to trust entirely to their tell-tale magnetised needles, he had devised a plan for making these worse than useless. As soon as the phonetic indicator told him that an enemy was coming, the commander of each of his battleships dropped a thin rope of insulated wire down thirty or forty feet into the water below him. The lower end of this cable was a powerful electro-magnet, through which a current of electricity was kept passing along the wires. The attraction of this magnet was far stronger than that of the hull of the vessel, and consequently the needles of the enemy were deflected downwards, and gave a totally erroneous idea as to the depth at which the Federation ship was floating.