The _Ithuriel_ and her consorts crossed the northern coast of Africa soon after daybreak on the 27th, in the longitude of Alexandria, at an elevation of nearly 4000 feet. From thence they pursued almost the same course as that steered by the deserters, as Natas had rightly judged that they would first make for Russia, probably St. Petersburg, and there hand the air-ship over to the representatives of the Tsar. There was, of course, another alternative, and that was the supposition that they had stolen the _Lucifer_--the "fallen Angel," as Natasha had now re-named her--for purposes of piracy and private revenge; but that was negatived by the fact that Tamboff knew that he only had a certain supply of motive power which he could not renew, and which, once exhausted, left his air-ship as useless as a steamer without coal. His only reasonable course, therefore, would be to sell the vessel to the Tsar, and leave his Majesty's chemists to discover and renew the motive power if they could. These conclusions once arrived at, it was an easy matter for the keen and subtle intellect of Natas to deduce from them almost the exact sequence of events that had actually taken place. The _Lucifer_ had a sufficient supply of power-cylinders and shells for present use, and these would doubtless be employed at once by the Tsar, who would trust to his chemists and engineers to discover the nature of the agents employed. For this purpose it would be absolutely necessary for him to give them one or two of the shells, and at least two of the spare power-cylinders as subjects for their experiments. Now Natas knew that if there was one man in Russia who could discover the composition of the explosives, that man was Professor Volnow of the Imperial Arsenal Laboratory, and therefore the shells and cylinders would be sent to him at the Arsenal for examination. The whereabouts of the deserters for the present mattered nothing in comparison with the possible discovery of the secret on which the whole power of the Terrorists depended. That once revealed, the sole empire of the air was theirs no longer. The Tsar, with millions of money at his command, could very soon build an aërial fleet, not only equal, but, numerically at least, vastly superior to their own, and this would practically give him the command of the world. Natas therefore came to the conclusion that no measures could be too extreme to be justified by such a danger as this, and so, after a consultation with the commanders of the three vessels, it was decided to, if necessary, destroy the Arsenal at St. Petersburg, on the strength of the reasoning that had led to the logical conclusion that within its precincts the priceless secret either might be or had already been discovered.