Thirdly, the difficulty of accurate aiming over such distances and of locating your hits so that they destroy the comparative small area of the fort is got over by the use of aircraft, which fly above the fort, note the hits, and signal the results. Now, the Germans maintained that under these quite recently modified conditions not even the best handled and heaviest gunned permanent fort could hold out more than a few days. The French believed that it could, and they trusted in the stopping power not only of individual works (such as the fortress of Manonvilliers on the frontier), but more especially of great rings of forts, such as surround Liége, Namur, Verdun, etc., and enclose an area within the security of which large bodies of troops can be held ready, armies which no one would dare to leave behind them without having first reduced them to surrender. The very first days of the war proved that the German theory was right and the French wrong. The French theory, upon which such enormous funds had been expended, had been perfectly right until within quite recent years the conditions had changed. Port Arthur, for instance, only ten years ago, could hold out for months and months. In this war no individual fort has held out for more than eleven days. It might be imagined under such circumstances that the very existence of fortresses was doomed; yet we note that Verdun continues to make a big bulge in the German line four months after the first shots fell on its forts, and that the Germans are actively restoring the great Belgian rings they have captured at Liége, Antwerp, and Namur. Why is this? It is because another German theory has proved right in practice.