Look at this little diagram. A howitzer, A, is firing at a fort, B, from the other side of a hill. The howitzer is invisible from the fort, and the men firing the howitzer cannot see the mark at which they are aiming. But a man on or above the hill can see the fort easily, and can so direct the fire of the howitzer that it can lob its shells over the hill and drop them directly on the fort. The guns of the fort are incapable of hitting the howitzer. The shells which they throw pass right over the hill, and fall a long way behind it. Until recently the fortress gun was master of the howitzer, for several reasons. The howitzers then in use were small, and their range was much less than that of heavy guns mounted in forts. No matter how carefully the howitzers were concealed or how frequently they were shifted about, they were sure sooner or later to be "spotted" from the fortress, and put out of action. The explosives which were then used in the howitzer shells were not sufficiently powerful to destroy the steel and concrete of the forts, and there was much difficulty in discovering, whether the aim of these high-angle guns was true. The Austrian and the German General Staff had studied all this very carefully, and they had come to the conclusion that howitzers could be constructed of such a size that they would be more than a match for fortress guns. Former howitzers were 6 inches across at the mouth; but before the war the Austrians built howitzers of from 11 to 12-inch calibre. This doubling of the calibre meant that the new weapon was _eight times_ as powerful as the old one. Such howitzers could throw their shells from an immense distance, and could take advantage of steep cover so far off that the fortress guns could not reach them. Though the new howitzers were so heavy, they were capable of being moved from place to place as soon as they were "spotted". Not only was the howitzer made much bigger than formerly, but new and very powerful explosives were discovered, and huge shells were filled with them. Experiments led the German Staff to believe that these explosives would utterly destroy the forts, no matter how solidly they might be constructed. How to aim these howitzers correctly was the only remaining difficulty. Aeroplanes and balloons solved the problem. Observers could fly high enough to see the forts, note where the shots fell, and signal to the gunners so that they could find the exact range. The Germans believed that no fortress along the line of their advance could long resist their big howitzers and high explosive shells. The French, on the other hand, thought that such ring fortresses as those at Liége, Namur, and Verdun could hold out for a considerable time. During the Russo-Japanese War of 1904-5, Port Arthur, though attacked by howitzers, held out for eleven months. The French saw no reason why their forts should not resist for several weeks at least.