"But when the gun is there!" the master retorted. "When the invasion is accomplished, think of the advantage which such information gives! There is no need to send out scouting parties to bring back estimates of distances; there is no need to waste energy, time, and ammunition in trial shots, during which time the battery might be subjected to fire from the guns of the fort. None of that. Secretly and silently, probably during the night or behind a screen of cavalry, a howitzer may be dragged up to the place selected by the spy and marked in detail on a large scale map. The officer commanding the battery knows the exact direction in which the fort bears and has already worked out the exact angle of elevation for the range. He has nothing to do but to order the aim and elevation and to fire, knowing, in advance, that his shells can fall nowhere but on the fort itself. It is not marksmanship, it is mathematics." [Illustration: _Belgian Official Photograph._ ARMORED TRAIN DEFENDING ANTWERP.] [Illustration: _Belgian Official Photograph._ ARMORED CAR HARASSING INVADERS.] "You think this has been done with the forts at Liége!" ejaculated the doctor. "That is evident," was the reply. "See, this is a night bombardment. There are no advance posts, no aeroplanes to report back the results of gun-fire. Yet the German shells are falling on the forts with deadly precision, falling on forts which the gunners have never seen. I doubt if there is a single fortified place in Belgium of which the Germans do not possess accurate plans." "Then you think they will break through?" "We cannot hope to prevent it," the master answered. "The Kaiser's generals would never attack Liége unless they were confident of success. Since they know exactly what we possess for defense, they would not be sure of success unless they knew that they possessed an infinitely stronger force of attack." "But I have heard that the forts of Liége were impregnable!" "They were when they were built," the master answered, "but that is twenty years ago. Against the guns of that period, notably the 6-inch howitzer, they were impregnable, for every possible gun-position for a weapon of that range was covered by the guns of the fort. But if pieces of heavier power can bombard the forts from positions outside the range of the fortress guns, then impregnability is gone. You must remember, Doctor, that the power of a gun increases as the cube of its caliber or diameter of its bore. Thus a 12-inch gun is not twice as powerful as a 6-inch gun, but eight times as powerful." "Are there such heavy guns?" "There are," was the answer. "Field guns of 8.4-inch and 10-inch caliber are known to exist, and the German War Party is reported to hold the secret of still more powerful engines of destruction, of which, as yet, the outer world knows nothing."