The bombing planes which replaced them had manifest advantages over their predecessors. They were speedy, less liable to be sighted by the enemy, and large enough to carry a heavy com plement of bombs. Their military value was enormous in breaking up enemy bases and depots and preventing the concentration of troops. During eight days of the German drive in 1918, French airmen dropped 317 tons of bombs in the German lines, and produced a demoralization that greatly increased the effectiveness of Foch's counter offensive.
Airplane bombs are constructed with great care, and so shaped that they offer the least possible resistance to the air. They have fins on their tails to assure a perpendicular fall. They are carried on the planes either suspended under the wings or fuselage of the planes or car ried in the fuselage itself. A special mechanism is employed to release them at the will of the aviator. He can release one or many at a time, according to whether he wants a salvo or just a "trail fire." A small lever in the fuselage effects the release. Care must be exer cised to release them alternately when they are carried beneath the wings so as not to disturb too much the equilibrium of the machine. He can drop them so as to explode or not, for sometimes an occasion arises when he must unload over his own lines.
There are three distinct types of bombs — demolition, incendiary, and fragmentation. The demolition bombs have a light steel shell, and are filled with T. N. T. or some other explosive of great destructive power. They are used against heavy structures like depots, railways, and ammunition dumps. The charge is set off by a detonator, separated from the contents of the bomb by a pin. When the bomb is released, the pin Is pulled out automatically, and the detonator slides into position to explode the bomb the instant it strikes. Some of the demolition bombs weigh a thousand pounds and carry five hundred and seventy pounds of explosive.
Fragmentation bombs are designed on explosion to scatter showers of frag ments. They carry smaller charges, because their walls are thicker. They are designed to be used against troops and are timed to explode when but a few inches above the ground. The 5—Vol. I—Cyc timing mechanism has to have an ac curacy of less than a thousandth of a second.
Incendiary bombs are intended to set fire to inflammable structures. They
weigh about 40 pounds and contain a combination of chemicals that develops an intense heat. A sodium element makes it difficult to extinguish the fire, because sodium explodes when water is poured on it.
The task of dropping a bomb so that it will hit the object aimed at is a difficult one, especially when the aviator relies exclusively on his own judgment and eyesight. Many things influence the fall of the bomb—the height above the ground, the rate of speed at which the plane is traveling, the air currents, and the shape of the bomb. It moves in a parabolic curve. When it is first released it moves almost horizontally, as it shares the motion if the plane. Then gravity asserts itself, the bomb gradually curves, and as the velocity in creases assumes a perpendicular position. All this requires expert calculation, as the bomb has to be dropped some time before the aviator is directly above the object he desires to hit. To remedy miscalculations, sights were adjusted to the height, speed, and other conditions. When the two sighting points came in line with the target, the aviator could release his bomb with the probability that an accurate hit would be registered.
When by these improvements the bombing plane had been practically perfected, it proved a most formidable weapon of offense. More and more the armies came to rely upon them as an effective and almost indispensable ad junct in large operations. Especially when the enemy was in retreat, were the bombing planes useful in harassing his flight and increasing his demoralization. The work of the Allied aviators during the retreat of the Crown Prince's army to the Vesle and Aisne in July and August of 1918,, and later in the driving of the Germans from France and Bel gium just prior to the armistice, can scarcely be overestimated.
Air raids over cities during the war were initiated by the Germans. Paris and London were the principal objectives. In the raids on England by planes and Zeppelins, 5,511 persons were killed and injured, of whom 4,750 were civilians. The raids by Zeppelins numbered 51 and those Ey bombing planes 59. Paris suffered to a lesser degree, but still heavily. An incidental feature of one of the Paris raids was the suffocation of about three-score people who had taken refuge in a subway tube.