The destructive power of the air raids was steadily lessened, however, as defensive measures were adopted and perfected. Airplane squadrons were kept in reserve at London and Paris, ready to ascend aloft at an instant's notice to repel the invaders. Anti-aircraft guns of great range and accuracy brought down both Zeppelins and planes. Searchlights of enormous candle power swept the skies at night in every direction. Sirens were stationed on the tops of buildings so that their shrill warnings might prompt the population to seek shelter. Bells and whistles were used for the same purpose. Underground passages were constructed at various points, to which the people flocked when the alarm was given. By these measures of precaution the casualties were sensibly lessened.
The Allies for a long time were unwilling to retaliate in kind. But later, as in the case of asphyxiating gases, they were forced in self-defense to adopt the same methods as the enemy, although they never attacked hospitals, as the Germans did repeatedly through the war. It was some time however before the bombing operations of the Allies rivaled those of Germany in effectiveness. The latter had organized their service to a high point of efficiency. They employed two types of planes, triplanes equipped with machine guns and automatic bomb throwers; the Gothas with two Mercedes engines of 260 horse power each and carrying 1,200 pounds of explosives, with gasoline sufficient for a five hours' flight, and the Friedrichshafens, with two Benz motors of 225 horse power each and carrying half a ton of bombs and gasoline supplies for four hours.
By the summer of 1918, the Allies had perfected bombing planes that rivaled those of the Germans in size and power and greatly exceeded them in number. They then began a system of reprisals over the cities of the Rhine valleys, selecting as their targets munition factories, railway sidings, barracks, and steel works. Despite their efforts to limit their targets, much damage was done inevitably to civilian property and considerable loss of life resulted in Coblenz, Düsseldorf, Cologne,
