"What is the difference between the Taube and the monoplane?" asked Ralph.
"The Taube is a monoplane. The word is the German name for dove. That name was given to it on account of its shape. See the broadly-spreading tail, and the peculiar wing-formation of the main planes."
After passing above the quarry the machine flew to the south, and then circled around so as to get a view of the tier of forts.
"See, there is another one off to the left," exclaimed Ralph.
In the distance, and in the direction from which the boys had come, in their wanderings, they noticed another ship of the same character. These were used for the purpose of ascertaining the locations, not only of the forts themselves, but to spy out the most convenient elevations in the vicinity of the fortifications.
The most important duty of the airplanes is to watch the movement of troops from one vicinity to the other, and to take particular note of the effect of the shells. In this respect they have an undoubted advantage over any other method ever used in warfare.
Heretofore the only way in which an attacking party could determine whether the shells took effect was indicated by the failure on the part of the fort to answer with their guns. But this was not the most satisfactory thing to judge from, because, in many instances, the forts would purposely cease firing, and thus delude the attackers into the belief that they were silenced by the exploding shells.
There is no mistaking the explosions of shells, as they fall around a fort. The flying machines are usually manned by a military observer, who has powerful glasses. He also has a large flag with a white center, and dark border. With this he can readily signal the effect of the shots to the officer at the battery, the latter being provided with field glasses.
The system of signals vary. Obviously, there are only four directions necessary in order to tell the gunners where to shoot. That is, if the shot should, for instance, go over the fort, the flag would be raised far over the head to indicate that fact. If the shot fell short, the flag would be lowered. In like manner, should the shot strike to the right, the flag would be waved in that direction, and so on.
If the shots are properly placed the flag is waved around the head, to show demonstration of approval.