"What do you mean by verifying them day after day?" asked Alfred.
"I mean by that to see what changes have taken place in the same view between the taking of the two pictures," replied the officer.
"Well, what possible changes could take place in a day or two?" asked Ralph in astonishment.
"A great many, and very important ones, too," answered the lieutenant. "The camera sees what the eye does not notice. If a picture is taken of a certain section today, and the following day another picture is snapped, a comparison of the two will show whether or not there has been a movement of troops, or, whether or not a certain undistinguishable object was something movable, or not."
"Is that the way the map of the enemy's country is made up?" asked Alfred.
"Yes, and every detail is noted, so that when you commence the work be careful to notice something which will distinguish the exact place where you are located; also put down the direction of the flight over that same area, and the altitude at which we are flying."
As the machine glided over the treetops and the village on the northern border of the Meuse, the sun shone out with intense brilliancy. They were going directly east, and both boys turned toward the lieutenant, their eyes seeming to say: "So you are going to take a peep at the front, by way of experiment?" which was exactly what the lieutenant purposed doing.
The altimeter now registered 1800 meters, considerably over a mile high. From that point they could see to Verdun, and thirty miles east of Verdun, could be noted the blurred outlines and surroundings of Metz, that greatest of all fortified German defences. On they went for nearly three-quarters of an hour until a stream of some magnitude appeared to the east, which they soon crossed.
They afterwards learned that this was the Mosel, which flows through Metz. The latter city now loomed up so it could be plainly seen, and to their right was another town, Nancy, a fortified French place. They were about midway between the two places.
Suddenly an aeroplane appeared to the right, and the lieutenant turned the machine in that direction. The boys were relieved to see the great circle on the control planes of the machine in the distance, which showed them it was a French reconnoitering airplane. In the east several other machines were noticed hovering about, and the lieutenant at first headed directly for them, but he soon turned to the north.