The camp was many miles behind the battle line, but not so far as to entirely shut out the almost incessant roar that was taking place all day long. This consisted first of the explosion of great shells that threw up the earth like a geyser in Yellowstone Park does water and steam. Then came the steady rattle of the French 75's, for all the world like a snare-drum when mellowed in part by distance. Finally the deep-throated boom of the monster guns would cause the earth to quiver.
By degrees both of the boys were becoming accustomed to various things that in the beginning had filled them with awe. They could even witness a savage fight up in the air, and figure on what the two pilots would be apt to do next; for it was always a part of their education to put themselves in the place of the one who represented their side.
Then came the day when Tom was told to accompany the grizzled French sergeant up in a double-seated plane, to send back or relay information coming from some more advanced pilot who soared high above the German lines.
Both boys had been found very clever at signal work, and just then it chanced there was a shortage of observers and signal men, owing to an unusual number of accidents. Jack was sent up with another member of the French esca-
