The sea-plane was not alone on her errand. In her wake flew two more, for the actual bombardment had now ceased, and the air-craft were at liberty to engage upon a raid several miles inland.
The Huns had not constructed their strategic railway close to the Dutch frontier without a cunning reason. Extreme care had to be exercised by British airmen, since it was an easy matter for a bomb to drop across the border. Nothing would please the Germans better, for at once there would be a case of violation of Dutch territory. On the other hand, the Huns had no scruple in mounting a battery of anti-aircraft guns, training them in such a manner that the earthward flight of spent shrapnel would assuredly fall upon the Dutch village of Venterloos, which was separated from Zwilhuit by a distance of less than four hundred yards.
In twenty minutes the sea-plane's objective came in sight: a broad line of railway crossing a canal by means of a steel bridge. It was evident that the Germans meant this base to be a permanent one, for the bridge was of massive construction, strong enough to bear the transport of the heavy 42-centimetre guns, and yet sufficiently high above the waterway to admit the passage of large lighters with towering deck-cargoes.
"Stand by!" cautioned the Flight-Sub. "Keep cool. Do as well as you have already done, and everything will go like greased lightning."
Volplaning at an acute angle, the sea-plane swooped down upon her quarry. Shrapnel shells burst over, in front, behind, and underneath her. It seemed impossible that such a frail object could escape destruction.
At five hundred feet the Flight-Sub checked her downward course.
"Now!" he ordered. "And again!"
Two puffs of white smoke marked the points of explosion of the powerful bombs. One had fallen fifty yards short of the bridge; the other had burst almost at the junction of the railway lines.
Round spun the sea-plane. As she turned Ross could discern the second of the aerial raiders gliding down, while the third was still at a great altitude. Before the one in which Ross was flying could again soar over its target the second sea-plane had dropped three of her missiles. All fell close to the bridge. The work of demolition was accomplished, for when the smoke and dust cleared away the substantial fabric had been precipitated, a mass of twisted steel, into the canal.