That night my "flight"--each squadron is divided into three flights consisting of six men each--got ready to go out again. As I started to put on my tunic I noticed that I was not marked up for duty as usual. I asked the commanding officer, a major, what the reason for that was, and he replied that he thought I had done enough for one day. However, I knew that if I did not go, some one else from another "flight" would have to take my place, and I insisted upon going up with my patrol as usual, and the major reluctantly consented. Had he known what was in store for me I am sure he wouldn't have changed his mind so readily. As it was, we had only five machines for this patrol, anyway, because as we crossed the lines one of them had to drop out on account of motor trouble. Our patrol was up at 8 P.M., and up to within ten minutes of that hour it had been entirely uneventful. At 7.50 P.M., however, while we were flying at a height of sixteen thousand feet, we observed three other English machines which were about three thousand feet below us pick a fight with nine Hun machines. I knew right then that we were in for it, because I could see over toward the ocean a whole flock of Hun machines which evidently had escaped the attention of our scrappy comrades below us. So we dove down on those nine Huns. At first the fight was fairly even. There were eight of us to nine of them. But soon the other machines which I had seen in the distance, and which were flying even higher than we were, arrived on the scene, and when they, in turn, dove down on us, there was just twenty of them to our eight! [Illustration: THE AEROPLANE WHICH LIEUTENANT O'BRIEN USED IN HIS LAST BATTLE WITH THE HUNS WHEN HE WAS BROUGHT DOWN AND MADE PRISONER] Four of them singled me out. I was diving and they dove right down after me, shooting as they came. Their tracer-bullets were coming closer to me every moment. These tracer-bullets are balls of fire which enable the shooter to follow the course his bullets are taking and to correct his aim accordingly. They do no more harm to a pilot if he is hit than an ordinary bullet, but if they hit the petrol-tank, good night! When a machine catches fire in flight there is no way of putting it out. It takes less than a minute for the fabric to burn off the wings, and then the machine drops like an arrow, leaving a trail of smoke like a comet. As their tracer-bullets came closer and closer to me I realized that my chances of escape were nil. Their very next shot, I felt, must hit me. Once, some days before, when I was flying over the line I had watched a fight above me. A German machine was set on fire and dove down through our formation in flame on its way to the ground. The Hun was diving at such a sharp angle that both his wings came off, and as he passed within a few hundred feet of me I saw the look of horror upon his face.