Submarines occasionally escape by changing their course after the nets have been set; but there appears to have been no instance of the destroyers themselves having been picked up by the periscope. This because they set pretty nearly as low as a submarine, and with their oil-burning propulsion give forth no telltale cloud of smoke. Other nets are hung from hollow glass balls, which the periscope cannot pick up against the sea water. These nets are set in profusion in the English Channel, the North Sea, or wherever submarines lurk, and they are tended just as the North River shad fishermen tend their nets. When a destroyer, making the rounds, sees that a glass ball has disappeared, there is more than presumptive evidence that something very valuable has been netted. Naval Lieutenant Weddingen, of the German submarine U-17, has related the following experience with the British net system. The U-17 had left her base early in the morning and had passed into the North Sea, the boat being under water with periscope awash. "I looked through the periscope," said Weddingen, "and could see a red buoy behind my boat. When, ten minutes later, I looked I saw the buoy again, still at the same distance behind us. I steered to the right and then to the left, but the buoy kept on following us. I descended deeply into the water, but still saw the buoy floating on the surface above us. At last I discovered that we had caught the chain of the buoy and that we were dragging it along with us. "At the same time, also, I saw through the periscope that a strange small steamer was steering a course directly behind us and the buoy. At this time my sounding apparatus indicated that a screw steamer was in the vicinity. Observation revealed that five enemy torpedo-boats were approaching from the north. I increased the speed of the boat in the expectation of being able to attack one of them. The five torpedo-boats arranged themselves in a circle. I sank still deeper and got ready for eventualities. "At this juncture my boat began to roll in a most incomprehensible manner. We began to rise and sink alternately. The steering-gear apparently was out of order. Soon afterward I discovered that we had encountered a wire netting and were hopelessly entangled in it. We had, in fact, got into the net of one of the hunters surrounding us.