49.the surface of display sweep coil contaminated by dust. The echo sighals in some sectors missing on the screen.

由于显示器扫描线图表面不清洁，荧屏上回波信号部分区域丢失。

This one is not less than a hundred feet long, and I can judge of its size when it sweeps over the waters the vertical coils of its tail.

这条鱼的长度不少于一百英尺，当它用垂直卷起的尾巴扫过水面时， 我就能判断出它的大小。