Illustration from 1911 Encyclopædia Britannica, article BUOYS.

Original text: The Courtenay whistling buoy (fig. 13) is actuated by the undulating movement of the waves. A hollow cylinder extends from the lower part of the buoy to still water below the movement of the waves, ensuring that the water inside keeps at mean level, whilst the buoy follows the movements of the waves. By a special apparatus the compressed air is forced through the whistle at the top of the buoy, and the air is replenished by two tubes at the upper part of the buoy. It is fitted with a rudder and secured in the usual manner.

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