In some river mouths and estuaries the incoming high-tide wave crest overtakes the preceding low-tide trough. This results in a wall of water proceeding upstream，and is called a（）.

In the Northern Hemisphere，a wind that shifts counterclockwise is a（）.

A swift current occurring in a narrow passage connecting two large bodies of water which is produced by the continuously changing difference in height of tide at the two ends of the passage is called a(n)（）.

In modern fathometers the sonic or ultrasonic sound waves are produced electrically by means of a(n)（）.

In the Northern Hemisphere you are caught in the dangerous semicircle with plenty of sea room available. The best course of action is to bring the wind on the（）.

In a tropical cyclone，in the Northern Hemisphere，a vessel hove to with the wind shifting counterclockwise is（）.

The edge of a hurricane has overtaken your vessel in the Gulf of Mexico，and the northwest wind of a few hours ago has shifted to the west. This is an indication that you are located in the（）.

In the Northern Hemisphere the major ocean currents tend to flow（）.

The apparent wind’s speed can be zero only when two conditions are present. One condition is that the true wind（）.

The ocean bottom that extends from the shoreline out to an area where there is a marked change in slope to a greater depth is the（）.