Tide, Tide and Half-tide

In the open sea high water and low water succeed each other 6 hours and 12 minutes apart, and this is known as a tide; but in channels where the stream continues to flow up for 3 hours after it is high water it is said to make a tide and half-tide, and if it continues to flow up for only 1 hour and 30 minutes it is said to make a tide and quarter-tide.