A steamer employing the same time (ten hours) for the
journey, and starting at high water in New York, would carry a flood
tide and highest water all the way, and have an up-river current of
about three miles an hour helping her. On the other hand, the same
steamer starting six hours later, or at low tide, would have dead low
water and an ebb tide current of about three miles against her the
entire way. The average rise and fall of the tides in New York is five
and one-half feet, and in Troy, about two feet.
Flood tide may carry salt water, under the most favorable
circumstances, so that it can be detected at Poughkeepsie; ordinarily
the water is fresh at Newburgh.
To those who have not studied the tides the following will also be of
interest.
The tides are the semi-diurnal oscillations of the ocean, caused by
the attraction of the moon and sun.
The influence of the moon's attraction is the preponderating one in
the tide rising force, while that of the sun is about two-fifths as
much as that of the moon. The tides therefore follow the motion of the
moon, and the average interval between the times of high water is the
half length of the lunar day, or about twelve hours and twenty-five
minutes.
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