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Observations and Collections at Sea
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Flow Methods

From an anchored vessel or float, currents can be measured by stationary instruments past which the current flows, turning a propeller of some type or exerting a pressure that can be determined by various methods. The advantage of these instruments is that observations need not be limited to the currents of the surface layers but can be extended to any depth. The obvious difficulty is to retain the instrument in a fixed locality so that the absolute flow of the water may be measured and not merely the flow relative to a moving instrument. In shallow water a vessel can be anchored so that the motion of the vessel is small enough to be insignificant or of such nature that it can be eliminated. In deep water, current measurements were first made from anchored boats, but in later years the technique of deep-sea anchoring has been advanced (p. 361) to such an extent that vessels like the Meteor, Armauer Hansen, and Atlantis have remained anchored in depths of from 4000 to 5500 m for days and weeks. In other instances, relative currents have been measured from slowly drifting vessels.

Maintaining a vessel at anchor for a long time is expensive, and devices have therefore been developed for anchoring automatic recording current meters that can be left for weeks at a time (p. 370). Measurements of currents very close to the sea bottom cannot be made safely from an anchored vessel, no matter how securely it is kept in position, because an instrument suspended from the vessel cannot be retained at a constant distance from the bottom owing to the motion due to swells and tides. This difficulty was first overcome by Nansen, who lowered a tripod to the sea bottom and suspended a current meter from the top of the tripod. The same method was later used by Stetson (1937), Revelle and Fleming (p. 480), and Revelle and Shepard (in press). The latter suspended three current meters from the top of the tripod and thus were able to obtain simultaneous measurements of currents at three levels within less than 2 m of the bottom.


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Observations and Collections at Sea
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