RIVERS IN THE SEA — SMITH 437 



depths from the surface to the bottom. Special weights sent down the 

 cable will release trigger devices so as to start and stop the meters at 

 precise times after lowering the cable and before hauling it back to 

 the surface. Variations of this type of meter are also designed so as 

 to make mechanical records, or to record current velocity and direc- 

 tion on the deck of the ship by electrical means. 



Among the ingenious devices applied to current meters are those 

 designed to keep the recording or electrical parts in a waterproof 

 housing. One type of meter, designed to operate for long periods 

 without attention, has the propeller outside of the main instrument. 

 Instead of a shaft entering the instrument through a watertight seal, 

 the propeller carries a magnet. As this rotates it actuates the record- 

 ing mechanism within the watertight shell, so that no shaft need pene- 

 trate and the problem of a seal resistant to the high pressures of deep 

 water is sidetracked. Some instruments of this type are designed so 

 that at regular intervals of time part of a strip of photographic film 

 is exposed, while a light illuminates the dials showing the time and 

 the velocity and direction of current. In an instrument developed 

 at Miami which can be left unattended on the bottom of bays and 

 estuaries, the photographic record also includes the salinity of the sea 

 water measured by its density, and the tidal depth of the water. 



There are simpler devices that tell the speed of currents at the sur- 

 face only. These depend usually upon the drag of a float upon a 

 pendulum or cable, which is pulled at an angle from the vertical ac- 

 cording to the speed of flow. For measuring current at various depths 

 the more complicated meters are needed. Even here the problem 

 arises of the back-and-forth motions of the vessel as it rides to its 

 anchor, especially in deep water. Fortunately, the motion of the ship 

 is rhythmical in nature and can be subtracted from the meter records 

 by careful analysis. 



DRIFT BOTTLES AND FLOATS 



When measurements are needed over a wide area, anchored ships 

 do not offer a practical solution and, instead, use is made of drifting 

 bottles or floats. The simplest observation of this kind must have been 

 the discovery of tropical woods or fruits on European shores, which 

 indicated that currents reaching northern Europe must have origi- 

 nated in the Tropics and to the west. Perhaps this influenced the 

 Norse explorations long before the time of Columbus. Today various 

 types of floats are set free upon the water so that when recovered 

 they may indicate something of the nature of the currents that carried 

 them. Glass bottles with addressed postcards and directions for filling 

 out details of time and place of recapture have long been used. Some 

 types of bottles are weighted and have a wire trailer below so that 



