44 OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



a graph, with temperature and salinity as the coordinates, and are joined 

 by a curve in order of increasing depth (p. 87). In any given area the 

 shape of the T-S curves has a fairly definite form, and hence errors in 

 observations may sometimes be detected from such a graph. 



The interpolated values of temperature and salinity at standard 

 depths are read off before the density, specific volume anomaly, and other 

 calculations based upon these data are made (see chapter II). As a 

 check on the correctness of the data, it is often advisable to plot the 

 vertical distribution of density (o-<) and the specific volume anomahes for 

 each station as functions of depth. 



Current Measurements 



Units and Terms. In scientific literature the velocity of a current 

 is given in centimeters per second (cm/sec) or occasionally in meters per 

 second (m/sec), but in publications on navigation the velocity is stated 

 in knots (nautical miles per hour) or in nautical miles per 24 hours. The 

 direction is always given as the direction toward which the current flows, 

 because a navigator is interested in knowing the direction in which his 

 vessel is carried by the current. The direction is indicated by compass 

 points (for example, NNW, SE), by degrees reckoned from north or south 

 toward east or west (for example, N 60° W, S 30° E), or in degrees from 

 0° to 360°, counting current toward north as 0° (or 360°) and current 

 toward south as 180°. 



Drift Methods. Information as to the general direction of surface 

 currents is obtained from the drift of floating objects such as logs, wreck- 

 age from vessels, and fishermen's implements. In most instances, con- 

 clusions as to currents from the finding of accidental drifting bodies are 

 incomplete because the locality and the time at which the drift started are 

 not known, nor is it known how long the object might have been lying 

 on the beach before discovery. Neither is it known to what extent such 

 drifting bodies have ''sailed " through the water, being driven forward by 

 winds. 



More than a century ago, in order to overcome such uncertainties, 

 drift bottles were introduced. These are weighted down with sand so that 

 they will be nearly immersed, offering only a very small surface for the 

 wind to act on, and they are carefully sealed. They contain cards giving 

 the number of the bottle, which establishes the locality and time of 

 release, and requesting the finder to fill in information as to place and 

 time of finding. 



The interpretation of results of drift-bottle experiments presents diffi- 

 culties. In general, a bottle does not follow a straight course from the 

 place of release to the place of finding, and conclusions as to the probable 

 drift must be guided by knowledge of the temperature and salinity 

 distribution in the surface layers. Fairly accurate estimates of the 



