This implies that the waves must have different modes and fre- 

 quencies and are probably moving in different directions. 



The establishment of the characteristics of internal waves 

 and three-dimensional isothermal surfaces by use of the chain is 

 not as difficult as might be supposed. If the waves are moving 

 one-fourth as fast as the ship, the results can be treated as spatial 

 distributions rather than as time variations. In the specific case 

 cited above, the distortion would be in error by only 25 per cent 

 at the most, and would become less with increased relative speeds 

 and by towing in directions nearly normal to the directions of 

 propagation of the dominant wave. 



It is therefore possible to construct a fairly reliable repre- 

 sentation of internal waves and isothermal surfaces by considering 

 their space distributions at a given time. 



If we consider, for example, the depth of the 9 °C isotherm 

 recorded during the two circular traverses (fig. 25) of 6-mile 

 diameter, assume a ship drift of 0. 5 knot down the coast, and dis- 

 regard the wave propagation and time of traverse, an approxima- 

 tion of a three-dimensional representation of the 9°C isothermal 

 surface can be constructed (fig. 26). This artist's conception, 

 based on the isotherm depths for the two circles, appears to be a 

 rough irregular pattern similar to the sea surface or a mountain 

 range. 



SUMMARY AND CONCLUSIONS 



With the thermistor chain it has been possible to record a 

 two-dimensional picture of the thermal structure. In addition, by 

 towing the chain in rectangular and circular paths, an attempt has 

 been made to gain knowledge of the spatial changes in isotherm 

 depth and the direction of propagation of internal waves. 



The record of the rectangular tow, away from, parallel to, 

 and toward shore, was analyzed by computing the isotherm depth 

 differences at half-minute intervals. These were used to study 



41 



