the cast with the reversing thermometers; in figure 35 where the 

 record is interpreted as indicating failure to extend the downtrace 

 after reaching about 750 meters with no uptrace; and in figure 32 

 where the trace cannot be followed beyond about 230 meters on the 

 downtrace. 



WHiere the uptrace and downtrace are similar in shape but do not 

 coincide such as at the secondary minimum and maximum at 250 

 and 325 meters in figure 22 the reason may be time lag in the instru- 

 ment (slow response) or time variation in the temperature of the 

 water. However, if it were due to slow response of the instrument 

 the uptrace and downtrace would not coincide as they do at such 

 abrupt changes as the temperature minimum at about 65 meters in 

 figure 29. Also such temperature differences as are shown by the 

 two traces in figure 24 at the temperature minimum at 375 meters 

 cannot be explained by slow response of the instrument. As the area 

 of operation is one of great contrasts possibly some of the discrepancies 

 are the recording of geographical (horizontal) differences. Most of 

 the differences between the uptraces and downtraces, however, are 

 considered to be the result of time variations such as would be pro- 

 duced by internal waves since there are numerous cases in which the 

 uptrace and the downtrace are both higher or both lower tlian the 

 temperature indicated by reversing thermometer. As the reversing 

 thermometer is a slow response instrument which gives a reading 

 depending on the average temperature of its surroundings for the 

 several minutes prior to its reversal, cases in which the temperature 

 by reversing thermometer is not included between the traces are 

 indicative of short period internal waves where the period is of the 

 order of magnitude of the length of time required for the reversing 

 thermometer to attain equilibrium. Where the reversing thermometer 

 temperature is included between the traces there is no indication as 

 to the length of period of the internal waves involved. 



The temperature difl"erences between the uptraces and downtraces 

 and the reversing thermometer readings are small in the deeper water. 

 Geographically, it would seem that the large variations extend deeper 

 at stations in the margins of the Atlantic Current than at stations in 

 Labrador Current water with intermediate depths at stations in 

 mixed water. This apparent difference may not be real, however, 

 since the vertical temperature gradient at a given depth is greater 

 in Atlantic Current water than in Labrador Current water and for 

 a given amplitude of vertical movement the temperature changes 

 would be greater. 



Inasmuch as the internal wave involves a periodic vertical dis- 

 placement of water the result is not only a fluctuation in temperature 

 at a particular point but presumably also a fluctuation in salinity and 

 density with a resulting undulation of the dynamic topography of the 

 sea surface and of the assumed motionless surface to which it is re- 



75 



