that changes in temperature are conservative. Consequently, 

 both salinity and density surfaces undergo the same variance 

 over time and space. 



The most widely used device for measuring temper- 

 ature continuously in the ocean has been the thermistor. 

 The thermocouple has been used with less success because 

 it requires much more sensitive equipment for sensing and 

 recording the output. The bathythermograph and the re- 

 versing thermometer have also been employed, but these 

 instruments introduce aliasing. Other devices, such as 

 platinum resistance thermometers and vibrotrons, have 

 been utilized, but to a considerably less extent. 



Current meters provide perhaps the most direct 

 way to measure internal waves. They have had limited 

 use, and there are a number of problems associated with 

 the employment of them, such as motion of the device and 

 vortex shedding, which introduce erroneous data into the 

 records. It is desirable to overcome these problems be- 

 cause there are serious handicaps in measuring internal 

 waves by the use of temperature devices. 



There appears to be no lag in the development of 

 electronic equipment capable of handling signals from 

 these transducers and recording them. Once a method of 

 measuring has been selected, there is no problem evident 

 in obtaining the necessary electronic equipment to apply 

 it, although integrating components and adapting the system 

 for a particular use usually involve a period of several 

 months . 



Width of the spectrum and the structure of the 

 modes must be given due regard in studies of internal 

 waves. The latter requires that temperature measurements 

 be made from surface to bottom. In deeper layers of the 

 ocean, temperature gradients are weak -- as low as 

 0.0010° per meter. Although many claims about accuracy 

 and stability are made, it is probable that considerable 

 equipment development is required in order to meet the 

 sensitivity, accuracy, and stability requirements. 



In addition, it is necessary for the equipment to 

 record for long periods of time under the rigors of the 

 marine environment (constant motion, solvency of sea 

 water, and extreme pressure) and with the usual power 

 limitation of a small, isolated system. 



