INTRODUCTION 
A heavy fluid with a free surface, having a density variation 
only in the vertical direction (for the undisturbed medium), is sub- 
ject to two basically different types of gravity wave excitation: a 
surface wave, and internal waves. Internal waves are character- 
ized by having, for every mode, the maximum vertical particle 
displacement below the free surface. 
Most of the ocean for most of the year consists of two parts. 
The first part is the surface layer consisting of well mixed, near- 
ly isothermal water; the second part is the lower, more dense 
water in which the temperature and density vary slowly. Separat- 
ing these two parts of the ocean is the (seasonal) thermocline re- 
gion in which the vertical temperature gradient is sizable. Since 
the horizontal salinity gradient is very slight in this region, iso- 
thermal surfaces nearly coincide with isopycnal surfaces. Thus, 
an obvious way to measure internal waves is to measure the ver- 
tical temperature structure in the vicinity of the thermocline. 
A vertical string of thermistor temperature sensors for de- 
tection of the passage of internal waves was first used by Ufford; * 
such arrays are commonly used today. This technique is fine in 
shallow water, and can even be used in deep water through the 
utilization of deep-sea mooring devices. When it is necessary to 
survey an area, an instrument on a moving platform is desirable. 
One such scheme is the towed chain, which is used to measure the 
thermal microstructure of the sea.** The one described in refer- 
ence 2 and also those devised since then cover an extensive range 
of depth with sizable spacing between sensor elements. Although 
provision is made for determining the depth of tow, this is usually 
done at only one point; the depth of individual sensor elements can 
be inferred from the shape of the chain. 
*Reference 1, page 55 
**Reference 2, page 55 
