Mtcroscopte Structures of Wind Waves 
some discussion is included on mechanism of wave generation by 
wind through comparing the cutoff wavelength of the slope data with 
the calculated neutrally stable wavelength. 
II. EQUIPMENT AND EXPERIMENTAL PROCEDURE 
Il, 1 Wind-wave tank and general instruments 
The wind-wave tank has a 1.5 x 1.55-m cross section and 
is 14 m long; see Figure la. The top of the tank is covered for 5.5m, 
up to the test section. Mounted at the upstream end of the tank is an 
axial-flow fan, and a permeable-type wave absorber is installed at 
the downstream end. The maximum obtainable wind velocity with a 
0. 35-m-deep air passage above 1.2-m-deep water is 14 m/sec. 
The wind-velocity profile in the tunnel is determined by the 
vertical traverse of a pitot-static tube. The drift current is measured 
by timing the motion of floats of various sizes. The surface drift cur- 
rent is determined by extrapolating the measured current-distribution 
curve to the water surface. The results of surface drift currents and 
of wind velocities are used together to obtain the wind velocity rela- 
tive to the water surface. 
Two types of instruments have been used simultaneously for 
wave measurements: a conductivity probe for recording gravity-wave 
profiles, and an optical instrument for surface-slope and surface- 
curvature measurements, The conductivity probe, a wave-height 
gauge, does not provide enough resolution for measuring wavelets 
that ride on top of gravity waves and have amplitudes only small frac- 
tions of the latter. A detailed description of the wave tank and its 
associated instruments has been given elsewhere (Wu 1968, 1971a) 
II, 2 Optical instrument 
The optical instrument, shown in figure lb, consists ofa 
light source, a telescope, and a photomultiplier unit. Supported over 
the wave tank, the instrument can be set at any desired inclination 
from the water surface. The photomultiplier receives reflected light 
only when the water surface is normal to the plane containing the light 
beam and the telescopic axis. The cross section of the light beam is 
rectangular, with a length-to-width ratio of 20 tol. The short side 
of the beam is aligned with the direction of the wind. Therefore, the 
angular sensitivity of the instrument in the traverse (cross-wind) di- 
rection is about 1/20 times that in the longitudinal (wind) direction. 
The angular tolerance of the instrument to the water-surface slope 
1437 
