Internal gravity waves of small amplitude propagate in a Boussinesq inviscid, adiabatic liquid in which the mean horizontal velocity U ( z ) depends on height z only. If the Richardson number R is everywhere larger than t, the waves are attenuated by a factor exp(-2n(R -if&) as they pass through a critical level at which U is equal to the horizontal phase speed, and momentum is transferred to the mean flow there. This effect is considered in relation to lee waves in the airflow over a mountain, and in relation to transient localized disturbances. It is significant in considering the propagation of gravity waves from the troposphere to the ionosphere, and possibly in transferring horizontal momentum into the deep ocean without substantial mixing. 33Fluid Mech. 37 33-2

Figures 1 to 3 represent a condensed account of some wave pulse experiments carried out in a long tank equipped with a wavemaker. The object of the experiments was to observe the effects of finite amplitude on the shape and frequency distribution of wave pulses propagated on deep water. Each pulse was generated by varying the wavemaker stroke smoothly from an amplitude of zero to some maximum value and back down to zero again. The resulting surface displacement was then recorded electrically at various positions along the centreline of the wave tank. The wavemaker frequency was 2·5 c/s throughout (corresponding to a regular wavelength of about 10in.). The pulse periods, which were selected to represent rapid and slow rates of modulation, were 12½ and 25 wavemaker periods.