Packets of surface gravity waves of constant frequency and similar raised cosine envelopes were generated with a variety of amplitudes by means of a hydraulic paddle. The behaviour of each packet was observed with two capacitance probes, one near the paddle and the other at some distance along the tank. In the time domain each packet was observed to evolve into a soliton and a "tail" as it traveled down the tank. In the frequency domain the initial spectral peak was observed to split into two

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... aks at frequencies on either side of the initial frequency in accordance with the well known "sideband" instability. In addition, for packets in excess of a particular threshold, wave breaking also occurred. The sideband instability was observed both above and below the threshold of breaking. At amplitudes below the threshold, energy was partitioned almost equally between the upper and lower sidebands but with the onset of breaking a smaller fraction of the initially energy was transferred to the upper sideband. The spectral behaviour of these evolving wave packets shows a number of features in common with wind waves observed in the open sea.