Observation of Sommerfeld precursors on a fluid surface.

We report the observation of two types of Sommerfeld precursors (or forerunners) on the surface of a layer of mercury. When the fluid depth increases, we observe a transition between these two precursor surface waves in good agreement with the predictions of asymptotic analysis. At depths thin enough compared to the capillary length, high frequency precursors propagate ahead of the "main signal" and their period and amplitude, measured at a fixed point, increase in time. For larger depths, low frequency "precursors" follow the main signal with a decreasing period and amplitude. These behaviors are understood in the framework of the analysis first introduced for linear transient electromagnetic waves in a dielectric medium by Sommerfeld [Ann. Phys. (Leipzig) 44, 177 (1914)]] and Brillouin [Ann. Phys. (Leipzig) 44, 203 (1914)]].