A gas jet directed on to a liquid surface causes considerable agitation of the liquid, a fact which is largely responsible for the rapidity of the steel-making reactions in the side-blown converter⁽¹⁾ as compared to the open-hearth furnace. As a start for laboratory investigation of this effect, the disturbance caused by an air jet playing on a water surface was photographed. The maximum depth of penetration was measured and plotted against the calculated reaction thrust, ρQ²/A, for a range of jet direction, orifice diameter, distance of orifice from water surface and air velocity. The consistency of the results within the practical limits of experimental error suggested an empirical relationship between the variables for angles between jet and surface above about 25°, and the following equation was derived expressing penetration as a function of thrust and angle of jet: P = 53τsin θ /[x²ω + 19(ωτ²)^1/3]