A study of the fundamental mechanics of the droplet and gas motion in liquid sprays is presented in this paper. Only vertical sprays without any externally applied gas flow are considered. First a detailed account of the origin of induced air motion within spray jets is given, and this lays the basis for a new one-dimensional model for predicting the induced axial air velocity. Two main flow zones (zone I and zone II) are identified, where the droplet velocity is much greater and of the same order as the induced air velocity respectively. The 1D model was applied to a rose-head axisymmetric spray and a flat-fan agricultural spray. The calculations agree closely with experimental observations. To calculate the radial variation of the air velocity a 2D axisymmetric model was developed where the air velocity was obtained in the form of a similarity solution. (from Authors)