  The Vaiśeṣikas extend this model to explain certain other kinds of motion. (1) The initial cause of the flowing of water is alleged to result from a force called ‘fluidity’ (rather than the weight of the water). The subsequent flowing is the result of the water’s acquired impetus. (2) Interestingly, the impetus of a body moving in a straight line was thought to be ontologically of the same type as the elasticity of a bent stick—a fact which reveals that impetus was thought of as a kind of internal force, not as inertia. (3) The Vaiśeṣikas mention various other kinds of motion: the movement of an iron needle towards a magnet, the upward motion of flames, the movement of air, and the initial motion of the atoms at the beginning of creation. Given the above model, there must be some ‘force’ which initially gives rise to each of these motions, yet none of the ‘forces’ so far isolated (weight/gravity, fluidity, elasticity) will do. The Vaiśeṣikas therefore speak of a new force, ‘adṛṣṭa’ (the ‘unseen’ force), alleged to account for such motions.