To illustrate: we find that if we let go of an object held in the hand it will fall. The first time this is observed it is simply a fact of experience or observation. But a further examination of the relations involved leads to the general statement that any body which is not supported will fall. We may, perhaps, call this a law, but it is still a comparatively imperfect statement of the existing relations. A further study teaches us that all bodies fall, in a vacuum, at the same rate, irrespective of their size or weight and that the velocity of a falling body varies as the time during which it has fallen. These may be called the empirical laws of gravitation, that is.