“You are perfectly right; and gravity may either act in occasioning the one or the other; our experiment at the well this morning afforded you an example of gravity producing a regularly accelerated motion. I did not fully explain the fact at the time, because I was desirous of avoiding too many new ideas at once; we must win our way slowly and cautiously through the mazes of philosophy: I will, however, now endeavour to give you as clear an explanation as the subject will allow.--It is, I think, evident, that if, at the moment you dropped the stone from your hand, the force of gravity could have been suspended, it would have descended to the bottom of the well with a uniform velocity; because there could have been nothing either to accelerate or retard its motion. But this was not the case, for the power of gravity was in constant operation; and, if you keep this fact in mind, you will readily understand how the velocity became accelerated: for, suppose the impulse given by gravity to the stone, during the first instant of its descent, be equal to _one_, the next instant we shall find that an additional impulse gives the stone an additional velocity equal to _one_, so that the accumulated velocity is now equal to two; the following instant, again, increases the velocity to _three_, and so on till the stone reaches the bottom.” Mr. Twaddleton observed, the fact might be shortly expressed by saying, that “the effects of preceding impulses must be added to subsequent velocities.” Mr. Seymour then remarked that the same explanation would apply to _retarded_ velocity. “If,” said he, “you throw a stone perpendicularly upwards, the velocity will be as much _retarded_, as it was in the other case _accelerated_, by gravity; the consequence of which is, that it will be exactly the same length of time ascending that it was descending.” “I should have thought the very reverse,” cried Louisa, “and that it would have fallen quicker than it rose.” “You have forgotten to take into account the force with which the stone is projected upwards, and which is destroyed by gravity before it begins to descend.” “Certainly,” answered Louisa; “but the force given to a stone in throwing it upwards, cannot always be equal to the force of gravity in bringing it down again; for the force of gravity is always the same, while the force given to the stone is entirely optional. I may throw it up gently or otherwise, as I please.”