"It does not go so far round."
"No, it does not. The extremities of the sails go through more space in the same time than the part near the centre."
By conversations like these, the technical meaning of the word velocity may be made quite familiar to a child much younger than what has been mentioned; he may not only comprehend that velocity means time and space considered together, but if he is sufficiently advanced in arithmetic, he may be readily taught how to express and compare in numbers velocities composed of certain portions of time and space. He will not inquire about the abstract meaning of the word space; he has seen space measured on paper, on timber, on the water, in the air, and he perceives distinctly that it is a term equally applicable to all distances that can exist between objects of any sort, or that he can see, feel, or imagine.
Momentum, a less common word, the meaning of which is not quite so easy to convey to a child, may, by degrees, be explained to him: at every instant he feels the effect of momentum in his own motions, and in the motions of every thing that strikes against him; his feelings and experience require only proper terms to become the subject of his conversation. When he begins to inquire, it is the proper time to instruct him. For instance, a boy of ten years old, who had acquired the meaning of some other terms in science, this morning asked the meaning of the word momentum; he was desired to explain what he thought it meant.
He answered, "Force."
"What do you mean by force?"
"Effort."
"Of what?"
"Of gravity."
"Do you mean that force by which a body is drawn down to the earth?"