“The gradually increasing breeze carried us rapidly out of the Straits of Chio. The different objects on the shore,--mountains,--valleys,--villages,--and steeples,--seemed in swift succession, first advancing to meet us, then halting an instant alongside our vessel, as if to greet us on our passage, and, lastly, again gliding off with equal speed; till, launched into the open main, we saw the whole line of coast gradually dissolve in distant darkness.” “That is indeed a beautiful and very apposite illustration,” said Mr. Seymour; “and I think Louisa will now admit, that it is not quite so easy, as she at first imagined, to distinguish between _Absolute_ and _Relative_ motion.” As the children now understood what was meant by the term _Motion_, their father asked them whether they could tell him what produced it. “I can make a body move by various means,” answered Tom. “But they may all be reduced to one,” said Mr. Seymour; “viz. some exertion which is called _Force_; thus the _force_ of my hand put your ball in motion; while gravitation was the _force_ which made it fall to the earth; and I must, moreover, inform you, that a body always moves in the direction of the force which impels it, and with a velocity, or rate of motion, which is proportional to its degree, or strength; and, were there no other forces in action but that which originally produced the motion, the body would proceed onwards in a right line, and with a _uniform_ velocity for ever.” “For ever!” exclaimed Louisa. “Ay, my dear, _for ever_: but we will discuss that question presently; you must first tell me whether you understand what is meant by _uniform_ velocity.” “I suppose that _uniform_ velocity is that which is regular, and of an equal rate throughout.” “Philosophers,” replied her father, “call the motion of a body _uniform_, when it passes over equal spaces in equal times.--Now, Tom, it is your turn to answer a question. Can you describe the meaning of the terms _Accelerated_ and _Retarded_ motion?” “I conclude that motion is said to be _accelerated_ when it moves every moment quicker and quicker; and to be _retarded_ when it moves slower and slower.”