you were moving: for you could not feel the motion, and you could only see it by observing the change of place in the different objects on the shore; and then it would be almost impossible, without the aid of reason and experience, to believe that the shore itself was not in motion, and that you were at rest: I shall, however, be able to explain this subject more clearly by an optical toy which I have in preparation." Mrs. Seymour here repeated the following passage from that interesting novel "Anastasius," which she observed was beautifully descriptive of the illusive appearance to which their papa had just referred:

"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."

"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 an 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 ac`cumulated 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, " a stone perpendicularly upwards, the velocity will be as much you throw 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."

"If you throw it gently," said her father, "it will not rise high, and gravity will soon bring it down again; if you throw it with violence, it will rise much higher, and gravity will be longer in bringing it back to the ground. Suppose, for instance, that you throw it with a force that will make it rise only sixteen feet; in that case, you know, it will fall in one second of time. Now it is proved by experiment, that an impulse requisite to project a body sixteen feet upwards, will make it ascend that height in one second of time; here, then, the times of ascent and descent are equal. But supposing it be required to throw a stone twice that height, the force must be proportionally greater. You see, then, that the impulse of projection, in throwing a body upwards, is always equal to the action of the force of gravity during its descent; and that it is the greater or less distance to which the body rises that makes these two forces balance each other."

"Thank you, dear рара, for the pains yon have taken in explaining this subject to us."

"Nay," replied Mr. Seymour, "bestow your thanks upon those to whom they are more justly due; Mrs. Marcet is entitled to the merit of this explanation; for I obtained it from her 'Conversations.' Before I quit this subject, I would just observe that, when we come to the consideration of the bow and arrow, you will, by the application of the law I have endeavoured to expound, be enabled to ascer tain the height to which your arrow may ascend, with the same facility as you discovered the depth of the well; for, since the times of ascent and descent are equal, you have only to determine the number of seconds which intervene between the instant at which the arrow quits the bow to that at which it falls to the ground, and having them, to make the usual calculation.-But let us proceed to another subject. Roll the ball hither, Tom; roll the ball hither, I say! you stand as if you thought it would advance to us of its own accord.”

“I know a little better than that, too," cried Tom; “no body can move without the application of some force."

"Nor stop, either," added Mr. Seymour, "when it is once in motion; for matter is equally indifferent to both rest and motion."

"And yet, papa," cried Louisa, "unfortunately for your assertion, the ball stopped just now, and I am sure that no force was used to make it do so."

"And pray, Miss Pert, why are you so sure that no force was opposed to its progress? I begin to fear that my lesson has been thrown away upon you, or you would not, surely, have concluded so falsely."

The vicar here interposed, observing that, simple as the question might appear to those who had studied it, the fact was so contrary to everything that passed before us, that Mr. Seymour ought not to feel any surprise at the scepticism of his daughter; he begged to remind him that the truth, apparent as it doubtless now was, lay hid for ages before the sagacity of Galileo brought it to light.

Mr. Seymour admitted the justice of this remark, and proceeded in his explanation.

"I think," said he, "you will readily allow that matter cannot, in itself, possess any power of changing its condition; it can, therefore, no more destroy, than it can originate its own motion; when it is at rest, it must ever remain so, unless some force be applied that can impart to it activity; and when once in motion, it must continue to move until some counteracting force stops it. To believe otherwise you must suppose that matter possesses in itself a power to alter its condition, which is perfectly absurd."

"And yet," said Tom, "when I see my ball or marble stop of its own accord, how can you blame me for believing it possible?"

"Your difficulty arises from your ignorance of the existence of certain forces which act upon the rolling ball or marble. Its progress, as it rolls along, is impeded and ultimately stopped by the rubbing, or friction, occasioned by its passage over the ground; and this will be greater or less, according to the degree of roughness of the surface; if it be small, the ball will continue for a longer time in motion; you must have observed that your marble has always rolled much further on a smooth pavement than on a rough gravel walk."

66

Certainly," said Tom, "and I well remember, that when we played at ring-taw last winter on the ice, we were

obliged, for this very reason, to extend the usual boundaries."

"Is it not evident, then, that the motion of a body is stopped by some opposing force; and that, if this could be entirely removed, the body would continue to move for 225

ever

"What a provoking thing this friction is!" said Tom; "it is always interfering with our experiments.'

"Provoking, is it? I fancy," said Mr. Seymour, "that you would be much more provoked by the loss of it; without it, you could not walk, nor even hold an object in your hands; and yet everything around you would be in perpetual motion, performing one universal and interminable dance."

"I can readily understand, from what you have said, that, if friction were removed, motion might continue; but pray how is it that we should be unable to walk, or to hold anything in our hands?" inquired Louisa.

"It is the friction of the ground which, at every step we take, prevents the foot from sliding back, and thus enables us to push the body forwards. Everybody must have felt how difficult it is to walk on ice, where the friction is only diminished, not entirely removed," answered her father; "and as to holding any object," continued he, “it is the friction of the body to which we apply our hands that enables us to hold it firmly."

"To be sure," exclaimed the vicar; "why, my boy, you must surely remember, that in ancient combats it was the custom to rub the body with oil, that the adversary might not be able to keep his grasp."

66

66 Well," said Tom, our houses, I suppose, would remain firm, and we might sit quietly in our chairs, at all events."

"Not so," replied Mr. Seymour; "for even granting that you had houses and chairs, which, without the existence of friction would never exist, the stability of the structures could never be secured; the slightest breath would be sufficient to make the stones or bricks slide off from each other, and to reduce your dwellings into dancing ruins."

Tom and Louisa, after some further discussion, both admitted the justness of the argument; but, at the same time, would have been better satisfied if the fact could have been proved by actual experiment. Mr. Seymour told them that the perpetual revolution of the earth and heavenly bodies,