or"di na ry, _common; usual_.
knowl"edge, _that which is known through study_.
de gree", _measure, as of s.p.a.ce or time_.
spent, _used up; exhausted_.
snapped, _broken off_.
de tached", _taken away from_.
WHY AN APPLE FALLS.
"Father," said Lucy, "I have been reading to-day that Sir Isaac Newton was led to make a great discovery, by seeing an apple fall from a tree.
What was there wonderful about the apple falling?"
"Nothing very wonderful in that," replied her father; "but it set him to thinking of what made it fall."
"Why, I could have told him that," said Lucy; "because the stem snapped and there was nothing to support it."
"And what then?" asked her father.
"Why, then, of course it must fall."
"Ah!" said her father, "that is the point: why must it fall?"
"I am sure I don"t know," said Lucy. "I presume it was because there was nothing to keep it up."
"Well, Lucy, suppose there was not--does it follow that it must come to the ground?"
"Yes, certainly," replied Lucy, wonderingly.
"Let us see," said her father; "but first answer this question: What is an animate object?"
"Any thing that has animal life, and power to move at will," replied Lucy.
"Very good," said her father; "now, what is an inanimate object?"
"Any thing that does not possess animal life, or can not move at will."
"Very good again," said her father. "Now an apple is, of course, an inanimate object; and therefore it could not move itself, and Sir Isaac Newton thought that he would try to find out what power moved it."
"Well, then," said Lucy; "did he find that the apple fell, because it was forced to fall?"
"Yes," replied her father; "he found that there was some force outside of the apple itself that acted upon it, otherwise it would have remained forever where it was, no matter if it were detached from the tree."
"Would it, indeed?" asked Lucy.
"Yes, without doubt," replied her father, "for there are only two ways in which it could be moved--by its own power of motion, or the power of something else moving it. Now the first power, you know it does not have; so the cause of its motion must be the second."
"But every thing falls to the ground as well as an apple, when there is nothing to keep it up," said Lucy.
"True. There must therefore be some power or force which causes things to fall," said her father.
"And what is it?" asked Lucy.
"If things away from the earth can not move themselves to it," said her father, "there can be no other cause of their falling than that the earth pulls them."
"But," said Lucy, "the earth is no more animate than they are; so how can it pull?"
"That is not an ordinary question, but I will try an explanation," said her father. "Sir Isaac Newton discovered that there was a law in nature called attraction, and that all bodies exert this force upon each other. The greater the body, the greater is its power of attraction.
"Now, the earth is an immense ma.s.s of matter, with which nothing near it can compare in size. It draws therefore with mighty force all things within its reach, which is the cause of their falling. Do you understand this?"
"I think that I do," said Lucy; "the earth is like a great magnet."
"Yes," said her father; "but the attraction of the magnet is of a particular kind and is only over iron, while the attraction of the earth acts upon every thing alike."
"Then it is pulling you and me at this moment!" said Lucy.
"Certainly it is," replied her father; "and as I am the larger, it is pulling me with more force than it is pulling you. This attraction is what gives every thing weight.
"If I lift up any thing, I am acting against this force, for which reason the article seems heavy; and the more matter it contains, the greater is the force of attraction and the heavier it appears to me."
"Then," said Lucy, "if this attraction is so powerful, why do we not stick to the ground?"
"Because," replied her father, "we are animate beings, and have the power of motion, by which, to a limited degree, we overcome the attraction of the earth."
"Well then, father," said Lucy, "if our power of motion can overcome the attraction, why can not we jump a mile high as well as a foot?"
"Because," replied her father, "as I said before, we can only overcome the attraction to a certain extent. As soon as the force our muscles give to the jump is spent, the attraction of the earth pulls us back."
"Did Sir Isaac Newton think of all these things, because he saw the apple fall?" inquired Lucy.
"Yes; of all these and many more. He was a man of great knowledge. The name by which the force he discovered is generally known, is the Attraction of Gravitation, and some time you will learn how this force keeps the earth, and the sun, moon, and stars, all in their places."