“It is, doubtless, much larger, and since the force of attraction is in proportion to the mass, or quantity of matter, you cannot be surprised at not perceiving the earth rise to meet the ball, the attraction of the latter being so infinitely small, in comparison with that of the former, as to render its effect wholly nugatory; but with regard to the earth being heavier than the ball, what will you say when I tell you that it has no weight at all?”
“No weight at all!”
Tom begged that his father would explain to him how it could possibly be that the earth should not possess any weight.
“Weight, my dear boy, you will readily understand, can be nothing more than an effect arising out of the resisted attraction of a body for the earth: you have just stated, that all bodies have a tendency to fall, in consequence of the attraction of gravitation; but if they be supported, and prevented from approaching the earth, either by the hand, or any other appropriate means, their tendency will be felt, and is called weight.”
Tom understood this explanation, and observed, that “since attraction was always in proportion to the quantity of matter, so, of course, a larger body must be more powerfully attracted, or be heavier, than a smaller one.”
“Magnitude, or size, my dear, has nothing whatever to do with quantity of matter: will not a small piece of lead weigh more than a large piece of sponge? In the one case, the particles of matter may be supposed to be packed in a smaller compass; in the other, there must exist a greater number of pores or interstices.”
“I understand all you have said,” observed Louisa, “and yet I am unable to comprehend why the earth cannot be said to have any weight.”
“Cannot you discover,” answered Mr. Seymour, “that, since the earth has nothing to attract it, it cannot have any attraction to resist, and, consequently, according to the ordinary acceptation of the term, it cannot be correctly said to possess weight? although I confess that, when viewed in relation to the solar system, a question will arise upon this subject, since it is attracted by the sun.”
The children declared themselves satisfied with this explanation, and Mr. Seymour proceeded to put another question: “Since,” continued he, “you now understand the nature of that force by which bodies fall to the earth, can you tell me the degree of velocity with which they fall?”
Tom asserted that the weight of the body, or its quantity of matter, and its distance from the surface of the earth, must, in every case, determine that circumstance; but Mr. Seymour excited his surprise by saying, that it would not be influenced by either of those conditions; he informed them, for instance, that a cannon-ball, and a marble, would fall through the same number of feet in a given time, and that, whether the experiment were tried from the top of a house, or from the summit of Saint Paul’s, the same result would be obtained.