“Certainly,” said Tom.
“The top, then, being in motion, all its parts tend to recede from the axis, and with greater force the more rapidly it revolves: hence it follows that these parts are like so many powers acting in a direction perpendicular to the axis; but, as they are all equal, and as they pass all round with rapidity by the rotation, the result must be that the top is in equilibrio on its point of support, or on the extremity of the axis on which it turns. But see, your top is down.”
“And what is the reason,” asked Tom, “of its motion being stopped?”
“I can answer that question, papa,” said Louisa; “is it not owing to the friction of the ground?”
“Certainly; that has, doubtless, its influence: but the resistance of the air is also a powerful force upon this occasion. A top has been made to spin in vacuo as long as two hours and sixteen minutes.[[24]] But come, Tom, spin your top once more. Observe,” exclaimed Mr. Seymour, “how obliquely the top is spinning. It is now gradually rising out of an oblique position;--now it is steadily spinning on a vertical axis;--and now its motion is so steady, that it scarcely seems to move.”
“It is sleeping, as we call it,” said Tom.
“Its centre of gravity is now situated perpendicularly over its point of support, which is the extremity of the axis of rotation: but attend to me,” continued Mr. Seymour, “for I am about to attempt the explanation of a phenomenon which has puzzled many older and wiser philosophers than yourselves. It is evident that the top, in rising from an oblique to a vertical position, must have its centre of gravity raised; what can have been the force which effected this change?”
“Was it the centrifugal force?” asked Tom.
“Certainly not,” said Mr. Seymour, “as I will presently convince you.”
“Then it must have been the resistance of the air,” said Louisa.