The subject of Rotatory Motion continued.--A Ball, by having a peculiar spinning motion imparted to it, may be made to stop short, or to retrograde, though it meets not with any apparent obstacle.--The rectilinear path of a Spherical Body influenced by its rotatory motion.--Bilboquet, or Cup and Ball.--The joint forces which enable the Balancer to throw up and catch his Balls on the full gallop.--The Hoop.--The Centre of Percussion.--The Whip and Peg-top.--Historical Notices.--The power by which the Top is enabled to sustain its vertical position during the act of spinning.--The sleeping of the Top explained.--The force which enables it to rise from an oblique into a vertical position.--Its gyration.
“Tom, do you remember that I told you a few days ago,” said Mr. Seymour, “that, by giving a revolving body a peculiar spinning motion, certain effects were produced, which I should, on some future occasion, take into consideration?”
“To be sure I do,” replied Tom.
“Well, then, attend to me.”
Mr. Seymour took a marble, and, placing it on the ground, gave it an impulse forward by pressing his forefinger upon it: the marble darted forward a few paces, after which it rolled back again.
“That is most extraordinary!” cried Tom; “the marble came back to your hand, as it were, of its own accord, and without having met with any obstacle.”
“And you, no doubt,” said Mr. Seymour, “regard it as contrary to the well known law, that a body once put in motion, in any direction, will continue to move in that direction until some foreign cause oppose it.”
“It really would appear so.”
“It is, however, far otherwise; the force which I imparted to the marble communicated to it two kinds of motion; the one projecting it forward, the other producing a rotatory motion round its axis, in a direction opposite to that of its rectilinear course; and the consequence was simply this, that when the former motion, on account of the friction of the marble on the ground, was destroyed, the rotatory motion continued, and by thus establishing an action in an opposite direction, caused the marble to retrograde. If, however, you will fetch your hoop, I will demonstrate the fact on a larger scale.”
Tom accordingly produced the hoop; and Mr. Seymour projected it forward, giving to it, at the same instant, a spinning motion in an opposite direction. The hoop proceeded forward to a certain distance, when it stopped, and then ran back to the hand.