Because, in being thrown from the hand, the top is delivered a little out of the perpendicular, but the spill is rounded off at the point, and when the top is rotating rapidly, the gravitative force which attracts the top to the ground continually acting upon it, draws the weight of the top on to the extreme centre of the round point. When the rotation subsides, and the centrifugal force is weakened, then the top is no longer balanced upon the extreme point of the spill, but falls upon its sides, until the force of gravitation is exerted beyond the line of the spill, upon the body of the top, and then it falls to the ground.
"Even a child is known by his doings, whether his work be pure, and whether it be right."—Proverbs xx.
Because at that period of its spinning, which is called "sleeping," the centrifugal and the gravitative forces acting upon the top, are nearly balanced; and the top, obeying chiefly the rotatory force, appears to be in a state of comparative rest.
817. Why does the top cease to spin?
Because the friction of the air against its sides, and the friction of the spill against the ground, act in opposition to the rotatory force, which is a temporary impulse applied by external means—the hand of the person who spins it—and as soon as this applied force is expended, the top yields to the law of gravitation, which is a permanent and ever-prevailing force.
818. Why does a marble revolve, as it is propelled along the ground?
Because, in propelling the marble, the thumb impels the upper surface forward, and the finger draws the under surface backward. This gives a tendency to the upper and lower hemispheres of the marble to separate, which they would do, but for the cohesion of the atoms of the marble. The upper part of the marble, therefore, rolls forward, drawing after it the under part, which acquires a forward motion by the force with which it is drawn upward, and in this way the opposite portions of the marble act upon each other in the successive revolutions.