The other force influencing the top is _the attraction of gravitation_: the attraction which, were the top not spinning, would draw it towards the earth. The "spill" projecting from the bottom of the top _stands in the line in which the top is drawn towards the earth_ and keeps it from obeying the law of gravitation. Therefore the rotatory motion given to the top, by the rapid unwinding of the string, and the tendency of its atoms to fly outward, _balance the top_ upon the line in which it is drawn to the earth, and which is occupied by the spill, which prevents it falling to the ground. 815. _Why does a top first reel around upon the spill, then become upright, and "sleep," and then reel again, and fall?_ [Illustration: Fig. 25.--PEG-TOP "REELING."] 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. [Verse: "Even a child is known by his doings, whether his work be pure, and whether it be right."--PROVERBS XX.] 816. _Why does a top "sleep?"_ 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.