Because, when the trigger is struck at A, it is forced downwards, turning upon the fulcrum B, the opposite end, forming the spoon, is thereby forced upwards, describing a small _arc_, or curved line; but directly the ball is set free from the spoon, it rises in a _right line_ with the _direction it was taking, at the moment it was set free_. [Illustration: Fig. 46.--BAT AND BALL.] 867. _What principles of natural philosophy are illustrated by the results of bat and ball?_ _Percussion_, when the bat strikes the ball; _rotatory motion_, when the ball is sent whirling away; _momentum_, which it acquires by velocity; _elasticity_, when it rebounds from an object against which it strikes; _reflected motion_, when it is turned by a body upon which it impinges; _friction_, as it rolls along the ground; the _communication of force_, when it sets another body in motion against which it strikes; _gravitation_, when it falls to the earth; and _inertia_, when it lies in a state of rest. [Verse: "A wise son makes a glad father: but a foolish son is the heaviness of his mother."--PROVERBS X.] 868. _Why do pith-tumblers always pitch upon one end?_ Because the _lead_ B is _specifically heavier_ than the _pith_ to which it is attached; it therefore always falls undermost; and as the lead is rounded off, just like the spill of a top, after the head has oscillated a little, and expended the force of the momentum of its fall, it will settle upon its _centre of gravity_, or the point through which it is _attracted to the earth_. [Illustration: Fig. 47. PITH-TUMBLER.] 869. _Why do the figures upon the "Thaumatrope" appear to dance, when they are made to revolve before a mirror?_ Because the eye, in looking through the holes in the card, towards the reflections in the mirror, receives a _rapid succession of impressions_. As the figures upon the card are represented in a graduated series of positions--the _first_ one standing upright, the _second_ with his knees a little bent, the _third_ a little more bent, as in the act of springing, and so on, the _figure_ being in each case _the same_, but the position _slightly altered_, imparts an impression to the mind, through the eye, that _one figure_ is passing through a _series of motions_. _Thaumatrope._--From two Greek words, meaning _wonder_ and _to turn_.