[Illustration: Fig. 42.--"SUCKER."] [Illustration: Fig. 43.--HOOP.] 860. _Why does a "sucker" raise a stone?_ Because underneath the sucker _a vacuum_ is formed and the external air, pressing on all sides _against the vacuum_, lifts the stone. The term "sucker" is founded upon the mistaken notion that the leather "sucks," or "draws" the stone. That such is not the case is evident: if, when the stone is suspended, a pin's point be passed under the leather, so as to open a small passage for the air, the stone will _drop instantly_. 861. _Why does a hoop roll, without falling to the ground?_ Because the _centrifugal force_ gives it a motion which is called the _tangent to a circle_--that is, a tendency in all its parts _to fly off in a straight line_. When a piece of clay adhering to the hoop flies off, it leaves the hoop in a line which is straight with the part of the surface from which it was propelled; this line is _the tangent to the circle of the hoop_; and the tendency of all the parts of the hoop to fly off in this manner, counteracts the attraction of the earth, so long as the hoop is kept in motion. [Verse: "Children obey your parents in all things: for this is well-pleasing unto the Lord."--COLOSSIANS III.] 862. _Why does the hoop, in falling, make several side revolutions?_ Because its onward movement, not being quite expended, influences the _centre of gravity of the hoop_, and changes its line of direction. The hoop is also elastic, and when its sides strike the earth, they spring up again, and continue turning until the opposing forces are overcome by the _attraction of gravitation_. 863. _Why will a little boy balance a large boy on a see-saw?_ Because the "see-saw" may be placed so that its ends are at _unequal distances from the centre_. This gives the little boy the power of _leverage_, by which is meant the increase of power, or weight, by _mechanical means_. [Illustration: Fig. 44.--BOYS AND "SEE-SAW."] 864. _Why does the little boy sink to the ground when the larger boy slightly kicks the earth?_ Because the larger boy, by kicking against the earth, opposes by mechanical force the _attraction of gravitation_ acting upon him, and he becomes _temporarily_ less attracted to the earth than the little boy. 865. _Why can the little boy, if he choose, keep the big boy up, when once he is up?_ Because, as the big boy is then on _an inclined plane_ with the _fulcrum_, or centre upon which the see-saw moves, the arm of _the lever_, upon which the big boy sits, is _relatively shortened_, and he has then _less mechanical power_. Also, a portion of the weight of the larger boy is transmitted along the lever _to the arm upon which the little boy sits_. [Verse: "Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous."--1 JOHN III.] [Illustration: Fig. 45.--TRAP AND BALL.] 866. _Why is the ball propelled upward, in the game of trap and ball, when the trigger is struck?_