THE KEASON WHY. 



193 



'' Honour thy father and thy mother * * That it may be well with thee, ana 

 thou mayest be long on the earth." EPHESIANS vi. 



mouse in a trap when the card is rapidly twirled upon a 

 string ? 



Because the image of the mouse is brought to the retina of the 

 eye before the image of the trap has passed away. The two 

 impressions, therefore, unite upon the retina, and produce the 

 image of a mouse in a trap. 



3 '.CARD WITH 

 MOUSE-TRAP. 



Fig. 37. BHTERSE OP CARD 

 WITH MOUSE. 



847. Why ivill a bow stretched out of its natural position, 

 propel an arrow through the air ? 



Because its substance, being highly elastic, the particles thereof 

 seek to restore themselves to their former state, as soon as the 

 resisting power is withdrawn. The force derived from this elas- 

 ticity, is communicated to the arrow by the string against which it 

 is placed. 



848. Why is the arrow propelled forward ? 



Because the elasticity of the bow, acting equally upon its two 

 ends, to which the string is fastened, produce a line of force in a 

 diagonal direction. It thus illustrates the law, that when a body 

 is acted upon by two forces at the same time, whose directions are 

 inclined to each other, it will not follow either of them, but will 

 describe a line between the two. 



849. What forces tend to arrest the flight of the arrow? 



The friction of the air, and the attraction of gravitation. 

 9 



