SCIENCE OF COMMON THINGS. 27 



Action of birds in flying. Action and reaction. 



the air; but the suspension is only momentary. In 

 flying 3 on the contrary, the body remains in the air 

 and acquires a progressive motion by repeated strokes 

 of the wings on the surrounding fluid. 



151 Why do birds stretch out their necks when flying f 



In order that they may act as a wedge, dividing the 

 air and diminishing the resistance. 



152 Why are the strongest feathers of birds in the pinions and tail f 



Because when the wing is expanded, the pinion- 

 feathers may form, as it were, broad fans, by which 

 the bird is enabled to raise itself in the air and fly; 

 while its tail-feathers direct its course. 



153 Why can a person safely skate with great rapidity over ice which 

 would not support his weight if he moved over it more slowly f 



From the fact that time is required for producing 

 the fracture of the ice : as soon as the weight of the 

 skater begins to act on any point, the ice, supported by 

 the water, bends slowly under him ; but if the skater's 

 Velocity is great, he has passed off from the spot which 

 was loaded before the bending has reached the point 

 which would cause the ice to break. 



154 It sometimes happens when persons are knocked down "by carriages 

 that the wheels pass over them with scarcely any injury, though if the weight 

 of the carriage had rested on the body, even for a few seconds, it would have 

 crushed them to death. What explanation can be given of this fact f 



The wheel moves with such rapidity, that the weight 

 has not time sufficient to exert its full effect. 



155 When two equal bodies meet, moving with equal velocities in oppo- 

 site directions, what will be the effect ? 



They will both come to rest for their motion being 

 equal and contrary, will be mutually destroyed. 



150 When two persons strike their heads together, one being in motion 

 and the other at rest, why are both equally hurt f 



Because, when bodies strike each other, action and 

 reaction are equal ; the head that is at rest returns the 

 blow with equal force to the head that strikes. 



157 When an elastic batt is thrown against the side of a house with a 

 certain force, why does it rebound f 



