26 SCIENCE OF COMMON THINGS. 



Phenomena of jumping. Flying of birds. Flying and leaping. 



145 Why does a person who is about to leap over a ditch or chasm first 

 make a run of a little distance ? 



In order that the impetus he acquires in running 

 may help him in the jump. 



140 Why is a standing leap always shorter than a running one f 



Because in the running leap, in addition to the force 

 acquired by the contraction of the muscles, we have 

 added the force of the motion acquired by running. 



147 Why do we kick against the door-post to shake the snow or dust 

 from our shoes ? 



The forward motion of the foot is arrested by the 

 impact against the post ; but this is not the case with 

 respect to the particles of dust or snow, which are not 

 attached to the foot ; but the motion imparted to them 

 equally with the foot is continued, and causes them to 

 fly off. 



148 Why do we beat a coat or carpet to expel the dust f 



The cause which arrests the motion imparted to the 

 coat or carpet by the blow does not arrest the particles 

 of dust, and their motion being continued, they fly off. 



149 Why can birds fly f 



Because they have the largest bones of all animals 

 in proportion to their weight. Air-vessels also enable 

 them to blow out the hollow parts of their bodies, 

 when they wish to make their descent slower, rise 

 more swiftly, or float in the air. The muscles that 

 move the wings of birds downwards, in many instan- 

 ces, are a sixth part of the weight of the whole body ; 

 whereas those of a man are not, in proportion, one- 

 hundredth part so large. 



It is an erroneous idea, still taught in many educational works, that 

 the bones of birds are hollow and filled with air. This is not the case. 

 Recent investigations have shown that the bones of birds, as a general 

 thing, are not more hollow than those of other animals, and do not con- 

 tain air. 



150 Why does flying differ from leaping 1 



Because flying is the continued suspension and pro- 

 gress of the whole body in the air, by the action of the 

 wings. In leaping, the body is equally suspended in 



