THE CONQUEST OF TIME AND SPACE 



the surfaces. On a windy day the kite-flyer holding the 

 string and standing still will have his kite pushed up- 

 ward into the air by the current rushing beneath its 

 surface. On a still day he may accomplish the same 

 thing by running forward with the kite-string, thus 

 causing the surface of the kite to "slide over" the op- 

 posing atmosphere. In short, it makes no diflterence 

 whether the air or kite is moving, so long as the effect 

 of the current rushing against the lower surface is 

 produced. Obviously, then, if in place of the kite-flyer 

 holding the string and running at a certain speed, some 

 kind of a motor could be attached to the kite that 

 would push it forward at a rate of speed corresponding 

 to the speed of the runner, the kite would rise — in short, 

 would be converted into a flying-machine. 



Looked at in another way, the action of the air in 

 sustaining a body in motion in the air has been com- 

 pared by Professor Langley to the sustaining power of 

 thin ice, which does not break under the weight of a 

 swiftly gliding skater, although it would sustain only a 

 small fraction of his weight if he were stationary. 

 Supposing, for example, the skater were to stand upon 

 a cake of ice a foot square for a single second ; he would 

 sink, let us say, to his waist in the water. On a cake 

 having twice the surface area, or two square feet, he 

 would sink only to his knees; while if the area of the 

 cake is multiplied ten times the original size, he would 

 scarcely wet his feet in the period of a second. Now 

 supposing the cake to be cut into ten cakes of one 

 square foot each, placed together in a line so that the 

 skater could glide over the entire ten feet in length in 



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