MECHANICAL PARADOXES. 



which supports the traveller may be, and 

 often is, in rapid motion itself. 



When this is the case, the contrivance for 

 travelling is at the mercy of the supporting 

 fluid, unless it can travel through the fluid 

 as fast as the fluid itself is likely to travel. 

 And to have real independence of movement 

 it must be able to do a good deal more. It 

 must be able to move through the fluid much 

 faster than it is ever likely to find the fluid 

 moving. 



A ship, for instance, which could only travel 

 at the rate of four miles an hour would be liable 

 to be dashed upon rocks or banks by ocean 

 currents of six miles an hour. And if a river 

 flowed at the rate of six miles an hour, a 

 steamer which could not go more than five 

 miles an hour would never be able to travel 

 any way but down stream ; and if it were only 

 capable of six miles an hour, all it could do 

 against the stream would be to remain stationary 

 opposite the wharf. To be able to travel freely 

 up and down the river it must be able to travel 

 through the water a good deal faster than the 

 water itself travels through the country. 



The same condition is equally imperative 

 for a machine which is to travel by swimming 

 in and through the air. It must not only be 

 able to rise and keep aloft as long as necessary, 

 but to avoid destruction it must be able to 

 travel through the supporting ^fluid as fast as 



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