MECHANICAL PARADOXES. 



is the one that offers least surface and least 

 weight of structural materials in proportion 

 to volume, and is therefore the one that gives 

 the greatest floating power. The further this 

 shape is departed from the greater the loss of 

 floating power, and the limits of improvement 

 in this direction have already been reached. 



The limits have also been reached, and, 

 for safety, exceeded, in the direction of light- 

 ness in the frame, the cords, and the gas-bag ; 

 and though reductions may yet be made in 

 the weight of the motor, so little, proportion- 

 ally, remains to be done in this direction that 

 the volume necessary to give the balloon its 

 floating power can never be substantially re- 

 duced. 



Now this volume is such that balloons 

 with practicable floating power can never be 

 driven fast enough to be even safe in strong 

 winds, much less to have free and independent 

 movement. In a hurricane the wind moves 

 at a speed approaching one hundred miles an 

 hour, and a balloon should be able, for safety, 

 to neutralise a wind of fifty miles an hour ; 

 that is, to travel at the same speed itself. To 

 have voluntary movement in any direction, 

 against even strong winds, it should have a 

 speed of its own equal to that of an express 

 train, fifty, sixty, or seventy miles an hour. 



Many birds, such as pigeons and swifts, 

 have such a speed. But they do not float 



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