324 THE BORDERLAND OF SCIENCE. 



balloon can be navigated. A balloon must be large, 

 many times larger than any machine to which it can 

 be attached. If we take even the case of one man 

 raised by a balloon, and inquire how large the balloon 

 should be, we at once see how disproportioned the size 

 of a balloon must needs be to the bodies of a heavier 

 nature which it is intended to raise. We know that 

 a man can barely float in water, so that he is about 

 equal in weight to an equal volume of water. But a 

 volume of water is more than eight hundred times 

 heavier than an equal volume of air, even at the sea- 

 level, where the air is densest. So that the weight of 

 a man is more than eight hundred times greater than 

 that of the air he displaces. It follows that if a very 

 light hollow vessel could be made, which should be 

 more than eight hundred times as large as a man, and 

 which could be perfectly exhausted of air without col- 

 lapsing (a thing wholly impossible), the buoyancy of 

 that vessel would barely enable it to support the weight 

 of a man. But the balloonist is unable to obtain any 

 vessel of this sort. He cannot employ the buoyancy of 

 a perfect vacuum to raise him. What he has to do, is 

 to fill a silken bag with a gas lighter than air, but still 

 not weightless, and to trust to the difference between 

 the weight of this gas and that of the air the balloon 

 displaces, to raise him from the ground. So that such 

 a balloon, in order to raise a man, must be considerably 

 larger than the hollow vessel just referred to. But 

 further, the balloon must rise above the denser parts of 

 the air ; it must carry its own weight as well as that of 



