AERIAL LOCOMOTION 427 



she is able to face a sixteen-mile white-cap breeze, and make 

 headway against it, instead of drifting backwards with the wind. 

 Under such circumstances her speed is materially reduced ; but 

 the point I would direct attention to is this, that she is not stopped 

 by a current of air moving with very much greater velocity than 

 her maximum possible speed in a calm. Of course there would 

 be nothing remarkable about this if her propellers were acting 

 in the water instead of the air, but they were not. They acted 

 exclusively in the air, and the water was only an additional 

 resistance to be overcome. 



It is worthy of note in this connection that the rapid rotation 

 of the propellers yield a theoretical efficiency of thirty or forty 

 miles an hour, and that the mass of the machine and the resis- 

 tance of the water drag this down to an actual performance of 

 only four miles, so that at first sight it appears probable that the 

 effect noted may be a result of the greater slip of the propellers 

 acting in a calm. I am inclined to think however that this 

 explanation is insufficient ; and would suggest the following as 

 more probable. 



The enormous mass of the moving body enables it to acquire 

 very considerable momentum with slight velocity ; whereas, the 

 opposing current of air has such slight mass, that it cannot 

 acquire an equal momentum with a very much higher velocity. 



If two bodies of unequal mass, moving with equal but oppo- 

 site velocities, come into collision with one another, then the 

 heavier body will not be completely stopped by the lighter. It 

 will make headway against the resistance of the other even 

 though the lighter should possess superior velocity, provided, 

 of course, that it has a sufficient superiority of mass. We are 

 here dealing with momentum (»/;■), not velocity (v) alone. The 

 body having the greatest momentum will be the victor in the 

 struggle whatever the actual velocities may be. 



The suggestiveness of this result lies in its application to the 

 flying machine problem. A balloon, on account of its slight 

 specific gravity, must ever be at the mercy of the wind. In 

 order to make an)' headway against a current of air it must itself 

 acquire a velocity superior to the wind that opposes it. On the 

 other hand it is probable that a flying machine of the heavier- 



