AERIAL LOCOMOTION 44I 



mz = m Q v 9 -f (F — F')t 



in which mv is the momentum at the time /, w Q v the initial momentum, 

 F — F' the resultant of the average propulsive and opposing forces. 

 If mv is positive for large values of /, the equation shows that F must 

 at least equal F' . But Dr. Bell observed, that the kites continued al- 

 ways to advance, or that mv remained positive. Therefore the pro- 

 pulsive force continued, on the average, at least equal to the resistance. 

 In other words, it was the propeller thrust, rather than the momentum, 

 that maintained the indefinite forward progression. 



But how, it may be asked, could the propeller thrust maintain head- 

 way against a sixteen-mile wind, if, in calm weather, it could support a 

 speed of only four miles an hour ? I would answer : first, that the water 

 resistance was not greater in the sixteen-mile wind, but probably less ; 

 second, that the propeller thrust might be not very different in a calm 

 and in a sixteen-mile wind, as Maxim found. This latter point Mr. 

 Manly can elucidate readily from his extensive study of both the theory 

 and actual working of screw-propellers. 



It is well for the world when a man of Dr. Bell's fertility espouses 

 some favorite science. He took up the kite as a toy, and now presents 

 these wonderful structures; light and beautiful as butterflies, yet strong 

 and stable enough for human life. If next he incline to magnificence, 

 what lovely air-castles will follow ! Serenely, one day, may he soar 

 in a gossamer palace, when the blue waves blossom, and the wind sings 

 over the sea ! 



Appendix A. 



Details Concerning the Kite "Frost King." 



Total number of cells, 1,300 



