v "' l ,,^ x | Fobbes, Concerning the Flight of Quite. 363 



wing wit 1 1< 1 raw from the wind's pressure but will <lo the reverse. 

 The fallacy in the wind-gravity conception lies in likening gravity 

 to u kite string which, unlike gravity, holds the kite from drifting 

 to leeward. 

 The gull, being a very perfect gliding machine, can soar at a 



w i o/D 



Fig. 1. 



comparatively slight downward angle, and in this way can doubtless 

 make bead againsl a strong wind by soaring at a moderate incline. 

 With an upward current of air the possibilities are wholly changed. 

 Through such a current a bird might readily glide downward with- 

 out descending at all in relation to the earth's surface. The down- 

 ward motion through the air might suffice to render gravity an 

 effective motive force and yet lie so counterbalanced by the ascent 

 <<f the air that the bird would remain at the same level above the 

 earth. The air eiirreni need not rise vertically, but there must be 

 some upward component in its direction, provided its motion be 

 uniform, to make horizontal soaring possible. 



The positions about the ship in which Mr. Brewster observed 

 horizontal gliding pre lent such a variety that at first sighl it. seems 

 difficult i<> explain the presence of persistent ascending currents 

 in all of them. Andyel I believe thai reasonable explanations are 



available in every CS e. The places noted were, (I) a short dis- 



