362 Forbes, Concerning the Flight of (lulls. [juiy 



earth. Suppose, then, a bird in still air: in what directions can he 

 glide without wing boats? Motion in any direction will be met by 

 friction from the air. To overcome this, energy must he expended. 

 This may be supplied by muscular action through the bird's wing 

 beats, or by gravity. In the present case we. have eliminated wing 

 beats as we are dealing with gliding on set wings, and the only 

 possible source of energy which remains to drive the bird is gravity. 

 Gravity can only work effectively by inducing motion in a direction 

 with a downward component; in a, direction which satisfies its 

 demands, so to speak, by bringing the object downward. The 

 direction may deviate from the horizontal by ever so slight a slope, 

 but it must have some downward component. For a, bird with 

 nothing but the resistance of his wing expanse to keep him from 

 falling there must be a considerable downward component, especi- 

 ally if the gliding is to be rapid. The only case in which t he energy 

 imparted by gravity can carry a bird in still air in a direction with- 

 out a downward component is when the bird soars for a short dis- 

 tance horizontally or even upwards with the momentum acquired 

 in a previous downward swoop. This case is clearly ruled out of 

 the present problem. Now the horizontal movement of the atmos- 

 phere in a, wind without ascending currents or other irregularities 

 does not alter the case of still air except in that the bird tends to 

 be carried horizontally with the wind, and consequently must 

 glide more rapidly through the air if going to windward in order 

 to make headway over the earth's surface. In order to glide faster 

 through the air the downward component of the direction of glid- 

 ing must be increased. 



When analysed in this way I think it is evident that no combina- 

 tion of the forces of a uniform horizontal wind and gravity can 

 drive a soaring bird horizontally to windward. The fundamental 

 difference between the close-hauled sailboat and the soaring bird 

 with downward sloping flight quills may be considered in the 

 following way. If a force- is to do work the mass upon which it 

 aeis must nunc in such a way as to yield to the force. When a 

 boat, close-hauled, sails to windward she moves in such a way that 

 the sail is withdrawn front the wind's pressure (Fig. 1). When the 

 gull soars with a horizontal wind bearing against the under surface 

 of the downward sloping wing forward motion will not make the 



