|fe£g 



135 



another series of movements. Now, as the velocity of the wind near the surface of the sea is diminished by 

 the friction of the waves, when the bird ascends into the more rapidly moving upper current its vis inertice 

 makes the wind blow past it, and so its stock of energy is increased. When it descends it will be moving 

 faster than the lower stratum of wind, and will again develop new energy if its inertia is sufficient to prevent 



— 



FIG. 3. — ALBATROS COMMENCING- TO RISE. 



.:.-;, -:■;,'' -M,^ 



FIG. 4. ALBATROS DESCENDING AND MAKING A BROAD CURVE TO THE LEFT, 



its attaining the new velocity of the wind at once. So that the bird must fly against the wind when 

 ascending and with it when descending. Thus the energy constantly lost by the friction of the air is partially 

 renewed by these manoeuvres. This explains why the birds can sail longer in a high wind than in a calm. It 

 is because in a high wind and with a high sea there is much greater difference between the velocities of 



