°i9i3 ' ] Nichols, Notes on Offshore Birds. 511 



Albatross can maintain his momentum by utilizing gusts of wind, 

 merely turning his upper surfaces to its impact, or swooping down 

 from time to time. It' by inadvertence he does lose this momen- 

 tum, — begin to drift (frequently observed in light and moderate 

 breezes) — he can readily regain it by a few flaps of his wings 

 (commonly observed in such cases). Care should be taken not 

 to consider the upward parachute pressure of the air a force, for 

 it is a resistance, not to confuse it with the pressure of the wind, 

 and not to draw false analogies between an object in compressible 

 air and one in non-compressible water. The above hypothesis 

 will explain sailing close to the wind as well as across it. There are 

 doubtless complications and adjustments which it does not cover, 

 just as sailing birds doubtless experience and utilize more or less 

 vertical currents which may at times make easier, but do not irt 

 general explain their sailing. It is also a conservation hypothesis 

 in that it allows for a bird's shooting a long distance in still air by 

 utilizing, instead of opposing, its elasticity. 



As corollary we would expect Albatrosses, Shearwaters, circling 

 Gulls and Hawks to attain their greatest velocity with their upper 

 surfaces inclined to the wind. That my memory fails me as to 

 whether or not they do so, shows how meager our observations 

 often are without a theory to direct them. In a way it is an ad- 

 vantage to draw up the hypothesis without these observations, as. 

 tin 11 when made they will be new facts to test it by. 



The compressed air hypothesis does not preclude a bird's being 

 lifted by the wind below its wings while losing momentum, nor of 

 its attaining momentum by being braced upward against the air 

 pressure, if its wing be flat and firm enough to meet perpendicular 

 resistant air pressure above, which the wings of most birds proba- 

 bly are not, though quite capable of utilizing the glancing impact 

 of the wind. A bird with a wing which would admit of the former 

 would be expected, in its ordinary flapping flight, to raise its wing 

 edgewise, turn it and bring it down flatwise. It is possible that 

 Shearwaters do fly somewhat in this manner, and that it explains 

 a certain stiffness which has been observed in their motions. 



In conclusion it is quite possible for a bird to utilize the wind by 

 bracing it against more or less vertical air pressure, and vertical 

 air currents are unnecessary as they are ineffectual in explaining 

 it- sailing. 



