510 Nichols, Notes on Offshore Birds. [oct* 



the end of his stumpy tail, which seems not to balance his big neck 

 and shoulders. As he goes he leans far over, first to one side, then 

 to the other. Now the tip of his lower wing actually cuts a knife- 

 like furrow in the water, now he swings high into the air on a great 

 bow. At first sight you say, of course the wind is the motive and 

 supporting force. But on trying to explain how it is such, difficul- 

 ties are apt to be experienced, and you would perhaps fall back 

 on the theory that the bird utilizes rising air currents, except that 

 this theory docs not satisfactorily explain the observed facts, for the 

 bird seems to go where it chooses and everywhere find the right 

 air currents. So universal a distribution of rising currents will not 

 meet the test of probability. It is believed that the difficulties 

 spoken of come from trying to apply the force of wind always to 

 the under side of the bird's wings and to brace it against gravity, 

 whereas in fact it is commonly applied to the upper surface in 

 gaining momentum. In order to use this force, the bird must 

 oppose it to a comparatively rigid resistance, as that of the water 

 against the flat side of a sailboat's hull. In the compressed air 

 beneath him he finds just such a force, familiar enough to us as- 

 holding up parachutes etc., and it is significant that the sailing 

 Albatross holds its wings somewhat downward, which would help 

 them to function as a parachute. By raising or lowering and turn- 

 ing his head, he may steer readily with this bow rudder and main- 

 tain or increase his elevation by directing his course slightly upward 

 through the air. It is an observed fact that he turns toward his 

 lower wing, and doubtless leaning to the side is the important factor 

 in steering to one side or the other. When his course is held upward 

 by pressure of air on his breast and lower head, and at the same 

 time he is being propelled forward by pressure on the upper side of 

 his wings, obviously a very long wing is an advantage, which we 

 find to be the type of wing possessed. It will be noted by our 

 hypothesis that all forces being braced against the upward para- 

 chute pressure of the air beneath the bird's wings, the downward 

 pressure opposed to it must be considerable, and the density of the 

 air increasing with the pressure, the sailing bird rests on an invisible 

 cushion of dense, compressed air. As he moves forward, fresh air 

 is feeding into this cushion in front and an equal amount escaping 

 from it behind, but the current through it is slight as in a lake 

 though with inlet and outlet. Braced against this air cushion, the 



