ON SOAKING FLIGHT. 203 



kept afloat by moving in circles so as to maintain a liigli velocity rela- 

 tive to the current, whereas in reality the bird is able to maintain a 

 steady course into the wind, and must consequently soon pass out of 

 any ascending current it may encounter. We of necessity, therefore, 

 are driven to look to some other hypothesis for an explanation of those 

 marvelous flights of the albatross and other sea birds whose evolutions 

 fairly bewilder the beholder; and, having worked out an hypothesis 

 which seems in harmony with all the phenomena of spiral flight, we 

 now come face to face with a new class of phenomena and a new kind 

 of flight under conditions which the present hypothesis does not demand. 

 A theory has been outlined by Mr. Laugley, in his treatise on The Inter- 

 nal Work of the Wind, which I think supplies us with a key to the 

 mystery, and my purpose is only to point out in what way that theory 

 finds its practical application in the flight of such birds as the alba- 

 tross, which, I aui informed by Professor Ridgway, never rises, as the 

 vultures do, to great heights in circles, and Avhicb rarely, if ever, resorts 

 to that mode of flight. M. Mouillard states that it can not soar in winds 

 having a velocity of less than 11 miles an hour, and while Professor Ridg- 

 way is confident he has seen the smaller species soar in much lighter 

 winds, the fact is well established that as the winds increase in violence 

 its powerof sustained flight without flapping increases accordingly. We 

 have here to look rather to the internal movements of the wind, to its 

 innumerable slight changes in direction and velocity, rather than to 

 ascending masses of air, for a solution of the problem. Some experi- 

 ments which I have made go to show that the changes in direction take 

 place with astonishing rapidity, amounting usually to many times each 

 second and varying several degrees from the general trend of the wind, 

 and there is reason for believing that they increase in rapidity and vio- 

 lence with the velocity of the wind. While the bird in light winds may 

 utilize ascending currents and avoid the descending, it is evident 

 that in direct flight it has no such choice, and while buoyed up by the 

 ascending it must deal with a like number which are descending and 

 which tend to depress it. It is here that the concavo-convex form of 

 the wing finds its advantage. Given an equal number of ascending 

 and descending currents, beating incessantly upon the wing, the con- 

 cave side will prove the more efficient surface, and there will be an 

 excess of pressure from below upward. We see the operation of the 

 same principle in the action of the cups of an anemometer and in the 

 wing of the flapping birds. The effect is in reality the same whether 

 the bird beats its wings in a still air or holds them rigid while the wind 

 beats against them. In the first case the necessary energy is supplied 

 by the bird, in the second by the wind, and in each case the energy 

 expended becomes effective through the peculiar form of the wing. The 

 albatross habitually flies low, seldom rising more than 50 feet above the 

 water, and it might be suggested that the waves act as wind breaks to 

 produce ascending currents, which serve to sustain the bird. But in 



