144 BRITISH BIRDS. 



downward glide they make their way to the region where 

 the upward deflection of the wind begins, a little to the 

 windward of the vessel. This lifts them rapidly and 

 easily. But sometimes there is no up-current in question. 

 Occasionally I have seen them advancing in this way over 

 a level plain. Then they have to trust solely to a 

 horizontal wind having different velocities at different 

 levels. 



Utilisation of Up-currents. 



As a rule when Gulls advance with wing-s held rio-id, 

 they are availing themselves of an up-current. Everyone 

 must have noticed how they will often keep pace with a 

 steamer, hanging over the stern, the only wing move- 

 ments discernible being such slight adjustments as are 

 required for the maintenance of balance. The Albatros 

 is an adept at the same method of progress. Some time 

 ago there was a letter in "Nature," the writer of Avhich 

 maintained that he had made a great discovery. The 

 Albatros kept all the while beating his wings, but the 

 human eye was unable to discern the movement, so rapid 

 was it, the movement of these wings stretching twelve 

 feet and more from tip to tip. This remarkable theory 

 owed its birth to a photograph taken by the wi'iter of the 

 letter. In this photograph the wings of the Albatros were 

 undoubtedly raised high above his back. The fact was 

 that, the up-current happening to fail for a moment, the 

 Albatros had given a vigorous stroke with his wings. 

 The camera had recorded this, but the photographer had 

 looked down at the moment he was pressing the button, 

 and so had failed to see it. When a Gull advances in 

 this way, there is always a wind coming from ahead. 

 When it strikes the vessel it is deflected upward, a fact 

 which can be detected by tying a handkerchief to a stick, 

 or by letting fly small pieces of ]3aper. It is this up- 

 current that at once lifts the Gull and propels him. The 

 bird inclines his body so that the general slope of its 

 various surfaces is slightly downward. Hence the wind 



