SAILING BIRDS ARE DEPENDENT ON WAVE-POWER. 125 



SAILING BIRDS are DEPENDENT on WAVE-POWER. 

 By L, Hargrave. 



[Received Aug. 24. Bead before the Royal Society of N.S. Wales, Sept. 6, 1899.'] 



There are many birds frequenting the southern oceans beyond 

 the limits of the S.E. trade winds that are not adapted for soaring, 

 and yet they circle, glide and swoop around without napping their 

 wings. These have been well called sailing birds ; and it is one 

 of their oft repeated evolutions that shows me, and I hope you, 

 that sailing flight is not at all incomprehensible. 



I will first point out that the tropics are the home of heavy 

 short-winged birds, such as gannets, boobies, divers and small gulls. 

 These seldom make any attempt to glide, much less sail or soar. 

 The only exceptions I know are the frigate bird, and the boatswain 

 bird ; these two soar at high altitudes for long distances on 

 motionless wings. 



It is worthy of remark that large flocks of sailing birds accom- 

 pany vessels running down their easting, and this gives an 

 opportunity for observing whether sailing birds really can work 

 to windward, this point can only be determined from on board 

 a steamer going westward, and south of what I believe is the 

 usual track from Australia to the Cape. 



My own opinion is that sailing birds cannot make anything to 

 windward except during the limited time that the sea is running 

 in an opposite direction to the wind : and as an argument I call 

 attention to the absence of sailing birds in the S.E. trades and 

 attribute the scarcity to their inability to get out of the trades by 

 standing to the S.W. if they get too far to leeward. 



The most ordinary conditions for observing sailing birds are 

 when the wind and sea are both aft. The waves are probably 

 overtaking the ship and passing at about six knots. Large num- 



