involved in the Sailing Flight of the Albatros. 135 



p. 152) and Dr. Pettigrew (Trans. Linn. Soc. vol. xxvi. p. 218) 

 that weight is absolutely essential for ordinary flight. The fact 

 of many birds diving and catching fish under water is a sufficient 

 refutation of this view, as diving is only flying in water, or in 

 a medium of greater specific gravity than the body of the bird ; 

 for all birds, even the Penguin, are lighter than water and float 

 upon it when shot; but, as Dr. Pettigrew has said (p. 214), 

 the wings must in this case act differently, as they have to over- 

 come an upward force of gravity instead of a downward one. 



As the resistance of the air decreases as the square of the ve- 

 locity, it is evident that low velocities are favourable for long- 

 continued sailing, although practically these velocities must be 

 regulated by the velocity of the wind that is necessary to sail 

 against. Now low forward velocities depend upon the bird having 

 a small terminal velocity, which in its turn depends to a great 

 extent upon a large under surface for the air to act upon, so that 

 it may be said that the sailing-powers of a bird depend upon its 

 weight and the expanse of its wing in proportion to its weight, — 

 weight enabling, indeed compelling, it to fly, and expanse of 

 wing enabling it to sail for a long time. For these reasons I 

 cannot agree with the Duke of Argyll (p. 157 et seq.) and Dr. 

 Pettigrew (pp. 216 & 257) that long narrow wings are essential 

 for sailing, and I appeal to the Condor, the Vulture, and the 

 Great Bustard to bear me out. In India I have often lain on 

 my back and watched through a telescope the vultures sailing 

 high up in the sky, and have never seen the slightest movement 

 of a wing ; and in the Crimea, on the plains of the Alma, I have 

 been astonished at the sailing-powers possessed by the Great 

 Bustard (Otis tarda), having once seen it wheeling round in 

 large circles for several minutes without moving its wings. 

 Long and pointed wings, however, are necessary for turning 

 quickly ; and the Albatros could not top the waves so neatly as he 

 does if his wings were shaped like those of the Condor, which, 

 soaring high in the air, has no necessity for sharp turns, and 

 consequently for sharp-pointed wings. I may here remark that 

 it is quite easy to understand, on these principles, that a bird 

 having a very large expanse of wing in proportion to its weight, 

 might sail for a very long time on a calm, or nearly calm day, 

 when there was no wind to carry it away, and when consequently 

 its velocity might be very slow. If, now, for the sake of compa- 

 rison, we take the Cape-pigeon (Procellaria capensis) and assume 

 the area of its under surface to be 2'5 square feet, and the area 

 of its front surface to be 0*25 square foot, its weight being, from 

 my own observations, 14 oz. or 0*88 lb., we find that it would 

 have a terminal velocity of 13 feet per second, which, when fly- 

 ing at the same angles as we have taken for the Albatros, will 



