240 NATURAL HISTORY SOCIETY OF DUBLIX. 



one of these birds sailing and wheeling about in all directions for more 

 than an hour without seeing the slightest movement of the wings. This, 

 however, is longer than usual. 



Wonderful as is this power of flight, it can all be explained by the 

 simple mechanical laws which govern the direction and magnitude of 

 pressures. Dr. Bennett states that he believes " that the whole sur- 

 face [of the body of the Albatross] is covered by numerous air cells, 

 capable of a voluntary inflation or diminution by means of a beautiful 



muscular apparatus By this power the birds can 



raise or depress themselves at will." Now, I do not for a moment doubt 

 the existence of this apparatus, for it is well known that all birds have 

 it to a greater or less extent ; but I do doubt its capability of doing the 

 dut) r assigned to it — viz., raising the bird in the air. The temperature 

 of the Albatross, as taken by Sir G. Gray, by placing a thermometer 

 under the tongue, is 98° F .; and if we add 10° F. to this, in order to 

 allow for the difference between the head and the body, we shall have 

 the temperature of the air cells at 108° F. The temperature of the 

 surrounding air cannot be taken lower than 48° F., as the mean winter 

 temperature of lat. 50° south is about 50° F. The bird, therefore, 

 could not raise the temperature of the air taken into these cells more 

 than 60° F. This would increase its volume not quite one eighth ; and 

 taking 100 cubic inches of air to weigh thirty-one grains, and the 

 average weight of an Albatross to be 171bs., as given by Gould, it 

 would be necessary, in order that the specific gravity of the bird might 

 be brought to that of the atmosphere, that these cells should contain 1820 

 cubic feet of air ; or, in other words, they must be more than 1200 

 times the size of the body itself of the bird, which, to say the least, 

 would give it when flying an aldermanic appearance, which I have 

 never observed. In fact, it would require a sphere of more than fifteen 

 feet in diameter to contain the necessary quantity of air. Even if it 

 could thus buoy itself up, it would entirely defeat its own object; for it 

 would at once destroy the whole of its momentum, and, unless propelled 

 forward by its wings, would drift helplessly to leeward. However, I 

 do not wish it to be inferred that I consider the air cells of no use. 

 The greater portion of them are situated round the neck, wings, and 

 forepart of the body of the bird ; and I believe that by their means he 

 is enabled to shift slightly the position of his centre of gravity, and 

 thus, with very slight muscular exertion, to vary the inclination of 

 his body to the horizon, according to the rate at which he is moving 

 through the air. 



Dr. Bennett also* gives a diagram explanatory of the flight of the 

 Albatross ; and, if I understand him rightly, says that it cannot sailf 



* " Gatherings of a Naturalist." 



t In this paper I use word " sail," for want of a better ore, to denote the power 

 these birds possess of flying for a considerable time without moving their wings, and the 

 following remarks relate to this mode of flying only. 



