14 THE VICTORIAN NATURALIST. 



yards, by throwing himself off a height against the wind, makes 

 the astonishing statement that the wind is always inclined upward; 

 but this position is untenable. 



Before passing on to the second theory it will be well to con- 

 sider how a bird is supported in the air, and what are the forces 

 to be overcome. Premising that the bird must always have some 

 velocity relatively to the surrounding air — that is, that the wind 

 must always be in its face (for nothing is more helpless than a bird 

 borne with the wind) — it is evident that there will always be a 

 certain amount of resistance to its progress. This resistance, ac- 

 cording to the known laws of wind pressure, is directly proportional 

 to the square of the relative velocity, and it must be remembered 

 that all motion is relative, so that the conditions are the same 

 whether the bird has a certain velocity relatively to still air or 

 the air has the same velocity and the bird is still. This atmos- 

 pheric resistance will reduce the velocity of the bird relatively to 

 the surrounding air, therefore a gain in relative velocity will 

 overcome the resistance. But there is another force to be over- 

 come, and that is gravity. To balance this, the pressure of the 

 wind underneath the bird must be equal to its weight, unless the 

 bird be rising, when it is greater, or falling, when it is less ; but 

 in rising or falling the pressures due to its vertical motion acting 

 on its full sail area have also to be taken into account. To 

 obtain a pressure underneath equal to its weight, an albatross 

 must incline its body against the wind till the pressure is equal 

 to 2}^ lbs. per square foot. This is exactly the weight of Mr. 

 Maxim's machine, and his experiments showed that it requires a 

 velocity of 40 miles an hour to lift it, the aeroplane being inclined 

 7^ degrees. Now, the pressure at right angles to the wind 

 would only be 6 lbs. per square foot, and Mr. Maxim accounts 

 for the large results obtained with slightly inclined planes to the 

 advantage of moving forward on to new air, the inertia of which 

 had not been disturbed. Professor Proctor give a similar 

 explanation, that when moving forward at a high velocity a 

 bird does not rest on the same air long enough for the air to be 

 set in motion. To Mr. Irminger, a Danish engineer, belongs 

 the credit of giving the true explanation — that the lifting force 

 is largely a negative pressure or suction on the upper surface 

 due to rarefaction of the air. He also determined the fact that a 

 narrow aeroplane, such as the wings of an albatross, is twice as 

 effective as a broad one. We are now in a position to under- 

 stand how a bird can by manoeuvring take advantage of a want of 

 uniformity in the wind to gain relative velocity, which we have 

 seen is necessary for the support of the bird in the air and can 

 also be utilized in overcoming atmospheric resistance. The 

 theory was first advanced in Nature, vol. xxix., by Lord Rayleigh, 

 and independently by Dr. Hubert Airy, to account for the con- 

 tinuous rising of eagles, pelicans, &c, as observed in Assam by 



