40G THE EMPIRE OF THE AIR. 



Griiiit .si/A' of tail always iiidicatt's Iceblc Hight, cspci-ially when the 

 appendage becomes very large. 



We may neglect to consider this organ, as giving l)at vague indica- 

 tions of its utility, yet if we must account for the final cause, for the 

 necessity for this organ, j)articularly when it is well developed, we 

 arrive at the following deduction: 



The tail of birds serves as either an ornament or as an organ for flight. 

 The ornamental consideration concerns us not, it may here be neglected. 

 As an organ of flight, we may be enlightened as to its use by the fol- 

 lowing description of a manceuver witnessed. 



A kestrel falcon was skimming close along a hedge, almost at ground 

 leAel; its speed was moderate and its direction straight; when all at 

 once, — as if moved by a released spring, it darted a right angle and 

 ]»ounced upon a lizard. The angle deviated was precise and the action 

 of incredible swiftness. To perform this, the bird used its tail; it 

 absolutely needed this rudder, so ample and powerful. 



Here we see the use of a great development of this organ of locomo- 

 tion; it permits surprising the prey by a sudden change of direction. 



It seems probable that the jjowerful tail of the gypaetus is destined 

 for the same function ; his mode of hunting among the rocks, deliver- 

 ing great body blows must be facilitated by its ample and powerful 

 tail. 



In tine, the tail ])est serves in pursuing the prey, but is not iiulis- 

 ])ensable for long-continued flight, as indeed may be proved by remov- 

 ing it from the bird. 



We then conclude that the tail's chief use is in producing rapid 

 changes of direction; and curiously enough when the bird does not 

 employ it, his flight is always straight. This may be formulated as 

 follows: Aptitude for changing the direction of flight is in direct 

 ratio with the amplitude and power of the tail. 



It is only from the theoretical point of view, from its application to 

 artificial flight, that the utility of this organ is here disregarded. The 

 equilibrium may be maintained ujion two points of support, as w^itness 

 our legs, stilts, the velocipede, etc, yet we must acknowledge that 

 in practice a third point of supi^ort becomes very useful; it introduces 

 Mi^sointe stability, and minimizes that constant strain on the attention 

 required to avoid falling. 



Therefore a third point of support obtains even among birds with 

 rudimentary tails. For examifle, the pelican dazzles not with the 

 development of his caudal ap])endage, but we may note that the gen- 

 eral form of his body sui)plies the deficiency. When in full flight he 

 presents the following attitude: (See Fig. 1.) 



It will be observed that his arm and his forearm form pronounced 

 angles, like those of a flattened letter M, and that he may shift his 

 center of gravity by playing these wings back and forth W'ithout com- 

 promising his equilibrium. This leads, incidentally, to another formula : 

 Birds without tails all have the forearm very long. 



