318 ON AERIAL LOCOMOTION. 



ing to the speed aud degree of propulsion required, and are thus self- 

 compensating; and could practical difficulties be overcome would prove 

 to be a form of propeller perfect in theory. 



In the flying mechanism of beetles there is a difference of arrange- 

 ment. When the elytra, or wiug-cases, are opened, they are checked 

 by a stop, which sets them at a fixed angle. It is probable that these 

 serve as "aero-planes," for carrying the weight of the insect, while a 

 delicate membrane that folds beneath acts more as a propelling than a 

 supporting organ. A beetle can not fly with the elytra removed. 



The wing of a bird or bat is both a supporting and propelling organ, 

 and flight is performed in a rapid course, as follows: During the down- 

 stroke it can be easily imagined how the bird is sustained; but in the 

 up-stroke the weight is also equally well supported, for in raising the 

 wing it is slightly inclined upwards against the rapidly passing air, and 

 as this angle is somewhat in excess of the motion due to the raising of 

 the wing, the bird is sustained as much during the up as the down 

 stroke — in fact, though the wing may be rising, the bird is still pressing 

 against the air with a force equal to the weight of its body. The fac- 

 ulty of turning up the wing may be easily seen when a large bird 

 alights, for after gliding down its aerial gradient, on its approach to the 

 ground it turns up the plane of its wing against the air ; this checks its 

 descent, and it lands gently. 



It has before been shown how utterly inadequate the mere perpen- 

 dicular impulse of a plane is found to be in supporting a weight when 

 there is no horizontal motion at the time. There is no material weight 

 of air to be acted upon, and it yields to the slightest force, however great 

 the velocity of impulse may be. On the other hand, suppose that a 

 large bird in full flight can make 40 miles per hour, or 3,520 feet per 

 minute, and perform one stroke per second. Now, during every frac- 

 tional portion of that stroke the wing is acting upon and obtaining an 

 impulse from a fresh and undisturbed body of air, and if the vibration 

 of the wing is limited to an arc of 2 feet, this by no means represents 

 the small force of action that would be obtained when in a stationary 

 position, for the impulse is secured upon a stratum of 58 feet in length 

 of air at each stroke. So that the conditions of weight of air for obtain- 

 ing support, equally well apply to weight of air. and its reaction in pro- 

 ducing forward impulse. 



So necessary is the acquirement of this horizontal speed, even in 

 commencing flight, that most heavy birds, when possible, rise against 

 the wind, and even run at the top of their speed to make their wings 

 available, as in the example of the eagle, mentioned at the commence- 

 ment of this paper. It is stated that the Arabs on horseback can 

 approach near enough to spear these birds, when on the plain, before 

 they are able to rise. Their habit is to perch on an eminence where 

 possible. 



The tail of a bird is not necessary for Might. A pigeon can fly per- 



