C. GENERAL PHYSICS. 177 



it is evident that, with equal moving force, the resistance op- 

 posed to flight regulates the velocity of the wings ; and, since 

 the resistance varies as the square of the velocity, lie had to 

 admit that the air resisted from nine to sixteen times less 

 with his artificial bird than it did in the case of the actual 

 flying bird. In this dilemma he began to ponder on the re- 

 actions which the wing experienced during one of its blows 

 upon the air. To obtain a correct idea of this action, he took 

 a light disk and gave it a uniform motion in a direction 



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perpendicular to its plane, and, by means of a dynamometer 

 placed behind the disk, he noted the resistance of the air at 

 different instants during this motion. He thus found, 1st, a 

 considerable resistance at the beginning of the motion : this 



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is the effect of the inertia of the column of air which the disk 

 begins to displace ; 2d, a more feeble pressure, which re- 

 mains constant during the motion ; and, 3d, a tendency to 

 carry forward the disk when the latter is stopped. This ac- 

 tion on the disk is due to the velocity acquired by the col- 

 umn of air already set in motion by the disk. Hence it is 

 evident that if the bird's wing is always in that condition 

 when it is beginning to move a column of air, it will meet 

 with the greatest resistance ; or, in other words, the wings 

 will have the firmest support to act against. Now this is 

 really the condition of the wings during a bird's flight; for, 

 by reason of the translation of the bird, the wing, at each in- 

 stant of its descent, acts on a new column of air, which it 

 tends to depress ; but, on account of the short duration of the 

 pressure which it receives, any one of these columns has not 

 the time to acquire the velocity of the wing. These col- 

 umns are, therefore, successively compressed, and offer the 

 maximum, or initial resistance to the wing. 



But Marey, as is his custom, was not satisfied with this rea- 

 sonable explanation alone ; he also actually showed its truth 

 by precise and conclusive experiments. He constructed an 

 artificial bird, whose wings were set in perfectly regular mo- 

 tion by an air-pump worked in a uniform manner by a small 

 steam-engine. This artificial bird was attached to a long 



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horizontal arm, which revolved around a vertical pivot, so 

 that the wings could beat when the bird was at rest and 

 when it was carried round at the end of the arm. Now Ma- 

 rey observed that when the bird was at rest its wings beat 



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