276 



PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



Fig. 25 represents the harrier flying with the apparatus in ques- 

 tion ; below hang the transmitting tubes of the registering apparatus. 



After a great many fruitless attempts and changes of construction of 

 the apparatus, which, being very fragile, broke at almost every flight of 

 the bird, I succeeded in obtaining satisfactory results. During flight 

 the registering lever described a kind of ellipse, but I was obliged to 

 give up registering this figure upon a stationary glass. The motions 

 of the wing differing at dilferent moments of flight, the style did not 

 pass over the same points, and I obtained a very confused tracing. I 

 then resolved to use a glass moving horizontally at a uniform rate in 

 order to obtain an extended figure, which I could afterward submit to 

 a geometri^c correction, and thus obtained as it should be, if traced on a 

 stationary surface, a figure for each instant of flight. 



Figure 2G represents one of the numerous 

 tracings which I have thus obtained. The i3er- 

 fect uniformity of these tracings gives me en- 

 tire confidence in their correctness. To ana- 

 lyze the meaning of this curve, it is necessary w 

 to know how the bird flies, how the apparatus ^ 

 is arranged, and in what direction the smoked § 

 glass moves while receiving the tracing. The g. 

 observer being placed opposite the glass, on 5' 

 the smoked side, sees it move from the right c^ 

 to the left ; between the glass and himself is a ® 

 tracing apparatus, with the lever rubbing upon § 

 the smoked surface directly in front of him. | 

 The bird flying from right to left, in a plane % 

 parallel with that of the glass, carries the lever ^' 

 of the apparatus on his right wing, so that the a 

 respective levers of the two machines are al- *§ 

 ways parallel to each other. This being 5' 

 known, the tracing should be read from left o 

 to right. We have seen that the tracing con- ^ 

 sists of a kind of ellipse, which the motion of ® 

 the glass extends into a spiral. The move ^. 

 ments, more extended at the beginning of (k 

 flight, gradually lose a little of their ampli- ^ 

 tude, and retain a uniform character for some p 

 time. & 



This figure somewhat resembles that which 3 

 we obtain from a Wheatstone's rod, according S 

 to the unison which traces the ellipse which |. 

 its lioint describes upon a surface moving from o 

 right to left. Fig. 27, showing the tracing » 

 of this rod, admits the comjjarison of the %■ 

 two. ^ 



The wing of a harrier thus describes a sort ox 

 ellipse, but it is necessary to determine more 

 exactly its shape, and to correct the error caused 

 by the motion of the glass plate. 



Such a correction is impossible, unless we know the elevation attained 

 by the wing at the end of successive and equal intervals of time. This 

 once obtained, if we trace parallel horizontal lines representing the 

 position of the wing at each of these successive moments, these lines 

 will cut the descending curve at points which correspond to the succes- 

 sive equal intervals of its course. It is clear that if these successive 



