214 



Carpenter, Height of Migrating Birds. 



[Auk 

 [.April 



F 



!E 



been drawn out of its true proportions, the horizontal dimension 

 being greatly exaggerated as compared with the vertical. 



The eye-piece of each telescope was provided with cross-hairs 

 which divided the field into octants, and 

 each observer had at hand an outline 

 drawing of the phase of the moon for 

 the night. On this chart were drawn 

 radiating lines corresponding to the 

 cross hairs of the eye-pieces. The 

 telescopes were centered on the moon's 

 disk, the whole of which appeared in 

 the fields of view, and each eye-piece 

 was rotated until one of the cross- 

 hairs was parallel with the diurnal 

 motion of the moon. Birds which 

 passed through the areas limited to 

 one telescope (A C E or B D E) were 

 merely counted by the observer who 

 saw them, but when a bird appeared 

 in the area covered by both telescopes 

 (E C D) its course across the face of 

 the moon was immediately marked on 

 the charts by straight lines, and the 

 hour was noted. The cross-hairs made 

 it possible to locate with considerable 

 accuracy the line described by the bird 

 in passing before the moon's disk. 

 These lines of flight were recorded by 

 the observers independently, and ap- 

 peared at different places on the two 

 charts, as can readily be seen by con- 

 sidering the projection from A and B of 

 any point (representing the bird) in the 

 area E C D upon the line C D, which 

 indicates the moon's disk. The distance 

 between these two projections of the 

 point will vary in accordance with vari- 

 ations in the vertical position of the 



East 



West 



Diagram showing areas of 

 observation. A and B, tele- 

 scopes; C D, moon's disk; 

 A C D, area of observation 

 for telescope A; BCD, area 

 of observation for telescope 

 B ; E C D, area in which birds 

 were simultaneously visible 

 through both telescopes; AC 

 E, area in which birds were 

 visible through telescope A 

 only; B D E, area in which 

 birds were visible through 

 telescope B only. 



