VOl 'l906' ni ] Carpenter, Height of Migrating Birds. 213 



nithological ends. Their readings indicated that the birds were 

 958 feet above the ground, and were riving at the rate of 47.8 miles 

 an hour. Under similar conditions in March of the following 

 year the same observer ascertained that a flock of geese migrating 

 northward was 905 feet high, and had a velocity of 44.3 miles an 

 hour. 



During the spring and autumn of 1905 Professor Joel Stebbins 

 of the Astronomical Department of the University of Illinois and 

 the writer made a series of observations on the nocturnal flight of 

 migrating birds. The work was done at the Astronomical Obser- 

 vatory in Urbana, Illinois. Professor Stebbins became interested 

 in the matter of determining the heights of the birds, and devised 

 a method which we believe furnishes the solution of this problem. 

 The entire credit for accomplishing this belongs to Professor 

 Stebbins, who has published in the February number of 'Popular 

 Astronomy' 1 a full account of his procedure with the mathematieal 

 data and calculations. The object of the present paper is to bring 

 to the attention of ornithologists the results of this successful 

 application of astronomical methods to a hitherto unsolved biolog- 

 ical problem. 



In order to obtain the necessary data for the computations two 

 telescopes were used in making the observations. These were 

 three-inch and four-inch equatorial instruments, placed from ten 

 to twenty-one feet apart on a line running east and west. The 

 magnifications were about twenty-five and thirty diameters re- 

 spectively, and the powers being low objects as near as one thou- 

 sand feet could be seen without difficulty. Both telescopes were 

 directed toward the moon, and an observer stationed himself at 

 each. Birds passing through a certain area between the telescopes 

 and the moon could be simultaneously seen by the two observers. 

 In the accompanying diagram this area is included by the triangle 

 E C D, through which the majority of the birds flew approximately 

 at right angles to the plane of the figure. There are also shown 

 the two areas, A C E and B D E, in which passing birds were 

 visible to one of the observers only. The figure has necessarily 



'Stebbins, J., : 06. A Method of Determining the Heights of Migrating Binls. 

 Popular Astronomy, Vol. XIV, No. 2, pp. 65-70, 2 fig. Also separate, pp. 1-6 

 (repaged). 



