Scott on the Migration of Birds. 99 



and. second, how near or how for distant from the glass the birds 

 would have to be in order to be seen at all, that is to be in focus. 

 The height of the moon above the horizon in degrees and the 

 two limits of the area of observation — that is how near or how far 

 the birds noted were from the glass— supply the data for determin- 

 ing how high the birds seen were flying, and this, combined with 

 the number noted as passing per minute through the held of obser- 

 vation, gives the basis for computing how many birds were pas- 

 sing through a square mile in a given time. 



In this connection it may be well to specify how the two limits 

 of observation were defined. ' The inferior limit, that is the near- 

 est point where objects could be seen with distinctness, was easily 

 determined by the power of the glass ; this is about one mile dis- 

 tant. The superior limit, or the most distant point, is provisionally 

 assumed to be not more than about four miles away, on the hy- 

 pothesis that the birds would not fly at a greater height than ten 

 thousand feet. It may appear, as future observations are made, 

 that this last limit is not correct but the reasons for assuming such 

 a height as the superior limit are sufficient to warrant its use 

 in this case, for birds were observed on this same night at a late 

 hour when the height of the moon above the horizon would make 

 the point at which the birds were noted almost at this great ele- 

 vation, viz., ten thousand feet. 



I am greatly indebted to Professor Charles A. Young for assis- 

 tance in these observations, and with his aid have arrived at 

 the conclusion that the average number of birds passing through 

 the field of observation per minute was four and one half. Pro- 

 fessor Young has also kindly assisted me with the details of the 

 problem in regard to the limits and area of the field; and the 

 following diagram and computations are from his study of the 



matter. 



Moon's altitude = 30 : moon's semidiameter = 15' 05." The 

 area of observation is a flat triangle = B, A, C. From this must 

 be deducted the small triangle b, A. c, the area within a mile of 

 the glass. The flight of the birds is thought to be nearly at right 

 angles to the field of observation. 



