Scott on the Migration of Birds. ■ 99 
and, second, how near or how far 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. 
