THE TRAVELS OF BIRDS 



Now in order to realize what a very small 

 part we saw of the birds that were traveling, we 

 have only to compare the size of the moon with 

 that of all the sky which we see when we look 

 at the moon. Let us imagine that I could have 

 seen just as well everywhere as I did in that long, 

 narrow wedge of air which stretched from the 

 telescope to the moon. What an amazing throng 

 of birds I should have beheld, all hurrying down 

 the air line to their winter homes! 



If we knew exactly how far the birds observed 

 were from us, it would be possible to tell how 

 high above the earth they were traveling. Those 

 that passed most quickly were, of course, the 

 nearest and lowest. Probably they were flying 

 at the height of those birds whose call-notes we 

 can hear so plainly as they go over us. This, 

 perhaps, may be anywhere between five hundred 

 and one thousand five hundred feet. But cal- 

 culations show that those which were farthest 

 away were probably three miles above us. As 

 we go upward from the earth we know that the 

 air becomes thinner and that what is called its 

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