Waver ley, Mass. 
May 9, 1890 
LETTER OP WALTER FAXON. 
Ycm remember our talking about the flight of the Chimney Swal- 
lows last surmier? I have always thought they moved the wings al- 
ternately. But, watching them lately- -by Jove! — I am more and 
more uncertain about it. At times it seems so, and then I think 
it is all an optical illusion. I wish you could decide the matter, 
for, if true it seems to be a remarkable method of aerial locomo- 
tion, and, a priori, most improbable. You remember that Miss Mer- 
riam felt sure enough of it to go into print, and I found this the 
other day anent the English Swift in Richard Jefferson’s Note Book? 
(quoted in Besant’s “Eulogy of R.J." page 177):— "The first (Swift) 
was flying low down against the wind; seemed to progress from. tip 
to tip of wing, alternately throwing himself along, now one tip 
downwards, now the other, like hand— over— hand swimming," Now it 
sometimes seems to me as if this appeara nce came merely from the 
rapid changes in the transverse plane of the bird's body in its 
swift and erratic course, not from an actual alternate stroke of 
the wings . What think you? 
Waverley . ’’ass . 
I y 
Ldf " fu 'i^) 
The only way that I kno w to settle the question in regard to 
the Chimney Swift's flight is by close observation through the op- 
era-glasses. I am now convinced that the flight is normal—both 
wings acting together. My test is to get a bird head or tail on. 
In such cases the wing-strokes are invariably together. It is only 
in profile view that the wings appear to flap alternately. I think 
I can see the reason for this:-- In side view, through the fore-, 
shortening of the alar extent of the bird, the tip of the near wing 
has to describe (to the eye) a larger arc than the off wing. does 
during the same time, or (which is the same thdgg) the off wing 
moveslfto the eye) slower than the nigh wing, ^his produces the 
optical effect of a cross or reverse motion of the two wings. In 
birds with slower wing-stroke the eye can folJ ow the synchronium 
of the strokes even in side view. — (Letter of W. Faxon, May I 2, 1890 ) 
