
800 The American Naturalist. [September, 
the cut ends of the ribs along a. You will see something like d. 
If we suppose an unpressed small portion of air to be represented 
by a small circle 0, and the same air when pressed, by a flat disk 
<>, and then mesh this disk with the sort of surface made by 
the manipulation, the nature of the front thrust will be made 
thinkable. I enlarge it below. That part of the air particle to 
JU 




j 
i 
! 
' 
1 
+ 
l 
' 
' 
' 
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-FIG 4. 
the rear of the dotted line as 4 slips easily over the ribs, while the 
front part, a, engages with them and tends to carry them to the 
front. I have worked out all this artificially with excellent 
results, and can give you a paper upon it if a large part of the 
actual size and shape of the ribs in different birds is left to mem- 
ory, as the rats have destroyed my samples. 




