68 INSECT OR REACTIVE-?ASSIVE LOCOMOTION. 
forces as the bird:—and its locomotion or darting move¬ 
ments are equally the refult of wind-preflures that caufe a 
pajftve locomotion fubftantially the fame with that of the 
bird. 
But although the principle according to which the bird 
performs its feat in the air is the fame with that of the fly, 
the modes vary. The art of the bird conflfts in accom¬ 
modating itfelf to wind-preflures over which it has other- 
wife no control; but the infedt is mafter both of itfelf and 
the wind-forces it creates. The upright pofltion of the bird 
and the folding of its wings do not neceflarily exift in 
the performance of the infedt—though I may remark in 
palling that the houfe-fly, when Handing nearly ftill in the 
air, holds its body in a manner approaching that of the gull, 
but not fo upright. 
The flapping of a bird’s wings, at right angles to its 
body, produces, as I have faid, an air-current in volume 
and ftrength proportioned to the ftretch of the creature’s 
wings and the rapidity of their movements : this air-current 
flowing from front to rear becaufe the flexible portions of 
the wings or fans look to the rear. 
That a bird has the power to a conflderable extent to 
change the pofltion of the fans fo as to make them fan the 
air fomewhat differently, cannot be doubted. We fee this 
when a pigeon, coming down in a hurry upon the ground, 
and wifhing to break the fall, throws its head well up to 
the perpendicular, gets his body as plumb as poflihle, and 
