What the vibrative motion of
the wings is, and after what manner they are moved, I have endeavoured by
many trials to find out: And first for the manner of their motion, I
endeavoured to observe several of those kind of small Spinning Flies, which
will naturally suspend themselves, as it were, pois'd and steady in one
place of the air, without rising or falling, or moving forwards or
backwards; for by looking down on those, I could by a kind of faint shadow,
perceive the utmost extremes of the vibrative motion of their wings, which
shadow, whil'st they so endeavoured to suspend themselves, was not very
long, but when they endeavour'd to flie forwards, it was somewhat longer;
next, I tried it, by fixing the leggs of a Fly upon the top of the stalk of
a feather, with Glew, Wax, &c. and then making it endeavour to flie away;
for being thereby able to view it in any posture, I collected that the
motion of the wing was after this manner. The extreme limits of the
vibrations were usually somewhat about the length of the body distant from
one another, oftentimes shorter, and sometimes also longer; that the
formost limit was usually a little above the back, and the hinder somwhat
beneath the belly; between which two limits, if one may ghess by the sound,
the wing seem'd to be mov'd forwards and backwards with an equal velocity:
And if one may (from the shadow or faint representation the wings afforded,
and from the consideration of the nature of the thing) ghess at the posture
or manner of the wings moving between them, it seem'd to be this: The wing
being suppos'd placed in the upmost limit, seems to be put so that the
plain of it lies almost _horizontal_, but onely the forepart does dip a
little, or is somewhat more deprest; in this position is the wing vibrated
or mov'd to the lower limit, being almost arrived at the lower limit, the
hinder part of the wing moving somewhat faster then the former, the _Area_,
of the wing begins to dip behind, and in that posture seems it to be mov'd
to the upper limit back again, and thence back again in the first posture,
the former part of the _Area_ dipping again, as it is moved downwards by
means of the quicker motion of the main stem which terminates or edges the
forepart of the wing.
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