4 8 Of Flies » 
the wing was moved forwards and backwards with 
an equal velocity, (and comparing it with a mufical 
flrifig tuned unifon to ir) the vibrations whereof are 
fo fwift, that it is probable there are many hundred, 
if not thoufand vibrations in one fecond of time, and 
fuppofes them the fwifteft vibrations in the world ; whence 
he reflects on the quicknefs of the animal fpirits, which 
ferve to fupply this motion. 
It is obfervable that moft infers are provided with 
a little ball, g or bladder, under each wing, fix’d at the 
top of a {lender flalk, moveable every way at pleafure; 
in fome they Hand alone, in others (as in the whole 
hefh fly tribe) they have little covers, under which 
they lie and move; with thefe poifes, and fecondary 
leffer wings, they obviate all the vacillations of their 
body, and poife it in flight, as a rope-dancer ballances 
himfelf by his pole loaden at each end with lead. 
If one of thefe be cut off, the creature hies auk- 
wardly for a while, and at lafl falls to the ground. 
Thefe bladders being hollow, may ferve likewife to 
produce the ncife many forts of hies make by flriking 
their wings againft them;, infers that have four wings 
ballance themfeives with the two leffer ones. 
The wings of hies are beft applied to the microfcope 
between two Mufcovy talcs, in an ivory Aider. 
If with an hair pencil, or point of a penknife, yon 
gently brufh or ftroke off fome of the minute fea¬ 
thers from the wings of butterhies, and fome fort of 
moths, then breathe upon a Angle talc in one of your 
Aiders, and apply it to the feathers, which feem only 
like £ fine dull, they will immediately adhere to it; if 
upon their application to the microfcope they lie not 
to your mind, wipe them off, and put on others in 
the 
£ Durham’s Phy. Theo. p, 377. 
