FLY 
wind, or water, or any other animate or in- 
animate power; and is ol great use in those 
rarts of an engine which have a quick circu- 
ar motion, and where the power or the resist- 
ance acts unequally in the different parts of a 
revolution. This Iras made some people 
imagine, that the lly adds a new power; but 
though it may be truly said t facilitate the 
motion, by making it more uniform, yet upon 
the whole it causes a loss of power, and not 
an increase ; for as the fly has no motion of 
its own, it certainly requires a constant force 
to keep it in motion ; not to mention the 
friction of the pivots of the axis, and the re- 
sistance of the air. The reason, therefore, 
why the fly becomes useful in many engines, 
is not that it adds a new force to them ; but 
because, in cases where the power acts un- 
equally, it serves as a moderator to make the 
motion of revolution almost every where 
equal: for as the fly has accumulated in 
itself a great degree of power, which it equally 
and gradually exerts, and as equally and 
gradually receives, it makes the motion in all 
parts of the revolution pretty nearly equal 
and uniform. The consequence of this is, 
that the engine becomes more easy and con- 
venient to be acted on and moved by the im- 
pelling force ; and this is the only benefit ob- 
tained by the fly. 
The best form for a fly, is that of a heavy 
wheel or circle, of a fit size, as this will not 
only meet with less resistance from the air, 
but being continuous, and the weight every 
where equally distributed through the peri- 
meter of the wheel, the motion will be more 
easy, uniform, and regular. In this form, 
the 'fly is most aptly applied to the perpen- 
dicular drill, which ‘it likewise serves to keep 
upright by its centrifugal force: also to a 
windlass or common winch, where the motion 
is quick; for in pulling upwards from the 
lower part, a person can exercise more power 
than in thrusting forward in the upper quar- 
ter; where, of course, part of his force would 
be lost, was it not accumulated and coit- 
i served in the equable motion ol the fly. 
Hence, by this means, a man may work ail 
day in drawing up a weight of 40lb. whereas 
30lb. would create him more labour in a day 
without the fly. 
In order to calculate the force of the fly 
joined to the screw for stamping the image 
upon coins, let us suppose the two arms of 
the fly to be each fifteen inches long, mea- 
suring from the centre of the weight to the 
axis of motion, the weights to be fifty pounds 
each, and the diameter of the axis pressing 
upon the dye, to be one inch. If every 
stroke is made in half a second, and the 
weights describe an half-circumference, 
which in this case will be four feet, the ve- 
locity will at the instant of the stroke be at 
the rate of eight feet in a second, so that the 
momentum of it will be 800 ; but the arms of 
the fly being as levers, each fifteen inches 
long, whilst the semi-axis is only half an inch 
we must increase this force thirty times, 
which will give 24000 ; an immense force 
equal to lOOib. falling 120 feet, or near two 
seconds in time ; or to a body of 750lb.^ fall- 
ing 16^- feet, or one second in time. Some 
of the engines for coining crown-pieces have 
the arms of the fly five times as long, and the 
weights twice as heavy ; so that the effect is 
ten times greater. 
Fly, in the sea-language, that part of the 
FLY 
mariner’s compass, on which the several 
winds or points are drawn. 
Let fly the sheet, is a word of command 
to let loose the sheet, in case of a gust of 
wind, lest the ship should overset, or spend 
her topsails and masts ; which is prevented 
by letting the sheet go amain, that it may 
hold no wind. 
Fly-boat, a large vessel with a double 
prow, carrying from seven to eight hundred- 
weight of goods. 
Fly, vegetable, a very curious natural 
production, chiefly found in the W est Indies. 
Excepting that it has no wings, it resembles 
the drone both in size and colour more than 
any other British insect. In the month of 
May it buries itself in the earth, and begins 
to vegetate. By the end of July, the tree 
is arrived at its full growth, and resem- 
bles a coral branch ; and is about three inches 
high, and bears several little pods, which 
dropping off become worms, and thence fl'.es, 
like the British caterpillar. Such was the 
account originally given of this extraordinary 
production. But several boxes of these flies 
having been sent to Dr. Hill for examina- 
tion, his report was this: “ There is in Mar- 
tinique a fungus of the clavariakind, different 
in species from those hitherto known. It 
produces soboles from its sides; I call it 
therefore clavaria sobolifera. It grows on 
putrid animal bodies, as our fungus ex pede 
equino, from the dead horse’s hoof. The 
cicada is common in Martinique, and in its 
nympha state, m which the old authors call 
it” tettigometra: it buries itself under dead 
leaves to wait its change ; and when the sea- 
son is unfavourable, many perish. The seeds 
of the clavaria find a proper bed in this dead 
insect, and grow. The tettigometra is among 
the cicadas in the British museum ; the cla- 
varia is just now known. This is the fact, 
and all the fact ; though the untaught inhabit- 
ants suppose a fly to vegetate, and though 
there is a Spanish drawing of the plants 
growing into a trifoliate tree, and it has been 
figured with the creature flying with this tree 
upon its back.” Edwards has taken notice of 
this extraordinary production in his Glean- 
ings of Natural History. 
FLYERS, in architecture, such stairs as 
go straight, and do not wind round. 
FLYING, the progressive motion of a 
bird, or other winged animal, in the liquid 
air. The parts of birds chiefly concerned 
in flying, are the wings, by which they are 
sustained or waited along. The tail, Messrs. 
Willughby, Ray, and many others, imagine 
to be principally employed in steering and 
turning the body in the air, as a rudder; 
but Borelli has put it beyond ail doubt, that 
this is the least use of it, which is chiefly to 
assist the bird in its ascent and descent in 
the air; and to obviate the vacillations of the 
body and wings : for, as to turning to this or 
that side, it is performed by the wings and 
and inclinations of the body, and but very lit- 
tle by the help of the tail. The flying of a 
bird, in effect, is quite a different thing from 
the rowing of a vessel. Birds do not vibrate 
their wing's towards the tail, as ours are struck 
towards the stern, but waft them downwards: 
nor does the tail of the bird cut the air at right 
angles, as the rudder does the water, but is 
disposed horizontally, and preserves the same 
situation what way soever the bird turns. 
In effect, as a vessel is turned about on 
4 
FLY 757 
its centre of gravity to the right, by a brisk 
application of the oars to the left, so a bird 
in beating the air with its right wing alone, 
towards - the tail, will turn its lore part to the 
left. Thus pigeons, changing their course- 
to the left, would labour with their right 
wing, keeping the other almost at rest. Birds 
of a long neck alter their course by the in- 
clinations of their head and neck, which al- 
tering the course of gravity, the bird will 
proceed in a new direction. 
The manner of flying is thus: The bird 
first bends his legs, and springs with a violent 
leap from the ground ; then opens and ex- 
pands the joints of his wings, so as to make 
a right line perpendicular to the sides of hi? 
body: thus the wings, with all the feathers 
in them, constitute one continued lamina. 
Being now raised a little above the horizon, 
and vibrating the wings with great force and. 
velocity perpendicularly against the subject 
air, that fluid resists those successions, both 
from its natural inactivity and elasticity, by 
means of which the whole body of the bird 
is protruded. The resistance the air makes 
to the withdrawing of the wings, and conse- 
quently the progress of the bird,, will be so 
much the greater, as the waft or stroke of 
the fan of the wing is longer; but as the 
force of the wing is continually diminished' 
by this resistance, when the two forces come 
to be in equilibrio, the bird will remain sus- 
pended in the same place: for the bird only 
ascends so long as the arch of air the wing 
describes, makes a resistance equal to the 
excess of the specific gravity of the bird 
above the air. If the air, therefore, is so rare 
as to give way with the same velocity as it is 
struck with, there will be no resistance, and 
consequently the bird can never mount. 
Birds never fly upwards in a perpendicular 
line, but always in a parabola. In a direct- 
ascent, the natural and artificial tendency 
would oppose and destroy each other, so that 
the progress would be very slow. In a di- 
rect descent they would aid one another, so 
that the fall would be too precipitate. 
Artificial Flying, that attemjatedby men, 
by the assistance of mechanics. 
The art of flying has been attempted by 
several persons in all ages. The Leucadians, 
out of superstition, are reported to hate had 
a custom of precipitating a man from a high 
cliff into the sea, first fixing feathers, vari- 
ously expanded, round his body, in order to 
break his fall. Friar Bacon, who lived five 
hundred years aro, not only affirms the art of 
flying possible, but assures us, that he him- 
self knew how to make an engine in which 
a man sitting might be able to convey him- 
self through the air, like a bird ; and further 
adds, that there was then one who had tried 
it with success ; but this method, which con- 
sisted of a couple of large, thin, hollow cop- 
per globes, exhausted of the air, and sustain- 
ing a person who sat thereon, Dr. Hook 
shews to be impracticable. The philosophers 
of king Charles the Second’s reign, were ex- 
ceedingly busied about this art. The famous 
bishop Wilkins was so confident of success in 
it, that he says, he does not question that, in- 
future ages, it will be as usual to hear a man 
call for his wings,, when he is going a journey,, 
as it is now to call for his boots. 
Flying army, a small body under a lieu- 
tenant or major-general, sent to harass the - 
country, intercept convoys, prevent the ene- 
