FLY 
wheel or circle, of a lit 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 
perpendicular 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 
tipper quarter 1 : where, of course, part of 
his force would be lost, were it not accu- 
mulated and conserved in the equable mo- 
tion of the fly. Hence, by this means, a 
man may work all day in drawing up a 
weight of 40 lb. whereas 30 lb. 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 50 pounds 
each, and the diameter of the axis pressing 
upon the dye, to be one inch. If every 
stroke be 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 30 times, 
which will give 24,000 ; an immense force, 
equal to 100 lb. falling 120 feet, or near two 
seconds in time; or to a body of 750 lb. 
falling 16Jj feet, or one second in time. 
Some engines, for coining crown-pieces, 
used to have the arms of the fly five times as 
long, and the weights twice as heavy, so 
that the effect is ten times greater. See 
Coining. 
Fly, in the sea language, that part of 
the 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 top sails 
and masts ; which is prevented by letting 
the sheet go a-main, that it may hold no 
wind. 
Fly boat, a large vessel with a double 
prow, carrying from four to six hundred 
tons. 
FLYERS, in architecture, such stairs as 
VOL. III. 
FLY 
go straight, and do not wind round ; nor 
have the steps made tapering, but the fore 
and back part of each stair, and the ends, 
respectively parallel to one another; so that 
if one flight do not carry you to your in- 
tended height, there is a broad half space, 
from whence you begin to fly again, with 
steps every where of the same length and 
breadth, as before. 
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 wafted along. The tail, 
Messeurs Willoughby, Ray and many 
others, imagine to be principally employ- 
ed in steering and turning the body in 
the air, as a rudder : but Borelli has put 
it beyond all 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 inclinations of the body, and but very 
little 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 wings towards the tail, 
as oars are stmck towards the stern, but 
waft them downwards : nor does the tail of 
file bird cut the air at right angles, as the 
rudder does the water; but is disposed 
horizontally, and preserves the same situa- 
tion what way soever the bird turns. 
In effect, as a vessel is turned about on 
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 fore part to 
the left. Thus pigeons, changing their 
course to the left, would labour it with 
their right wing, keeping the other almost 
at rest. Birds of a long neck alter their 
course by the inclinations of their head and 
neck, which altering 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 vio- 
lent leap from the ground ; then opens and 
expands the joints of his wings, so as to 
make a right line perpendicular to the sides 
of his body : thus the wings, with all the 
feathers therein, 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 m* 
, N 
p 
