fly fcet 
activity and elasticity, by means of which the country, intercept convoys, prevent the 
the whole body of the bird, is protruded, enemy’s incursions, cover its own garrisons. 
The resistance the air makes to the with- 
drawing of the wings, arid’ ''e&hseqturitly 
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 
suspended 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 ek»4ss of the specific gravity of the 
bird above/ the air. If the air, therefore, 
be so rare as to give way with the same 
velocity as it is struck withal, there will be 
no resistance and consequently the bird 
can ngver mount. Birds never fly upwards 
in a perpendicular line, but always in a 
parabola. In a direct ascent, the natural 
arid artificial tendency would oppose and 
destroy each Other, so that the progress 
would be very slow. In a direct descent 
they would aid one another, so that the fall 
would be too precipitate. 
Fying, artificial, that attempted by 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 have had a 
cifstom of precipitating a man from a high 
cliff into the sea, first fixing feathers, va- 
riously expanded, round his body, in order 
to break his fall. Friar Bacon, who lived 
near five hundred years ago, not only 
affirms the art of flying possible, but assures 
us, that he himself knew how’ to make an 
engine wherein a man sitting might be able 
to convey himself through the air, like a 
bird’; and further adds, that there was 
then one who had tried it with success: 
but this method, which consisted of a 
couple" of large, thin, Inflow copper globes, 
exhausted of the air, and sustaining a person 
who sat thereon, Dr. Hook shews to be 
impracticable. The philosophers of K. 
Charles the second’s reign, were mightily 
bits'ieil about this art. Bishop Wilkins 
was so confident of success in it, that he 
says, he does not question but, 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 r to call for his boots. 
The art of flying has in some measure 
been brought to bear in the construction 
and use of balloons. See /Erostaticn. 
Flying army, a small body under a 
lieutenant or major general, sent to harass 
and keep the enemy in continual alarm. 
Flying bridge. See Bridge. 
Ylving fish, a name given by the English 
writers to several species of fish, which, 
by means of their long fins, have a method 
of keeping themselves out of water some 
time. See ExoCoeTus, &C. 
FOCUS, in geometry and conic sections, 
is applied to certain points in the parabola, 
ellipsis, and hyperbola, where the rays re- 
flected from all parts of these curves con- 
cur and meet. 
Foci of an ellipsis, are two points in the 
longest axis, on which as centres the figure 
is described. See Ellipsis. 
If from the foci two right lines arc drawn, 
meeting one another in the periphery ol 
the ellipsis, their sum will be always equal 
to the longest axis; and therefore when an 
ellipsis and its two axis are given, and the 
foci are required, you need only take half 
the longest axis in your compasses, and 
setting one foot in the end of the shorter, 
the other foot will cut the longer in the 
focus required. 
Focus of an hyperbola, is that point in 
the axis, through which the latus rectum 
passes ; from. whence if any two right lines 
are drawn, meeting in either of the oppo- 
posite hyperbolas, their difference will be 
equal to the principal axis. See Hyper- 
bola. 
Focus of a parabola, a point in the axis 
within the figure, distant from the vertex- 
one fourth part of the latus rectum. See 
Parabola. 
Focus, in optics, is the point wherein 
rays are collected, after they have under- 
gone reflection or refraction. See Optics. 
FODDER, any kind of meat for horses, 
or other cattle. In some places, hay and 
straw, mingled together, is peculiarly de- 
nominated fodder. 
Fodder, in mining, a measure contain- 
ing twenty-two hundred and an half weight, 
though in London but twenty hundred 
weight. 
FCETUS, in anatomy, a term applied to 
the offspring of the human subject, or of 
animals, during its residence in the uterus. 
The term of ovum is applied to the foetus, 
with its membranes and placenta taken al- 
together. We shall consider under this ar- 
ticle the anatomy of the membranes which 
cover the foetus during its abode in the 
uterus; of the placenta, which forms the 
medium of connexion between the systems 
