A X I 
The axioms in use being derived from 
slender experience, and a few obvious par- 
ticulars, are generally applied in a corres- 
ponding manner. No wonder, therefore, 
they lead us to few particulars ; and if any 
instance, unobserved before, happen to 
turn up, the axiom is preserved by some 
trifling distinction, where it ought rather to 
be corrected. 
Axiom is also an established principle in 
some art or science. 
Thus it is an established axiom in phy- 
sics, that nature dees nothing in vain ; so it 
is in geometry, that if to equal things you 
add equals, the sums will be<*qual. It is an 
axiom in optics, that the angle of incidence 
is equal to the angle of reflection, &c. In 
which sense too, the general laws of mo- 
tion are called axioms : whence it may be 
observed, that these particular axioms are 
but deductions from certain hypotheses. 
AXIS, in geometry, the straight line in a 
plane figure, about which it revolves, to 
produce or generate a solid: thus, if a semi- 
circle be moved round its diameter at rest, 
it will generate a sphere, the axis of which is 
that diameter. 
Axis, in astronomy. 1 . Axis of the world, 
an imaginary right line conceived to pass 
through the centre of the earth, from one 
pole to the other, about which the sphere 
of the world in the Ptolemaic system re- 
volves in its diurnal rotation. 2. The axis 
of a planet, is that line drawn through the 
centre about which the planet revolves. The 
sun, together with all planets, except Mer- 
cury, Saturn, and Herschel, are known by 
observation to move about their respective 
axis. The axis of the earth, during its revo- 
lution round the sun, remains parallel to it- 
self, and is inclined to the plane of the eclip- 
tic, making with it an angle of 66i de- 
grees. 3. The axis of the equator, horizon, 
ecliptic, zodiac, &c. are right lines drawn 
through the centers of those circles perpen- 
dicular to their planes. 
Axis, in conic sections, a right line di- 
viding the section into two equal parts, and 
cutting all its ordinates at rightangles. See 
Conic Sections. 
Axis, in mechanics. The axis of a bal- 
lange is that line about which it moves, or 
rather turns about. Axis of oscillation is a 
right line parallel to the horizon, passing 
through the centre about which a pendulum 
vibrates. 
Axis in peritrochio, one of the five 
mechanical powers, consisting of a peritro- 
chium or wheel concentric with the base of 
AZA 
a cylinder, and moveable together with it 
about its axis. See Mechanics. 
Axis, in optics, is that ray, among all 
others that are sent to the eye, wliich falls 
perpendicularly upon it, and which conse- 
quently passes through the centre of the 
eye. 
Axis of a glass or lens, is a right line join- 
ing the middle points of the two opposite 
surfaces of the glass. 
Axis of incidence, in dioptrics, is a right 
line perpendicular in the point of incidence, 
to the refracting superficies, drawn in the 
same medium that the ray of incidence 
comes from. 
Axis of refraction is a right line drawn 
through the refracting medium, from the 
point of refraction, perpendicular to the 
refracting superficies. 
AXYRIS, in botany, a genus of the Mo- 
noecia Triaudria class of plants, in the male 
flowers of which the calyx is a perianthium 
composed of four patent, obtuse leaves, di- 
vided into three segments : there is no co- » 
rolla : in the female flowers the calyx is com- 
posed of five obtuse, concave, connivent, 
and permanent leaves, with the two ex- 
terior ones shorter than the rest ; there is no 
corolla ; nor is there any pericardium ; the 
seed is single, oblong, compressed obtuse, 
and contained in the cup. 
AYE -uye, in natural history, a singular 
quadruped discovered by Sonnerat, in the 
island of Madagascar, and described in his 
vo >‘ a g e t° the East Indies. Sonnini forms a 
new genus of this animal, under the name of 
“ Chieromys,” hut Gmelin ranks it under 
the genus “ Sciuras,” which see. 
AYENIA, in botany, so called in honour 
of the Duke d’Ayen, a great promoter of the 
science of botany, of the Gynandria Pen- 
tandria class and order. Natural order of 
Columniferae. Malvaceae Jussica. Essen- 
tia! character, monogynous. Calyx five- 
leaved. Petals united into a star, with long 
claws ; five anthers, under the star ; capsule 
five-celled. There are four species ; of 
which, three are natives of South America, 
and one of Jamaica. These plants are pro- 
pagated by seeds, sown in the spring in hot- 
beds, where they must continue ; but they 
will want a good portion of free air. If ex- 
posed to the open atmosphere, they will not 
thrive; and if too much drawn, they do not 
flower well. The plant will live through 
the winter, but as they perfect their seeds 
the first year, the old plants are seldom con- 
tinued. 
AZALEA, in botany, of the Pentandris 
