AXI 



AZA 



AXIOM is also an established principle 

 in some art or science. 



Thus, it is an established axiom in phy- 

 sics, that nature does nothing in vain ; so 

 it is in geometry, that if to equal things 

 you add equals, the sums will be equal 

 It is an axiom in optics, that the angle of 

 incidence is equal to the angle of reflec- 

 tion, &c. In which sense, too, the gene- 

 ral laws of motion 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 

 semicircle 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, revolves in its diurnal rotation. 

 2. The axis of a planet is that line drawn 

 through the centre about which the pla- 

 net revolves. The sun, together with all 

 planets, except Mercury, Saturn, and 

 JHerschel, are known, by observation, to 

 move about their respective axes. The 

 axis of the earth, during its revolution 

 round the sun, remains parallel to itself, 

 and is inclined to the plane of the eclip- 

 tic, making with it an angle of 66"^ de- 

 grees. *" 3. The axes of the equator, hori- 

 zon, ecliptic, zodiac, &c. are right lines 

 drawn through the centres of those cir- 

 cles perpendicular to their planes. 



Axis, in conic sections, a right line, di- 

 viding the section into two equal parts, 

 and cutting all its ordinates at right an- 

 gles. See CONIC SECTIONS. 



Axis, in mechanics. The axis of a ba- 

 lance is that line about which it moves, 

 or rather turns about. Axis of oscilla- 

 tion is a right line parallel to the horizon, 

 passing through the centre about which a 

 pendulum vibrates. 



Axis in peritroctdo, one of the five me- 

 chanical powers, consisting of a peritro- 

 chium or wheel, concentric with the base 

 of a cylinder, and move able together with 

 it about its axis. See MECHANIC?. 



Axis, in optics, is that ray, among all 

 others that are sent to the eye, which 

 falls perpendicularly tipon it, and which 

 consequently passes through the centre 

 of the eye. 



Axis of a glass or lens, is a right line 

 joining the middle points of the two op- 

 posite 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 Triandria class of plants, in the 

 male flowers of. which the calyx is a peri- 

 anthium, composed of four patent, obtuse 

 leaves, divided into three segments : there 

 is no corolla : in the female flowers the 

 calyx is composed of five obtuse, con- 

 cave, connivent, and permanent leaves, 

 with the two exterior ones shorter than 

 the rest ; there is no corolla ; nor is there 

 any pt- ricarpium the seed is single, ob- 

 long, compressed, obtuse, and contained 

 in the cup. 



AYE-loye, in natural history, a singular 

 quadruped, discovered by Sonnerat, in 

 the island of Madagascar, and described 

 in his voyage to the East Indies. Sonnini 

 forms a new genus of this animal, under 

 the name of " Chieromys," but Gtnelin 

 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 

 Pentandria class and order. Natural or- 

 der of Columniferae. Malvacese, Jussieu. 

 Essential character, monogynou.s. Citlyx 

 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 propagated by seeds, 

 sown in the spring, in hot-beds, where 

 they must continue ; but they will want 

 a good portion of free air. If exposed 

 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 continued. 



AZALEA, in botany, of the Pentan- 

 dria Monogynia class and order. Natural 

 order, Bicornes ; Rhododendria, Jussieu. 

 Essential character, corolla bell-shaped ; 

 stamina inserted into the receptacle ; 

 capsule five-celled. There are seven 

 species. A. Pontica much resembles the 

 rhododendron ponticum, but it has five 

 stamens and yellow corollas, not ten sta- 

 mens and violet-coloured corollas, as that 

 has. The leaves are smaller, ovate, and 



