A X I 



AZO 



a leaf-stalk with the stem, the normal 

 position of the bud, which is hence called 

 axillary. 



A'XINITE [d^lvt], an axe). Thumer- 

 stone. A mineral found at Thum in 

 Saxony, and named from the resemblance 

 of its crystals to an axe in the form and 

 sharpness of their edges. 



A'XIOM (af/w/xa, dignity). A self- 

 evident proposition, incapable of being 

 established by proof, and assumed as the 

 basis of demonstration, 



A'XIS (af 0)1/, the axle-tree of a chariot). 

 A term applied to an imaginary straight 

 line passing through the centre of a body, 

 and on which the body turns. The axis 

 of the earth is a line of this kind passing 

 from one pole to the other; and, if we 

 suppose this line to be extended in both 

 directions, it will become an axis of the 

 heavens, and will mark two points which 

 are also called poles, around which, or 

 rather their line of junction, all the fixed 

 stars appear to revolve. 



1. Axis, rotation on. The different 

 points of a body may at the same time 

 move in different directions ; this is 

 called a rotatory motion. Rotation on an 

 axis implies, in addition to this rotatory 

 motion, that all the parts of the body 

 shall revolve around an imaginary right 

 line in a state of rest, called the axis. 



2. Axis, free or moveable. A term 

 applied to an axis which is free from all 

 pressure, in consequence of the mass of 

 the body surrounding it being so equally 

 arranged about it, that the centrifugal 

 forces of its particles mutually counteract 

 one another. The axis of the earth is a 

 free or moveable axis. 



3. Axis of refraction. The right line 

 drawn perpendicular to the surface of the 

 refracting medium through the point of 

 incidence of the refracted ray. Of crys- 

 tals which possess the property of double 

 refraction, many, like Iceland spar, have 

 only one axis of double refraction ; others 

 have two axes, which are variously in- 

 clined to each other, as the topaz. "When 

 the ray of extraordinary refraction is in- 

 clined from the axis, the crystal is said 

 to haxe a. negative axis; when it is in- 

 clined towards it, more than the ordinary 

 ray, the crystal is said to have a positive 

 axis. 



4. Axes of crystals. The lines which 

 join the points, and pass through the 

 middle of a crystal, are called its axes. 

 Most of these bodies have three such 

 axes at right angles to each other: that 

 which is most unlike . the other two, is 



45 



called the principal axis, and the other 

 two secondary or subordinate axes. 



5. Axis of a lens. A right line drawn 

 through the optical centre of the lens, 

 and perpendicular to both its surfaces. 

 At this point no refraction is produced 

 on perpendicular rays. 



6. Axis of a spherical mirror. A right 

 line which passes through the geometri- 

 cal and the optical centres of the mirror. 



7. Axis of a cone. A right line drawn 

 from the vertex of the cone to the centre 

 of its circular base. 



8. Axes of an ellipse. These are called 

 the major and the minor axes, and they 

 are respectively synonymous with the 

 transverse and the conjugate diameters. 



9. Axis of a parabola. The line which, 

 passing through the vertex of the para- 

 bola, divides the figure into two equal 

 and similar portions. 



10. Axis of a magnet. The imaginary 

 line which connects the north and south 

 poles of a magnet. 



11. Axis anticlinal. The imaginary 

 line lying between the strata which dip 

 in opposite directions on the two sides of 

 a hill or of a valley. In a row of houses 

 with steep roofs facing the south, the 

 slates represent inclined strata dipping 

 north and south, and tlie ridge is an east 

 and west anticlinal axis. 



A'YMESTRY LIMESTONE. A group 

 of the Ludlow rocks, consisting of grey 

 or bluish argillaceous limestone, full of 

 the remains of shells and corals. 



A'ZIMUTH. An Arabic term de- 

 noting the angular distance of a celes- 

 tial object from the north or south point 

 of the horizon, when the object is re- 

 ferred to the horizon by a vertical circle. 

 Or, it is the angle comprised between 

 two vertical planes, one passing through 

 the elevated pole, the other through the 

 object. 



1. Azimuth circles. Great circles of 

 the sphere, passing through the zenith, 

 and intersecting the horizon at right 

 angles. 



2. Azimuth compass. A compass used 

 at sea for finding the horizontal distance 

 of the sun or a star from the magnetic 

 meridian. 



3. Azimuth dial. A dial of which the 

 style or gnomon is perpendicular to the 

 plane of the horizon, and so called from 

 its shadow marking the sun's azimuth. 



A'ZOTANE. Chloride of azote. The 

 term is derived from Davy, who pro- 

 posed to designate the compounds of 

 chlorine by the termination ane. 



