CROWN
, in Astronomy, a name given to two con stellations, the southern and the northern.
Crown, in Geomctry, a plane ring included between two parallel or concentric peripheries, of unequal circles.
The area of this is had, by multiplying its breadth by the length of a middle periphery, which is an arithmetical mean between the two perpheries that bound it; or by multiplying half the sum of the circumferences by half the difference of the diameters; or lastly by multiplying the sum of the diameters by the difference of the diameters, and this last product by .7854. See my Mensuration, pa. 148, 2d ed.
Crown-Post, is a post in some buildings standing upright in the middle, between two principal rafters; and from which proceed struts or braces to the middle of each rafter. It is otherwise called a king-post, or king's-piece, or joggle-piece.
Crown-Wheel, of a Watch, is the upper wheel next the balance, or that which drives the balance.
Crown-Work, in Fortification, is an out-work running into the field; designed to keep off the enemy, gain some hill, or advantageous post, and cover the other works of the place. It consists of two demi-bastions at the extremities, and an entire bastion in the middle, with curtains.
Crowned Horn-work, is a Horn-work with a crownwork before it.
CRYSTALLINE Humour, is a thick compact humour of the eye, in form of a flattish convex lens, placed in the middle of the eye, and serving to make that refraction of the rays of light which is necessary to have them meet in the retina, and form an image there, by which vision may be performed.
Crystalline Heavens, in the Old Astronomy, two orbs imagined between the primum mobile and the firmament, in the Ptolomaic system, which supposed the heavens solid, and only susceptible of a single motion.
King Alphonsus of Arragon, it is said, introduced the Crystallines, to explain what they called the motion of trepidation, or titubation.
The first Crystalline, according to Regiomontanus, | &c, serves to account for the slow motion of the fixed stars; by which they advance a degree in about 70 years, according to the order of the signs, or from west to east; which occasions a precession of the equinox. The 2d serves to account for the motion of libration, or trepidation; by which the celestial sphere librates from one pole towards the other, causing a difference in the sun's greatest declination.
