AST 



than at others, the tides will at those times 

 be greater, and at the opposite points of 

 her orbit less. Some variations likewise 

 take place in consequence of the different 

 declinations of the sun and moon at dif- 

 ferent times. For if either of these lumi- 

 naries were at the pole, it would occasion 

 a constant elevation both there and at the 

 opposite one, and a constant depression at 

 the equator ; so that as the sun and moon 

 gradually decline from the equator, they 

 lose their effect, and the tides become 

 less; and when they are both in the equa- 

 tor, the tides of course become greater. 

 Astronomy is sometimes divided, in 

 books, with respect to its different states, 

 into " new" and " old." The former re- 

 fers to the art, as it stood under Ptolemy 

 and his followers, with all the apparatus 

 of solid orbs, epicycles, &c. &c. By new 

 astronomy is meant the science, as it has 

 been cultivated since the period in which 

 Copernicus flourished. By that great 

 man the constitution of the heavens was 

 reduced to more simple, natural, and 

 certain principles. The substance of the 

 old astronomy is given by Tacquet, and 

 of the new by Whiston, in his " Prelec- 

 tiones Astronomic^," published in 1707. 

 The whole doctrine, both according to the 

 ancients and moderns, is explained by 

 Mercator, in his Institutiones Astron. 



Having concluded this brief sketch of 

 a very important science, we shall refer 

 to other articles, in which many subjects 

 will be discussed, that usually find place 

 in a treatise of astronomy. Under the word 

 Sen will be found some interesting specu- 

 lations of Dr. Herschel ; under that of 

 Moo*, an account of the methods, of 

 measuring its mountains, an explication 

 of the harvest moon and horizontal moon. 

 For equation of time, see TIKE ; see also 

 EARTH, figure of; ECLIPTIC ; EQUI- 

 NOXES, precession of; GALAXY; GRAVI- 

 TATION ; NEBUUE ; SATURN, ring of ; 

 ASTRONOMY, practical ; and ASTRONO- 

 MICAL instruments: See OBSERVATORY ; 

 SATELLITES; TRANSIT; &c. &c. &c. 



ASTROSCOPE, an instrument com- 

 posed of two cones, having the constella- 

 tions delineated on their surfaces, where- 

 by the stars may be easily known. 



ASYMETRY, in a general sense, the 

 want of proportion between the parts of 

 any thing, being the contrary of symme- 

 try. 



In mathematics it is used for what is 

 more commonly called incommensurabi- 

 lity, or the relation of two quantities which 

 have no common measure, as between one 



ATH 



and the Square root of two, or as 1 : ^/ 2, 

 or the side and diagonal of a square. 



ASYMPTOTE, in geometry, a line 

 which continually approaches nearer to 

 another, but though continued indefinitel) 

 will never meet with it : of these there 

 are many kinds. 



The term asymptotes is appropriated 

 to right lines, which approach nearer and 

 nearer to some curve, of which they are 

 said to be the asymptotes ; but if they 

 and their curves are indefinitely continu- 

 ed, they will never meet. 



Concerning asymptotes and asymptoti- 

 cal curves, it may be remarked, 1. That 

 although such curves as have asymptotes 

 are of the number of those which do not 

 include a space ; yet it is not true, on the 

 other hand, that, wherever we have a 

 curve of that nature, we have an asymp- 

 tote also. 2. Of these curves that have 

 an asymptote, some have only one, as the 

 conchoid, cissoid, and logarithmic curve ; 

 and others two, as the hyperbola. 3. As 

 a right line and a curve may be asymp- 

 totical to one another, so also may curves 

 and curves : such are two parabolas, 

 whose axes are in the same right line. 

 4. No right line can ever be an asymp- 

 tote to a curve that is every where con- 

 cave to that right line. 5. But a right 

 line may be an asymptote to a mixed 

 curve that is partly concave, and partly 

 convex, towards the same line. And, 6. 

 All curves that have one and the same 

 common asymptote, are also asymptotical 

 to one another. See CONIC SECTIONS. 



ASYNDETON, in grammar, a figure 

 which omits the conjunctions in a sen- 

 tence, as in that verse of Virgil, 



Ferte citi J!ammas t date vela, impellite 

 reinos. 



ATCHTEVEMENT, in heraldry, de- 

 notes the arms of a person, or family, to- 

 gether with all the exterior ornaments of 

 the shield, as helmet, mantle, crest, 

 scrolls, and motto, with such quarterings 

 as may have been acquired by alliances, 

 all marshalled in order. 



ATHAMANTA, in botany, a geuus of 

 the Pentandria Digynia class of plants, 

 the general corolla whereof is uniform ; 

 the partial one consists of five inflexo- 

 cordated unequal petals ; there is no peri- 

 carpium; the fruit is ovato-oblong, striat- 

 ed, and divisible into two parts ; the 

 seeds are two, oval, striated, and convex 

 on the one side, and plain on the olher : 

 There are ten species. 



ATHANAS1A, in botany, a genus of 

 the Syngenesiu Polygamia Equalis class 



