ABE 



ABU 



arch of the horizon, which is contained 

 between a line at right angles with the 

 keel and that point of the compass which 

 is directly opposite the ship's course. 



ABAS, a weight used in Persia for 

 weighing pearls, being one eighth part 

 lighter than the European carat. 



ABASED, in heraldry, is said of the 

 wings of eagles, ike. when the tip looks 

 downwards to the point of the shield, or 

 when the wings are shut; the natural way 

 of bearing them being spread. 



ABATE, in law, signifiesto breakdown 

 or.destroy, as to abate a nuisance, and to 

 abate a castle. It means to defeat and 

 overthrow, on account of some error or 

 exception. 



ABATEMENT, inheraldry, something 

 added to a coat of arms, in order to lessen 

 its true dignity, and point out some imper- 

 fection or stain in the character of the 

 person who bears it. 



A it \TF.MENT, in law, signifies the re- 

 jecting a suit, on account of some fault 

 either in the matter orproceeding. Hence, 

 plea in abatement is some exception al- 

 leged, and proved, against the plaintiff's 

 writ, declaration, &c. and praying that the 

 plaint may abate or cease ; which being 

 granted, all writs in the process mast be- 

 gin de vf>\'u, 



ABATOR,inlaw, one who enters into a 

 house or lands, void by the death of the last 

 possessor, before the true heir; and there- 

 fore keeps him out, till he brings the writ 

 mtrusione. 



ABDOMEN, in anatomy, the lower pail 

 of tire trunk of the body, reaching from 

 the thorax to the bottom of the pelvis. 

 See ANATOMY. 



ABDOMINALES, in natural hi story, an 

 order of fishes, having ventral fins placed 

 behind the pectoral in the abdomen, and 

 the branchia ossiculated. This order 

 comprehends sixteen genera, viz. 

 Amia Cobitis Atherina 



Clupea Esox Cyprimis 

 Elops Loricaria Exocoetus 



Fistularia Salmo Mugil 

 Polynemas Teuthis Silurus 

 Argentina 



ABDUCTOR, or ABDUCENT, in anato- 

 my, a name given to several muscles, on 

 account of their servingto withdraw, open, 

 or pull back the parts to which they are 

 affixed. See ANATOMY. 



ABERRATION, in astronomy, an ap- 

 parent motion of the heavenly bodies, pro- 

 duced by the progressive motion of light 

 and the earth's annual motion in her orbit. 

 Since light proceeds always in right lines, 

 when its motion is perfectly undisturbed, 



if a fine tube were placed so as to receive 

 a ray of light passing exactly through its 

 axis when at rest, and then, remaining in 

 the same direction, were moved trans- 

 versely with great velocity, it is evident 

 that the side of the tube would strike 

 against the ray of light in its passage, anil 

 that, in order to retain it in the axis, the 

 tube must be inclined, in the same man- 

 ner as if the light, instead of cominginits 

 actual direction, had also a transverse 

 motion, in a direction contrary to that of 

 the tube. The axis of a telescope, or even 

 of the eye, may be considered as resem- 

 bling such a tube, the passage of the light 

 through the refracting substances not al- 

 tering the necessary inclination of the axis. 

 In various parts of the earth's orbit, the 

 aberration of any one star must be differ- 

 ent in quantity and in direction; it never 

 exceeds 20" each way, and therefore in- 

 sensible in common observations. If AB 

 and AC ( Plate Acoustics, &c. fig. 1,) re- 

 present the comparative velocity of light 

 and of the earth, in their respective direc- 

 tions, a telescope must be placed in the 

 direction BC in order to see the star D, 

 and the star will appear at E. This dis- 

 covery was made by Dr. Bradley, in his 

 observations to determine the annual pa- 

 rallax of the fixed stars, or that which 

 arises from the motion of the earth in its 

 orbit round the sun. 



ABERRATION of the planets, is equal to 

 the geocentric motion of the planet, the 

 space which it appears to move, as seen 

 from the earth, during the time that light 

 employs in passingfrom the planet to the 

 earth. Thus, with regard to the sun, the 

 aberration in longitude is constantly 20", 

 which is the space moved by the earth in 

 the time 8' 7", which is the time that light 

 takes to pass from the sun to the earth. 

 Uence, the distance of the planet from the 

 earth being -known, it will be, as the dis- 

 tance of the sun is to the distance of the: 

 planet, so is 8' 7" to the time of light pass- 

 ing from the planet to the earth ; then 

 computingthc planet's geocentric motion 

 in this time, will give the aberration of the 

 planet, whether it be in longitude, lati- 

 tude, right ascension, or declination. The 

 aberration will be greatest in longitude, 

 and but very smallin latitude, because the 

 planets deviate very little from the plane 

 of the ecliptic. In Mercury it is only 4 1 ' 

 and much less in the other planets. The 

 aberration in declination and right ascen- 

 sion depends on the situation of the pla- 

 net in the zodiac. The aberration in lon- 

 gitude, being equal to the geocentric mo- 

 tion, will be more or le'ss, according a* 



