OBS 



OCC 



to 0". The principal uses of this equa- 

 torial are, 1. To find your meridian by 

 one observation only ; for this purpose, 

 elevate the equatorial circle to the co- 

 latitude of the place, and set the declina- 

 tion semi-circle to the sun's declination 

 for the day and hour of the day required ; 

 then move the azimuth and hour circles 

 both at the same time, either in the same 

 or contrary direction, till you bring the 

 centre of the cross hairs in the telescope 

 exactly to cover the centre of the sun ; 

 when that is done, the index of the hour 

 circle will give the apparent or solar time 

 at the instant of observation ; and thus 

 the time is gained, though the sun be at 

 a distance from the meridian ; then turn 

 the hour-circle till the index points pre- 

 cisely at twelve o'clock, and lower the te- 

 lescope to the horizon, in order to observe 

 some point there in the centre of your 

 glass, and that point is your meridian 

 mark found by one observation only ; the 

 best time for this operation is three hours 

 before or three hours after twelve at noon. 

 2. To point the telescope on a star, though 

 not on the meridian, in full day light. 

 Having elevated the equatorial circle to 

 the co-latitude of the place, and set the 

 declination semi-circle to the star's de- 

 clination, move the index of the hour cir- 

 cle till it shall point to the precise time at 

 which the star is then distant from the 

 meridian, found in tables of the right as- 

 cension of the stars, and the star will then 

 appear in the glass. Besides these uses 

 peculiar to this instrument, it is also ap- 

 plicable to all the purposes to which the 

 principal astronomical instruments, -viz. 

 a transit, a quadrant, and an equal alti- 

 tude instrument, are applied. See Vince's 

 " Practical Astronomy." 



OBSIDIAN, irt mineralogy, a genus of 

 the Pitch-stone family, found in nests in 

 the pearl-stone of Hungary. It is common 

 likewise in Iceland, Siberia, the Levant 

 islands, and in South America, and has 

 obtained the name of the Iceland agate. 

 The principal colour is velvet-black, but 

 it passes into greenish grey. It is often 

 striped and spotted. The specific gravity 

 is about 2.4: it melts into an opaque, 

 grey mass. Specimens have been analys- 

 ed, and found to contain 



Silica .... 69 74 



Alumina . . 22 2 



Oxide of iron 9 ...... 14 



100 



Loss 



90 

 10 



100 



It is on account of its great hardness 

 and opaque blackness, and of its capabili- 

 ty of receiving a high polish used as an 

 ornament in dress. In Peru, before the 

 conquest of the country by Spain, obsi- 

 dian was used as a mirror, and in Europe 

 it has been fashioned into reflectors for 

 telescopes. 



OBTUSE, signifies blunt, dull, &c. 

 in opposition to acute, sharp, &c.; thus 

 we say, obtuse angle, obtuse angled tri- 

 angle, &c. 



OCCIDENT, in geography, the west. 

 ward quarter of the horizon, or that 

 part of the horizon where the ecliptic, 

 or the sun therein, descends into the 

 lower hemisphere, in contradistinction to 

 orient. 



OCCIPITAL, in anatomy, a term ap- 

 plied to the parts of the occiput, or back 

 part of the skull. 



OCCULT, something secret, hidden, or 

 invisible. The occult sciences are, magic* 

 necromancy, cabbala, &c. 



OCCULT, in geometry, is used for a line 

 that is scarcely perceivable, drawn with 

 the point of the compasses, or a leaden 

 pencil. These lines are used in several 

 operations, as the raising of plans, de- 

 signs of building, pieces of perspective, 

 &c. Thejr are to ke effaced when the 

 work is finished. 



OCCULTATION, in astronomy, the 

 time a star or planet is hidden from our 

 sight, by the interposition of the body of 

 the moon, or of some other planet. 



OccriTATioif, Circle of perpetual, is a 

 parallel in an oblique sphere, as far dis- 

 tant from the depressed pole, as the ele- 

 vated pole is from the horizon. 



All the stars between this parallel and 

 the depressed pole, never rise, but lie 

 constantly hidden under the horizon of 

 the place. 



OCCUPANCY, in law, is a right which 

 one acquires to a thing by being the first 

 to gain possession of it. But this right is 

 now chiefly done away by the English 

 law. Formerly, if a tenant for a term of 

 another's life died, leaving the cestid que 

 vie; that is, during the life of the person 

 for whose life the estate was held; he 

 who first entered should hold the land 

 during the other man's life ; and he was 

 in law called an occupant, because his 

 title was by his first occupation. But 

 now this title is prevented by the statutes 

 29 Charles II. c. 3, s. 12, and 14 George 

 II. c. 20, s. 9, which make the estate 

 personal assets devisable, and chargeable 

 with the debts of the deceased, in the 

 hands of the heir, who enters as special 

 occupant 



