2S4 
O B S 
O B S 
"hour-circle ; then turn the declination-semi- 
circle to 90°, and adjust the bubble by raising 
or depressing the polar axis ; then turn the 
hour-circle 12 hours; and it the bubble is 
wrong, correct half the error by the polar 
axis, and the other half-error bv the two 
pair ot capstan-sc: vs at the feet of the two 
supports on one side ot the axis of motion of 
the telescope; and thus this axis will be at 
r-el’t angles to the polar axis. The next 
an;u., meet is to make the centre of cross 
hairs remain on the same object, while you 
turn toe eye-tube quite round by the pinion 
of the refraction apparatus : .for this adjust- 
ment, set the index on the slide to the first 
division of the dovetail ; and set the division 
marked 18" on the refraction-circle to its 
index ; then look through the telescope, and 
with the pinion turn the eye-tube quite 
round ; and if the centre of the hairs does 
not remain on the same spot during that re- 
volution, it must be corrected by the four 
small screws, two and two at a time (which 
you will find upon unscrewing the nearest 
end of the eye-tube that contains the first 
eye-glass) ; repeat this correction till the 
centre of the hairs remains on the spot you 
are looking at during an entire revolution. 
In order to make the line of collimation pa- 
rallel to the brass rod on which the level 
hangs, set the polar axis horizontal, and the 
declination-circle to 90’ ; adjust the level by 
the polar axis ; look through the telescope 
on some distant horizontal object, covered 
by the centre of the cross hairs ; then in- 
vert the telescope, which is done by turning 
the hour circle half-round; and if the centre 
of the cross hairs does not cover the same 
object as before, correct half the error bv 
the uppermost and lowermost of the four 
small screws at the eye-end of the large tube 
of the telescope: this correction will give a 
second object now covered by the centre of 
the hairs, which must be adopted instead of 
the first object: then invert the telescope as 
before ; and if the second object is not: co- 
vered by the centre of the hairs, correct half 
the error by the same two screws which 
were used before : this correction will give a 
third object, now covered by the centre of 
the hairs, which must be adopted instead of 
the second object ; repeat this operation till 
no error remains; then set the hour-circle 
exactly to 12 hours (the declination-circle 
remaining at 90 1 degrees as before) ; and if 
the centre of the cross hairs does not cover 
the last object fixed on, set it to that object 
by the two remaining small screws at the 
eye-end of the large tube, and then the line 
of collimation will be parallel to the brass 
rod. For rectifying the nonius of the decli- 
nation and equatorial circles, lower the tele- 
scope as many degrees, minutes, and se- 
conds, below 0° or JE on the declination- 
semicircle, as are equal to the complement of 
the latitude ; then elevate the polar axis till 
the bubble is horizontal, and thus the equa- 
torial circle will be elevated to the co-lati- 
tude of the place; set this circle to 6 hours ; 
-adjust the level by the pinion of the decli- 
nation-circle ; then turn the equatorial circle 
exactly 12 hours from the last position ; and 
if the level is not right, correct one half of 
the error by the equatorial circle, and the 
other half by the declination-circle; then 
turn the equatorial circle back again exactly 
12 hours from the last position ; and if the 
level is still wrong, repeat the correction as | 
before till it is right when turned to either 
position; that being done, set the nonius of 
the equatorial circle exactly to 6 hours, and 
the nonius of the declination circle exactly 
to 0". - 
The principal uses of this equatorial are, 
1. To find the meridian by one observa- 
tion only : for this purpose elevate the equa- 
torial circle to the co-latitude of the place, 
and set the declination-semicircle 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 directions, 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 is at a distance from 
the meridian ; then turn the hour-circle till 
the index points precisely at 12 o’clock, and 
lower the telescope to the horizon, in order 
to observe some point there in the centre 
of your glass, and that point is your meri- 
dian mark found by one observation only ; 
the best time for this operation is three hours 
before or three hours after 12 at noon. 
2. To point the telescope on a star, though 
not on the meridian, in full day-light. Hav- 
ing elevated the equatorial circle to the co- 
latitude of the place, and set the declination- 
semicircle to the star’s declination, move 
the index of the hour-circle till it shall point 
to the precise time at which the star is then 
distant from the meridian, found in tables of 
the right ascension of the stars, and the 
star will then appear in the glass. Besides 
these uses peculiar to this instrument, it is 
also applicable to all the purposes to which 
the principal astronomical instruments, viz. 
a transit, a quadrant, and an equal-altitude 
instrument, are applied. 
Of all the different sorts of chronometers 
or timekeepers, a pendulum-clock, when 
properly constructed, is undoubtedly capable 
of the greatest accuracy, it being liable to 
fewer causes of obstruction or irregularity ; 
therefore such machines are most recom- 
mendable for an observatory. The situation 
of this clock must be near the quadrant, 
and near the transit instrument ; so that the 
observer, whilst looking through the tele- 
scope of any of those instruments, may hear 
the beats of the clock and count the seconds. 
We need hardly observe with respect to te- 
lescopes, that they are of very great use in 
an observatory. Indeed a telescope for the 
same can never be too good or too large ; 
and it should be furnished with micrometers, 
with different eye-pieces, &c.;but as a large 
instrument of that sort is not easily managed, 
nor is always required, so there should be two 
or three telescopes of different sizes and dif- 
ferent powers in every observatory. One 
at least ought to be fixed upon an axis 
which may move parallel to the axis of 
the earth; for ia this construction the celes- 
tial bodies may, with the telescope, be easily 
followed in their movements ; as the hand of 
the observer is, in that case, obliged to move 
the telescope in one direction only. 
A pretty good telescope placed truly ver- 
tical in an observatory, is likewise a very use- 
o c c 
ful Instrument; as the aberration of the stars, 
latitude of the place, Sec. may be observed 
and determined by the use of such an instru- 
ment, with great ease and accuracy. 
1 he night telescope is a short telescope, 
which magnifies very little; but it collects a 
considerable quantity of light, and has a very 
great field of view ; it therefore renders visible 
several dim objects, which cannot be disco- 
vered with telescopes of considerably greater 
magnifying powers; and hence it is very 
useful for finding out nebulae, or small co- 
mets, or to see the arrangement of a great 
number of stars in one view. 
r l he principal instruments that are at pre- 
sent used for marine astronomy, or for the 
purposes of navigation, are that incompara- 
bly useful instrument called Hadley's sex- 
tant, or quadrant, or octant ; a portable chro- 
nometer; and a pretty good telescope. With 
these few instruments, the latitudes, longi- 
tudes, hours of the day or night, and several 
other problems useful to navigators, may b e 
accurately solved. SeeOpxics, and Quad- 
rant. 
OBSIDIAN, in mineralogy, called also 
the Iceland agate, is found either in detached 
masses, or forming a part of rocks. It has 
the appearance of black glass. It is usually 
invested with a grey or opaque crust. Its 
fracture is conehoidal. Specific gravity 
2.3:“, nearly. Colour black, or greyish-black ; 
when in very thin pieces green. Very brit- 
tle. It melts intfe an opaque grey mass. It 
is composed of 
69 silica 
22 alumina 
9 iron 
100. 
OR 1 U BAT O R . See An atomy. 
OCCi PI I ALES. See Anatomy. 
OCCULT, in geometry, is used for a line 
that is scarcely perceptible, drawn with the 
point of the compasses, or a leaden pencil. 
These lines are used in several operations 
as the raising of plans, designs of buildings* 
pieces of p mspective. See. They are to be 
effaced when the work is finished. 
. CC CU LTATION, circle of perpetual , 
is a parallel in an oblique sphere, as far dis- 
tant from the depressed pole, as the elevated 
pole is from the horizon. All the stars be- 
tween this parallel and the depressed pole, j 
never lise, but lie constantly hid under the 
horizon of the place. 
OCCUPATION, or Occupancy. The 
law of occupancy is founded upon the law of 
nature, and is simply the taking possession 
of those things, which before belonged to 
nobody ; and this is the true ground and 
foundation of all property. In the civil law 
it denotes the possession of such things as at 
present properly belong td no private person 
but are capable of being made so ; as by seiz- 
ing or taking of spoils in war, bv catching 
things wild by nature, as birds and beasts of 
game, &c. or by finding things before undis- 
covered, or lost by their proper owners. 
OCCUP1EHS of willing, a term in the 
salt-works for the persons who are the sworn 
officers that allot, in particular places, what 
quantity of salt is to be made, that the mar- 
kets may not be overstocked, and see that a 1 
is carried fairly and equally between the losd' 
and the tenant. 
