OBL 
CBS 
O B S 
881 
O. 
the fourteenth letter of our alphabet. 
"> As a numeral, it is sometimes used 
for eleven ; and with a dash over it thus O 
for eleven thousand. In the notes of the 
antients, O. CON. is read opus conductum ; 
O. C. Q. opere consilioque; O. D. M. 
opera, donum, muuus ; and O. L. O. opus 
locatum. 
In music, the O, or rather a circle, or 
double CD, is a note of time called by us a 
semi-breve ; and by the Italians circolo. The 
0 is also used as a mark of triple time, as 
being the most perfect of all figures. 
OAK. See Quercus. 
OAKAM, old ropes untwisted, and pulled 
out into loose hemp, in order to be used in 
caulking the seams, tree-nails, and bends of 
a ship, for stopping or preventing leaks. 
OAR, in navigation, a long piece of wood, 
for moving a vessel by rowing. Oars for 
ships are generally cut out of fir-timber, 
those for barges are made out of New Eng- 
land or Dantzic-rafters, and those for boats, 
either out of English ash, or fir rafters from 
Norway. 
OA'f. See Avena. 
OATH, an affirmation or denial of any 
thing before one or more persons, who have 
the authority to administer the same, for the 
discovery and advancement of truth and 
right. See Affidavit. 
OBELISK, a truncated, quadrangular, 
and slender pyramid, raised as an ornament, 
and frequently charged either with inscrip- 
tions or hieroglyphics. 
Obelisks appear to be of very great anti- 
quity, and to be first raised to transmit to 
posterity precepts of philosophy, which were 
cut in hieroglyphical characters : afterwards 
they were used to immortalize the great 
actions of heroes, and the memory of persons 
beloved. The first obelisk mentioned in his- 
tory was that of Raineses king of Egypt, in 
the time of the Trojan war, which was forty 
cubits high. Phius, another king of Egypt, 
raised one of forty-live cubits ; and Ptolemy 
Philadelphus, another of eighty-eight cubits, 
in memory of Arsinoe. Augustus erected 
one at Rome in the Campus Martins, which 
served to mark the hours on an horizontal 
dial, drawn on the pavement. They were 
called by the Egyptian priests the fingers of 
the sun, because they were made in Egypt 
atso, to serve as styles or gnomons to mark 
the hours on the ground. The Arabs still 
call them Pharoah's needles, whence the 
Italians call them aguglia, and the French 
aiguilles. 
'Phe proportions in the height and thick- 
ness are nearly the same in all obelisks ; 
their height be ng nine , or nine and a half, 
and sometimes ten times, their thickness ; and 
their diameter at the top never less than 
half, and never greater than three-fourths, of 
that at the bottom. 
OBLATE, flatted, or shortened ; as an 
oblate spheroid, having its axis shorter than 
VOL. II. 
its middle diameter, being formed by the 
rotation of an ellipse about the shorter axis. 
OBLATENESS. See Earth, figure of. 
OBLIGATION, a bond containing a pe- 
nalty, with a condition annexed, either for 
payment of money, performance of cove- 
nants, or the like. This security is called a 
specialty. Co. Lit. 172. See Bond, and 
Deed.' 
OBLIQUE, in geometry, something aslant, 
or that deviates from the perpendicular. 
Thus, an oblique angle, is either an acute or 
obtuse one; that is, any angle except a right 
one. 
Oblique Planes. See Dialling. 
OBLONGATA Medulla. See Ana- 
tomy. 
OBOLUS, in antiquity, an antient Athe- 
nian coin, worth a penny farthing. Among 
antient physicians ; obolus likewise denoted 
a weight, equal to ten grains. 
OBOLAKIA, a genus of the angiospermia 
order, in the didynamia class of plants ; and 
in the natural method ranking under the 
40th order, personate. The calyx is bifid ; 
the corolla campanulated and quadrifid ; 
the capsule unilocular, bivalved, and poly- 
spermous ; the stamina rising from the divi- 
sions of the corolla. There is one species, a 
herb of Virginia. 
OBSERVATORY, a place destined for 
observing the heavenly bodies; being gene- 
rally a building erected on some eminence, 
covered with a terrace for making astrono- 
mical observations. 
Hie principal instruments for a fixed ob- 
servatory are, a large fixed quadrant, or a 
circular divided instrument, chiefly for mea- 
suring vertical angles ; a transit instrument ; 
an equatorial instrument ; a chronometer, or 
regulator ; one or more powerful telescopes ; 
a fixed zenith telescope, and a night tele- 
scope. • 
The quadrant, or quarter of a circle, di- 
vided into 90°, “and each degree subdivided 
into minutes or smaller parts, has been made 
of various sizes ; some of them having a ra- 
dius even of eight or nine or more feet in 
length. When those quadrants do not ex- 
ceed one or two, or at most three feet, in 
radius, they are generally fixed upon their 
particular stands, which are furnished with 
various mechanical contrivances, that are 
nesessary to place the plane of the qua- 
drant perpendicular to tiie horizon, and for 
all the other necessary adjustments. But 
large quadrants are fixed upon a strong wall 
by means of proper clamps ; hence they have 
been commonly called mural quadrants, and 
are situated in the plane of the meridian 
of the observatory. In either of those qua- 
drants an index, which reaches from the 
centre to the edge of the arch, moves 
round that centre, or round a short axis which 
(lasses through that centre so as to be moveable 
with its extremity all round that arc, and 
thus point out on the divisions of the arch, 
the angle wlfich it forms with the horizon, or 
with the vertical line, in any given situation. 
This index carries a telescope, through 
which the observer looks at any particular 
object, whose altitude he wishes to de- 
termine. 
Plate Observatory, &c. fig. 1. represents a 
simple construction of a small moveable qua- 
drant, and fig. 2. represents a mural qua- 
drant. Of the quadrant fig. 1. CEB is the 
arch divided into 90°, and generally subdi- 
vided into smaller divisions, such as half 
degrees, on third parts of each degree, &:c. 
The centre of the arch is at A, and the whole 
is connected together by means of strong 
metallic bars, as is shewn between the letters 
ABC in the figure : in the centre A, a short 
axis is fixed perpendicular to the plane of 
the instrument, and to the upper part of this 
axis is fastened the index AD, which carries 
the telescope. This index generally has a 
small lateral projection, as at E, upon which 
the nonius or vernier is marked, by which 
means the minutes or smaller parts of each 
degree may be discerned. (See Vernier.) 
The screw P, commonly called the tdngent 
screw, with a nut that may be fastened to 
any part of the arch BC, screws likewise 
into the extremity of the index, and is use- 
ful for moving the index gently, or more ac- 
curately than by the immediate application 
of the hand to the index itself. 
Since the index is suspended at one end, 
viz. at A, if the other end D happens to be 
disengaged from the screw P, the lower end 
D of the index will naturally come down to 
C, on account of its own weight, and that of 
the telescope. Now, in order to avoid this 
tendency downwards, an arm Y of brass 
or iron, is frequently affixed to the upper 
part of the index, which carries the leaden 
weight Z, sufficient to balance the weight 
of the index and telescope ; so that by this 
means, even when disengaged from the 
screw P, the index will rernain in any situ- 
ation in which it may be left. 'Phe whole 
frame ABC is supported upon a strong ver- 
tical axis FS, the lower part of which turns 
into the pedestal OK m, and carries an index 
SX, which moves upon the divided horizon- 
tal circle O, fixed to the pedestal. This 
serves to fix the plane of the quadrant in 
any azimuth that may be required. 'Phe 
lower part of the pedestal lias three claws, 
with a screw m in each ; by which means 
the axis FS may be set truly perpendicular. 
The plummet AO, suspended at A, serves 
to shew when the edge AC of the instru- 
ment is truly perpendicular, or when the first 
division of the arch at C is exactly in the 
vertical which passes through the centre A 
of the quadrantul arc BC. The weight 
of the plummet generally moves in a glass 
of water, which is fixed upon the arm Git; 
, the object of which is to check the vibrations 
6f the pendulum ; which otherwise would be 
.easily moved by every breath of air, aad 
