SEG 
fine seed as the sea-pink ; or two as in um- 
belleferous plants; or three as in the spurge : 
or many as in the ranunculus, &c. The 
shape, structure, and sides of seeds are va- 
rious. Linnaeus denominates seeds the 
eggs of plants ; and the fecundity of plants 
is often astonishing : there are 4,000 seeds 
in a single sun-flower; more than 30,000 
in a poppy ; and in a single tobacco plant 
360,000 have been enumerated. The an- 
nual produce of a single stalk of spleen- 
worth has been estimated to be a million 
of seeds. Plants are disseminated in va- 
rious methods : some are carried along by 
rivers and torrents many hundred miles 
from their native soil, and cast upon a very 
different climate, to which, however, by de- 
grees they render themselves familar. Some 
are formed by wings to be borne before 
the wind to distant places. Birds, squir- 
rels, &c. swallow seeds, and void them 
whole and fit for vegetation, and thus dis- 
seminate them. There are others that dis- 
perse themselves by an elastic force, that 
resides either in the “ calyx”; as in oats and 
the ferns: in their “pappus”, as in the 
“ centaurea crupina”, or in their capsule, as 
in the geranium. 
Seed. See Semen. 
SEGMENT of a circle, in geometiy-, that 
part of the circle contained between a 
chord and an arch of the same circle. 
Segment of a sphere, is a part of a sphere 
terminated by a portion of its surface, and 
a plane which cuts it off, passing sopiewhere 
out of the centre; being more properly 
called the section of a sphere. The base 
of such a segment, it is evident, is always a 
circle. And the convex surfaces of different 
segments, are to each other as their alti- 
’ tudes, or vqrsed sines. And as the whole 
convex surface of the sphere is equal to 
four of its great circles, or four circles of 
the same diameter; so the surface of any 
segment is equal to four circles on a dia- 
meter equal to the chord of half the arc of 
the segment. So that if d denotes the dia- 
meter of the sphere, or the chord of half 
the circumference, and c the chord of half 
the arc of any other segment, also a the 
altitude or versed sine of the same ; then, 
S.Ulfid^ is the surface of the whole 
sphere, and 
3.1416c*, or 3 . 141 6d, the surface of the 
segment. 
For the solid content of a segment, there 
are two rules usually given; viz. 1. To 
three times the square of the radius of its 
SEL 
base, add the square of its height ; multiply 
the sum by the height, and the product by 
.5236. Or, 2dly, From three times the 
diameter of the sphere, subtract twice the 
height of thefrustrum; multiply the remain- 
der by the square of the height, and the pro- 
duct by ..6236. 
SEGUIERIA, in botany, so named in 
honour of Jean Franqois Seguier, a genus 
of the Polyandria Monogynia class and 
order. Essential character: calyx, five- 
leaved; corolla none; capsule, one-seeded, 
terminated by a large wing, having small 
lateral ones. There are two species, viz, 
S. Americana and S. Asiatica. 
SEISIN, is two-fold, seisin in law, and 
seisin in fact. Seisin in fact, is when an 
actual possession is taken; seisin in law, 
when something is done, which the law ac- 
counts a seisin. See Estate, Fee, &c. 
SELAGO, in botany, a genus of the Didy- 
namia Gymnospermia class and order. Na- 
tural order of Aggregatae. Vitices, Jussieu. 
Essential character : catyx, five-cleft ; co- 
rolla, tube capillary, border almost equal ; 
seed one or two. There are twenty species. 
These plants are herbaceous or shrubby; 
all natives of the Cape of Good Hope. 
Leaves alternate; flowers in most pf the 
.species, allied to those of eranthemum and 
verbena, irregular, tubular, in alternate 
terminating spikes, which are simple or 
manifold; in a few, the flowers are regular, 
with a short two seeded tube, in a sort of 
terminating corymb. 
SELENITE, in mineralogy, a species 
of the Calc genus, is of a snow white colour, 
passing into different shades of grey. It 
is most generally massive, and not unfre- 
quently crystallized; the crystals are sel- 
dom large; internally, its lustre shining and 
splendent. The fracture is perfectly foliat- 
ed. It is transparent, soft, and not par- 
ticularly frangible ; specific gravity 2.3 ; it 
is composed of lime, sulphuric acid, and 
water, and is found in Germany, France, 
and England. On account of its great 
purity it is employed in taking the most 
delicate impressions : it is also used for 
crayons, and when burnt and phwdered, 
it 'is used for cleansing silver. It is tlie same 
with Gypsum, under which word will be 
found some other particulars relating to it. 
SELEUCIDAi:, in chronology. JErvi of 
the Seleucidae, or the Syro-Macedonian mra, 
is a computation of time, commencing from 
the establishment of the Seleucidae, a race 
of Greek kings, who reigned as successors 
of Alexander the Great, in Syria, as the 
