U N I 
V O L 
S02 
ULNA. See Anatomy. 
ULVA, a genus of plants ofthe class cryp- 
togam ia, and order of alga*. The fructifica- 
tion is inclosed in a diaphanous membrane. 
There are 26 species of British plants. They 
are all sessile, and without roots, and grow in 
ditches, and on stones along the sea-coast. 
None of them are applied to any particular 
use different from the rest of the algce, except 
the umbilicalis, which in England is pickled 
with salt, and preserved in jars, and after- 
wards stewed and eaten with oil and lemon 
.juice. This species, called in English the 
navel laver, is flat, orbicular, sessile, and co- 
riaceous. 
UMBELLvE, umbels, among botanists, 
the round tufts or heads of certain plants set 
thick together, and all of the same height. 
UMBELLIFEROUS PLANTS, are such 
as have their tops branched and spread out 
like an umbrella, on each little subdivision 
of which there -is growing a small flower; such 
are fennel; dill, &c. "See Botany. 
UMBER, or Umbjre, umbria, among 
painters, &c. a kind of dry dusky-coloured 
■earth, which, diluted with water, serves to 
make a dark-brown colour, usually called 
with us a hair-colour. It is called umber, 
from umbra, a shadow, as serving chiefly for 
the shading ofohjects ; or, perhaps, from Um- 
bria a country of Italy, whence it used to be 
brought. 
Umber, or .grayling, in ichthyology. See 
• Salmo. 
UNCAR1A, a genus of plants of the class 
and order pentandria monogynia. The corolla 
is salver-shaped ; germ, crowned with a gland; 
stigma two-grooved ; peric. two-celled, many- 
seeded. There are two species. 
UNCIA, in general, a Latin term denot- 
ing the twelfth part of any thing, particularly 
the twelfth part of a pound, called in English 
an ounce ; or the twelfth part of a foot, call- 
ed an inch. See Measure, and Weight. 
UNCIjE, in algebra, the numbers prefixed 
before the letters of the members of any 
power produced from a binomial, residual, or 
multinomial root. Thus, in the fourth power 
of viz. « 4 -|-4 T/> -f-6 a 2 b 2 -\-Aub ! 4-/; 4 , the 
uncjai are4, 6, and 4, being the same with what 
others call co efficients. See Algebra. 
UNDECAGON, is a polygon of eleven 
sides. If the side of a regular undecagon is 
1, its area will be 9,3656399 = - 4 -X tang, of 
73 — 7 t degrees ; and therefore if this number 
is multiplied by the square of the side of any 
- other regular undecagon, the product will be 
the area of that undecagon. 
UNGUIS. See Anatomy. 
UNGULA, in geometry, is a part cutoff 
a cylinder, cone, &c. by a plane passing ob- 
liquely through the base, and part of the 
curve surface, so called from its resemblance 
To the (ungu)a) hoof of a horse, &c. 
U'NICORX-FISII. See Monodon. 
UNIO.LA, a genus of the friandria digy- 
•nia class of plants, the corolla whereof con- 
sists of a bivalve glume; the valves are of a 
lanceolate-compressed figure, like those of 
the cup; the inner valve appears somewhat 
higher than the outer one ; the corolla per- 
forins the office of a pericarpium, inclosing 
the seed, which is single, and of an ovated 
*)blong figure. There are three species. 
UNISON, in music, that consonance, or 
VO L . 
coincidence uf sounds, proceeding from an 
equality in the number of vibrations made in 
a given time by two sonorous bodies ; or the 
union of two sounds so directly similar to 
each other in respect of gravity, or acute- 
ness, that the ear perceiving no difference, 
receives them as one and the same. 
The antients were much divided in opinion 
respecting the question whether the unison is 
a consonance. Aristotle speaks in the nega- 
tive ; Muris Mersennus, and others, declare 
in the affirmative. The decision of the ques- 
tion, however, depends on the definition we 
give to the word consonance. If by a conso- 
nance we only understand two or more sounds 
agreeable to the ear, the unison is a conso- 
nance ; but if we include in the consonance 
sounds of a different pitch, i. e. sounds less 
or more acute with respect to each other, the 
unison, by its own definition, is not a conso- 
nance. 
UNISONI, (Ital. plu.,) a word implying 
that the parts in a score over which it is 
written, are in unison with each other ; as 
violini unisoni, the violins in unison ; flauti 
Unisoni, the flutes in unison. 
UNITY. See Poetry. 
UNONA, a genus of the pdyandria polv- 
gamia class and order of plants. The calyx 
is three-leaved, six-petals ; berries two or 
three seeded. There are four species, trees 
ofthe East and West Indies. 
UNXIA, a genus of plants ofthe class and 
order syngenesia polygamia superflua. The 
calyx is five-leaved, leaflets ovate ; florets of 
disk and ray five, recept. naked. There is 
one species. 
VOID and voidable, in the law. Some 
things are absolutely void, and others only 
voidable. A thing is void which is done 
against law at the very time of doing it, and 
no person shall be bound by such an act ; 
but a thing is only voidable which is done 
by a person who ought not to have done it, 
but who, nevertheless, cannot avoid it him- 
self after it is done; though it may be by 
some act in law made void by his heir, &c. 
2 Lil. Abr. 807. 
VOLCANO, in natural history, a burning 
mountain, or one that occasionally vomits ■ 
forth fire, flame, ashes, cinders, &c. Volca- 
noes are peculiar to no climate, and have no 
necessary connection with any other moun- 
tains, but seem to have some with the sea, 
being generally in its neighbourhood ; they 
frequently throw out matters which belong 
to the sea, as the relics of fishes, sea-weed, 
and sometimes sea-water itself. Sir William 
Hamilton observes in the Phil. 'Frans, for 
1776, that “ the operations of Vesuvius are 
very capricious and uncertain, except that 
the smoke increases considerably and con- 
stantly when the sea is agitated, and the wind 
blows from that quarter. Volcanic moun- 
tains are of all heights ; some, as tffiit of 
Tanna, so low as 450 feet ; Vesuvius is 3600 
feet high; and Etna, 11000. They in general 
form lofty spires ; and the volcano itself is 
internally shaped like an inverted cone, 
placed on a broader basis. This cone is 
called the crater, or bowl, and through it the 
lava generally passes, though sometimes it 
bursts through the sides, and even proceeds 
occasionally from the bottotn of the moun- 
tain. Sometimes the crater falls in, and is 
effaced ; sometimes, in extinguished volca- 
noes, it is filled with water. Submarine vol- 
canoes have been observed, and from these 
some islands have derived their origin. Vol- 
canic fires taking place at the bottom ofthe 
ocyan, would frequently, by the expansive 
force of the steams which are generated, 
elevate those parts which were once at the 
bottom of the deep, ami overflow those which 
were habitable earth. It is conjectured, that 
subterraneous convulsions operated more 
powerfully in the early ages of the world than 
at any later period ; and indeed such an hy- 
pothesis is supported by the most probable 
reasoning, since we may well conceive that 
at the first consolidation of the earth, much 
heterogeneous matter would be included in 
the different masses, which might produce 
more frequent fermentations than at any alter 
periods, when these have been, if we may so 
express it, purged off -by frequent eruptions, 
and in many parts, perhaps, rectified and 
assimilated by slow and secret processes in 
the bowels of the earth. But history was 
not cultivated till a very late period, and the 
most eventful ages of nature have passed un- 
recorded. 
The force of subterraneous fires, or rather 
of the steam which is generated by them, is 
so great, that considerable rocks have been 
projected by Vesuvius to the distance of 
eight miles. A stone was once thrown from 
the crater of that volcano twelve miles, and 
fell upon the marquis of Lauro’s house at 
Nola, which it set on fire. One also, which 
measured twelve feet in height and forty-five 
in circumference, was carried, in 1767, by 
the projectile force of the steam, a quarter of 
a mile from the crater. In an eruption of 
Etna, a stone, fifteen feet long, was ejected 
from the crater to the distance of a mile, and 
buried itself eight feet deep in the ground. 
A volcano broke forth in Peru in 1600, ac- 
companied with an earthquake, and the sand 
and ashes which were ejected covered the fields 
ninety miles one way, and one hundred and 
twenty another. Dreadful thunders and 
lightning were heard and seen for upwards of 
ninety miles round Araquapa during this 
eruption, which seemed to denote some con- 
nection between the electric matter and these 
volcanic fires ; and this fact is strongly con- 
firmed by the very accurate observations of 
sir William Hamilton, which we shall after- 
wards have occasion to notice more at large. 
Both the inside of the crater and the base of 
many volcanoes, consist of lava, either entire 
or decomposed, nearly as lovv as the level of 
the sea ; but they finally rest either oil gra- 
nite, as in Peru, or sehistuS, as the extin- 
guished volcanoes of Hesse and Bohemia, or 
on limestone, as those of Silesia, mount Ve- 
suvius, &c. No ore is found in these moun- 
tains, except that of iron, of which lava con- 
tains from twenty to twenty-five parts in the 
hundred, and some detached fragments of 
the ores of copper, antimony, and arsenic. 
Vesuvius ejected, from the year 1 779 to 
1783, 309,658,161 cubic feet of matter of 
different kinds; we must therefore conclude 
the seat of these fires to be several miles 
below the level of the sea ; and as iron makes 
from one-fourth to one-fifth of these ejections, 
we may infer that the internal parts of the 
earth abound much in this metal. 
The origin of these subterraneous fires is 
not easily explained, Iron-filings mixed witk 
