POR 
Also the transparency of bodies is supposed 
to arise from their pores being directly 
opposite to one another. And the matter of 
insensible perspiration is conveyed through 
the pores of the cutis. 
Sir Isaac Newton shows, that bodies are 
much more rare and porous than is com- 
monly believed. Water, for example, is 
nineteen times lighter and rarer than gold ; 
and gold itself is so rare, as very readily, and 
without the least opposition, to transmit 
magnetic effluvia, and easily to admit even 
quicksilver into its pores, and to let water 
pass through it: for a concave sphere of 
gold hath, when filled with water, and sol- 
dered np, upon pressing it with a great 
force, suffered the water to ouze through 
it, and stand all over its outside, in multi- 
tudes of small drops like dew, without 
bursting or cracking the gold. Whence it 
may he concluded, that gold has more 
pores than solid parts, and consequently 
that water has above forty times more pores 
than solid parts. Hence it is that the mag- 
netic effluvia passes freely through all cold 
bodies that are not magnetic ; and that 
the rays of light pass, in right lines, to the 
greatest distances through pellucid bodies. 
PORISM, in geometry, has been defined 
a general theorem, or canon, deduced from 
a geometrical locus, and serving for the 
solution of other general and difficult pro- 
blems. But Dr. Simson defines it a pro- 
position, either in the form of a problem or 
theorem, in which it is proposed either to 
investigate or demonstrate. Euclid wrote 
three books of Porisms, which are lost; 
and nothing remains in the works of the 
ancient geometricians on this subject, be- 
sides what Pappus has preserved in his 
matliematical collections. Dc. Simson, 
among the moderns, left behind him a con- 
siderable treatise on the subject of Porisms, 
which was printed at the expense of the 
late Earl Stanhope, who was himself a very 
able mathematician, and the patron of seve- 
ral persons who had distinguished them- 
selves in that branch of science. 
POROSTEMA, in botany, a genus of 
the Polyadelphia Polyandria class and or- 
der. Natural order of Lauri, Jussieu. Es- 
sential character: calyx six-parted, unequal; 
corolla none; filaments nine, with four an- 
tliers on each ; capsule covered, four or six- 
celled, many-seeded. 'There is but one 
species, viz. P. guianensis, a native of the 
woods of Guiana. 
PORPHYRY, in mineralogy, a name 
appropriated to that rock where grains or 
crystals of felspar are imbedded in a certain 
POR 
basis, as in horn-stone, pitch-stone, or in 
durated clay. There are five species of 
rocks belonging to the porphyritic forma- 
tion ; viz. 1. Horn-jstone porphyry ; the 
horn-stone, w'hich serves as the base of this 
substance, is generally red or green, and in- 
closes crystals of quartz and felspar. 2. Fel- 
spar porphyry ; the base of this is red com- 
pact felspar, inclosing crystals of felspar 
and quartz. 3. Scenitic porphyry ; contain- 
ing crystals of hornblende, in addition to 
the other ingredients. 4. Pitch-stone por- 
phyry; the base of which is red, green, 
brown, or black. 5. Clay porphyry; the 
base of which is indurated clay passing into 
horn-stone, it is of a reddish colour, and 
contains crystals of quartz and felspar. 
Horn-stone porphyry is the oldest of the 
class ; and clay porphysy the most recent. 
The red 'porphyries are employed in orna- 
mental architecture for columns. 
PORT, a commodious place situated on 
the sea coast, or at the mouth of a river, 
screened from the wind and the enterprizes 
of an enemy, with depth of water sufficient 
for ships of burden, and where vessels lie 
by to load and unload. 
Ports are either natural or artificial ; the 
natural are those formed by Providence, and 
the artificial such as are formed with moles 
running into the sea. The city of Constan- 
tinople is called “The Port,” from its having 
one of the finest ports in Europe. All the 
ports and havens in England are within the 
jurisdiction of the county ; and the Court 
of Admiralty cannot bold jurisdiction of any 
thing done in them. 30 Henry VI. 
Port holes, in a ship, are the holes in 
the sides of the vessel, through which are 
put the muzzles of the great gtms. These 
are shut up in storms to prevent the water 
from driving through them. The English, 
Dutch, and French ships, have the valves, 
or casements, fastened at the top of the 
port holes, and the Spanish vessels aside 
of them. ^ 
PORTAL, in architecture, a little gate 
where there are two gates of a different 
bigness; also a little square corner of a 
room cut off from the rest by the wainscot, 
and forming a short passage into tlie room. 
The sarnie name is also sometimes given to 
a kind of arch of joiners’ work before a 
door. 
PORTCULLICE, in fortification, is an 
assemblage of several large pieces of wood, 
joined across one another like a harrow, 
and each pointed with iron at the bottom. 
They are sometimes hung over the gateway 
of old fortified towns, ready to let down in 
