FOR 



681 



POS 



terminate, and so far it partakes of the 

 nature of a theorem ; and seeking to dis- 

 cover the conditions by which this may 

 be effected, it partakes of the nature of 

 a problem. 



POROS'ITT. A property of matter ; its 

 molecules not being in absolute contact, 

 but separated by pores. 



POR'PHYRY, from xoQtfvgec,, purple. The 

 porphyrites of the Latins ; a form of com- 

 pound rock, composed essentially of crys- 

 tals of felspar, disposed in a base of horn- 

 stone, which base in an Egyptian variety 

 of porphyry is of a beautiful red colour, 

 passing to a purple, and which, with its 

 snow-white crystals of felspar, forms one 

 of the most beautiful rocks in nature. 

 There are, however, several varieties, as 

 petrosiliceous porphyry, felspar porphyry, 

 clinkstone porphyry, &c. Porphyry is an 

 igneous rock, often forming masses of 

 enormous thickness, and sometimes dikes 

 like basalt. 



POB/PHYRT-SHELL. A species of murex. 

 The animal afforded the Tyrian purple. 



POR'POISE, from porcus-piscis, hog-fish. 

 The Delphinus phocana, Lin., so named 

 from its shape. 



POR'RECT, Lat. porrigo, I extend. In 

 zoology, when a part extends forth hori- 

 zontally. 



PORT. 1. Lat. portus, a harbour, either 

 natural or artificial, but usually applied to 

 such harbours as are much resorted to, 

 as the port of London ; whereas harbour 

 includes all places of safety for ships. 

 Among seamen, the term is further applied 

 to designate the larboard or left side of a 

 ship, as a heel to port, meaning an inclina- 

 tion to the larboard side. Port the helm, 

 is an order to put the helm to larboard. 

 Also the opening in the ship's side for a 

 .gun. 2. A kind of wine made in Por- 

 tugal, and so named from Oporto. 



POR'TAL. 1. Fr. portail, from Lat. porta, 

 a gate. The arch over a door or gate ; the 

 frame- work of the gate ; the lesser gate 

 when there are two of different dimen- 

 sions at an entrance. 2. A little square 



corner of a room, separated from the rest 

 of the apartment by wainscoting. 



PORT'CULLIS, Fr. port, and coulisse, a 

 groove. A strong grating of timber, 

 fenced with iron, and made to slide up 

 and down, in a groove of solid stone- work, 

 within the arch of the portal of old cas- 

 tles, to be let down in case of surprise, to 

 prevent the entrance of an enemy. 



POR'TER. 1. Fr. portmr, from porter, to 

 carry. A person who conveys burdens 



for hire. 2. Fr. portier, from Lat. porta, 



a gate ; a gate-keeper. 3. A malt liquor, 



which differs from ale in being made 

 with high-dried malt. It is so called from 

 its being at first regarded as a drink well 

 adapted for porters, and other labourers 

 about the metropolis. 



PORTE, SCBLIME. The official title of 

 the government of the Ottoman empire. 



PORT-FIRE. A pyrotechnic composition 

 of 4 nitrate of potash, 1 sulphur, 2 mealed 

 powder, and 1 sulphuret of antimony, well 

 rubbed together. This composition is 

 sometimes moistened with linseed oil. 



PORT'GRAVE, ) From port, and Sax. ge- 



PORT'GREVE. \ refa, a count or earl. 

 Formerly the chief magistrate of a port 

 or maritime town. This officer is now 

 called mayor or bailiff. 



PORT'-HOLES. The embrasures in the side 

 of a ship of war, through which the guns 

 are pointed. 



POR'TICO (Ital.) , from Lat. porticns, from 

 porta, a gate. A long covered place, com- 

 posed either of vaults supported by ar- 

 cades, or of flat roofs supported by pillars, 

 the sides being open. 



PORT'LAX-D BEDS. The name given by 

 geologists to a marine formation, occur- 

 ring in the island of Portland, and inWilt- 

 shire. The beds consist of coarse shelly 

 limestone, fine-grained white limestone, 

 and compact limestone (all having an 

 oolitic structure), and chert. 



PORT'LAND STONE, I A marine 



PORT'LAND LIMESTONE. / oolitic mem- 

 ber of the Portland Beds. It is soft when 

 quarried, but hardens by exposure to the 

 atmosphere. It was formerly highly va- 

 lued as a building material in London 

 and elsewhere, but it is not durable. 



PORT'LAND VASE. A celebrated cine-f 

 rary urn, presented by the Portland fa- 

 mily to the British Museum. It was 

 found in the tomb of the Emperor Alex- 

 ander Severus and his mother. It was 

 recently broken to pieces by an Irish- 

 man, but has been repaired in a masterly 

 manner, by Mr. Doubleday. 



PORT-SILLS (of a ship) are the upper 

 and lower edges of the ports. 



PORTDLA'CA. Purslane. A genus of 

 herbaceous plants. Dodecandna Mono- 

 gynia. Name from porto, to carry, and 

 lac, milk, because it (at least the garden- 

 purslane, P. oleracea) is said to increase 

 the flow of milk. 



POSE. In heraldry, the attitude of a 

 lion, horse, or other beast, standing still, 

 with all his four feet on the ground. 



POSI'TION, from positus. In arithmetic, 

 a rule called also the rv.lt of false, and 

 trial and error, because in calculating on 

 several false numbers, taken at random, 

 as if they were the true one, and from the 

 difference found, the number sought is 

 determined. Position, both single and 

 double, affords only imperfect and tedious 

 processes for working some of the sim- 

 pler cases of algebraical equations. 2. 



In geometry, when the direction of a line is 

 known, it is said to be grit-en in position, 

 and to be given in magnitude, when iU 

 length is known. The investigation of 



