P O R P R 521 Breton, the following books or tracts on the Forisms of Euclid : Aug. Richter, Porismcn nach Simson bcarbeitct (Elbing, 1837) ; Ch. Housel, " Les Porismes d Euclido," in Lioumlles Journal de mathe- matiques pures et appliquecs (2d ser., vol. i., 1856) ; M. Cantor, " Ueber die Porismen des Euklid und deren Divinatoren," in Schlomilch s Zeitsch. f. Math. u. Phy., 1857, and Literaturzeitung, 1861, p. 3 sq. ; Th. Leidenfrost, Die Porismen des Euklid (Pro- gramin der Realschule zu Weimar, 1863) ; Fr. Buchbinder, Euclids Porismen und Data (Programm der kgl. Landesschule Pforta, 1866). (T. L. H.) POROS, or PORO (" The Ford "), an island off the east coast of the Morea, separated at its western extremity by only a narrow channel from the mainland at Troezen, and consisting of a mass of limestone rock and of a mass of trachyte connected by a slight sandy isthmus. The town, which is at the head of an eparchy with 5414 inhabitants (1879), has its "houses perched among the volcanic rocks," and looks down on the beautiful harbour between the island and the mainland on the south, which between 1830 and 1877 was the seat of a national arsenal. The ancient Calauria, with which Poros is identified, was given, according to the myth, by Apollo to Poseidon in exchange for Delos ; and it became in historic times famous for a temple of the sea-god, which formed the centre of an amphictyony of seven maritime states Hermione, Epidaurus, .ffigina, Athens, Prasia 1 , Nauplia, and Orchomenus. It was there that Demosthenes took sanctuary with "gracious Poseidon," and, when this threatened to fail him, sought the more inviolable asylum of death. The build ing was of Doric architecture and lay on a plateau near the middle of the limestone part of the island, which now contains a mon astery. In the neighbourhood of Poros - Calauria are two small islands, the more westerly of which contains the ruins of a small temple, and is probably the ancient Sphreria 1 or Hiera mentioned by Pausanias as the seat of a temple of Athena Apaturia. It was at Poros that the English, French, and Russian plenipotentiaries met in 1828 to discuss the basis of the Greek government. See Chandler, Travels ; Leake, Morea ; Le Has, Voyage archeologique ; Curtius, Pdoponnesos; Pouillon-Boblaye, Jtecherches ; Bursian, Geographic von Griechen- land ; and Rangabe, "Em Ausflug nacli Poros," in Deutsche Revue, 1S83. PORPHYRY, a name originally applied to a reddish or purple rock (7rop</n peo5, purple) found in Upper Egypt, principally at Jebel Dokhan, and much used by the ancients as a decorative stone. This porphyry, the por- fido rosso antico of Italian antiquaries, consists of a dark crimson or chocolate-coloured felsitic base, with dissemi nated crystals of white felspar, probably oligoclase. It was a favourite material with Roman sculptors under the lower empire, and notwithstanding its excessive hardness was worked into large sarcophagi and other objects, ornamented in some cases in elaborate relief. This porphyry was also ingeniously used for the lower part of the busts of Roman emperors, the head being executed in another material, while the porphyry was used for the drapery, the colour of the stone suggesting that of the imperial purple. The antique red porphyry is often confounded with the rosso antico, which, being merely a red marble, is a much softer stone. The term "porphyry" has been gradually extended to a variety of rocks which contain distinct crystals of any mineral sprinkled through a fine-grained ground. Among the best known of the ancient porphyries is the porftdo i-erJe antico, or lapis Lacedsemonius, a beautiful rock with pale -green crystals of labrador- felspar, found at Mount Taygetus in the Morea. The meaning of the word "porphyry" has become so vague, in consequence of its application to many rocks widely differing from each other in composi tion, that there is a tendency among modern petrologists to abandon its use as a substantive, and merely to retain the adjective " porphyritic " as a convenient designation for all rocks which exhibit a structure like that of the ancient porphyry. Any rock, whatever its mineralogical composition, may therefore become porphyritic by contain ing isolated crystals developed in a compact or micro-crys talline matrix. Among the finest rocks of this class in 1 Some writers identify Calauria with one half only of Poros, and consider that the other half was in antiquity a separate island, to be identified with Sphseria. Britain are the porphyritic granites of Cornwall and of Shap in Westmoreland ; the elvans, or quartz-porphyries (see vol. x. p. 233), which occur as dykes cutting through the slates and granites of Cornwall ; the peculiar rock termed " luxullianite " (see vol. xi. p. 49); and the green and red porphyritic felstones of Cumberland. A beautiful brown porphyritic felstone occurs at Buchan Ness, on the coast of Aberdeenshire ; while a rock closely resembling the antique green porphyry is found on Lambay Island, near Dublin. For a description of porphyrite, see vol. x. p. 234. PORPHYRY (c. 233-306). See NEOPLATONISM, vol. xvii. p. 336 sq. PORPOISE (sometimes spelled PORPUS and PORPESSE). The word is apparently derived from the French pore and poisson, or the Italian porco and pesce, and thus corresponds with some of the English vernacular appellations, "hog- fish," "sea-hog," "herring-hog," and the German Meer- schwein, whence the usual modern French name of the animal, marsouin. "Porpoise" is commonly used by sailors to designate all the smaller cetaceans, especially those numerous species which naturalists call " dolphins " ; but in scientific language it is restricted to a particular form constituting the genus Phoc&na of Cuvier, of which the Common Porpoise of the British seas, Phoc&na communis, Cuvier (Delphinus phoceena, Linnaeus), is the type. The essential characters by which the genus is separated from the other members of the order Cetacea are described in the article MAMMALIA (vol . xv. p. 398). The common porpoise, when full grown, attains a length FIG. 1. Phoccena communis. of 5 feet or a little more. The dimensions of an adult female specimen from the English Channel were as follows : length in straight line from nose to median notch between the flukes of the tail, 62| inches; from the nose to the anterior edge of the dorsal fin, 29 inches ; height of dorsal fin, 4^ inches; length of base of dorsal fin, 8 inches; length of pectoral fin, 9^ inches ; breadth of pectoral fin, 3| inches ; breadth of tail flukes, 13 inches. The head is rounded in front, and differs from that of the true dolphins in not having the snout produced into a distinct " beak " separated from the frontal eminence by a groove. The under jaw projects about half an inch beyond the upper one. The aperture of the mouth is tolerably wide, and is bounded by stiff immobile lips, and curves slightly upwards at the hinder end. The eye is small, and the external ear represented by a minute aperture in the skin, scarcely larger than would be made by the puncture of a pin, situ ated about 2 inches behind the eye. The dorsal fin is placed near the middle of the back, and is low and triangular. The pectoral fins are of moderate size, and slightly falcate.
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