The approach to the curie, the basiliem, and the forums, were generally by porticos ; several ranges of porticos led to the capitol, and lined the sides of the declivity ; the Cam pus .NIartius was surrounded by an uninterrupted colonnade ; almost every einpenr distinguished himself by the erection of a new edifice of the kind ; and Nero is said, by Suetonius, to have lined the streets of Rome (those probably which he himself bad rebuilt) with a continued portico. Several por ticos were erected by later emperors, or astonishing extent ; such were that of Gallienus, extending nearly two miles along the Via Flaminia ; and that of Gordian in the Campus Mar tins, which was a mile in length, and formed of one range of pilasters and four of columns, opening upon plantations of box, cedar, and myrtle.
POlZTLAND srux E, (Susan?, ...1rrnarium Portlandicum, of Da Costa, and Psadurium lIebes,..11bidum,La.rins, of II ill), an alkaline sand-stone, of a dull whitish colour, heavy, moderately hard, of a somewhat flat texture, and composed of a large roundish grit, cemented together by an earthy spar, and intermixed with numerous glittering spangles of pure spar ; the grit splits in the cutting of the stone, so that it is capable of being brought to a surface very smooth and equal; it will not strike fire with steel, and burns to a slight ashen hue. The Portland stone belongs to the third variety of the compact limestone, under the calcareous genus, whose fracture is earthy, according to Kirwan's arrangement. Its
specific gravity is 2.461. There arc vast quarries of it in the island of Portland, in Dorsetshire, whence its name. It is brought from thence in large quantities to London, and is much used in building. This and all similar sorts of stone, composed of granules, and not of a laminated texture, will cut and rive in any direction, as well in a perpendicular, or in a diagonal, as horizontally and parallel to the site of the strata. For this reason they have obtained the name of free-stone. This stone is very soft when it comes out of the quarry, works very easily, but becomes in time very hard and durable.
Posrn oN, in architecture, the situation of a building with regard to the points of the horizon. Yitruvius directs the position of a building to be such, that the four corners may point directly to the four winds.
Posmos, in geometry, a term sometimes used in contra distinction to magnitude. Thus a line is said to be given in position (positione data) when its situation, bearing, or direc tion, with regard to some other line, is given : on the con trary, a line is given in magnitude, when its length is given, but not its situation.
Sir Isaac Newton shows how to find a point, from which three lines, perpendicularly let fall, to three other lines given in position, have any given ratio, &c.