A B. Chissels, hatchets, and other sharp instruments, which are sloped down to an edge on one side only, are to be referred to the inclined plane, and those instruments that are sloped down on both sides, act on the principle of the wedge. The chief use of the wedge consists in its being urged by the stroke of a hammer, wedge, &c. and not by mere pressure. By repeated blows the wedge is used to split wood, rocks, &c.

The last mechanical power is the screw, fig. 11, which is always turned with a handle, &c. acting like a lever, and which gains advantage in proa: portion as the circumference of the circle made by the lever is greater than the distance between two threads of the screw. The screw is used for pressing light bodies close together, as in presses. for paper-makers, book-binders, packers, &c. A common cork screw in passing through the cork acts upon this principle. .

We have, under the article FRICTION, shewan that one fourth, or even one third of the supposed power gained must, in practice, be allowed when we calculate upon what is really gained by the mechanical powers. The capital advantages of these powers are, that by their means we can raise and move from place to place large weights, as blocks of stone, which could not be moved without them: we can give almost any direction to the moving power, and apply its action at a distance from the body to be moved. See MoTION, POWERS moving.

MEAN, a middle state between two extremes : thus we have an arithmetical mean; a geometrical mean; mean distance ; mean motion, &c. VOL, III.

= 7.

An arithmetical mean, is half the sum of the extremes, thus, if 2 and 12 be the extremes, the mean is equal to

2 + 12

2 Geometrical mean, called a mean proportional, is the square root of the product of the two extremes: thus a mean proportional between 6 and 24 is equal v 6 X 24 = ~ 144 = 12.

MEDAL, a piece of metal in the form of coin, intended to convey to posterity the portrait of some great person, or the memory of some illustrious action. See MINT.

MEDALLION, a medal of an extraordinary size, supposed to be antiently struck by the emperors for their friends, and for foreign princes and embassadors.

MEDITERRANEAN, the Mediterranean, called by the Greeks, Mare Internum, and by the Jews, Mare Magnum, is a vast gulf of the Atlantic Ocean, with which it has a communication by a narrow strait at the southern extremity of Spain, and is bounded on the South by Africa, and on the North by Spain, France, Italy, European Turkey, and Asia Minor.

Its length from the strait of Gibraltar to the coast of Syria, is computed to be 2,300 miles, and its breadth in some places 900, in others not 800 miles.

As there is a constant influx of water from the Atlantic and Euxine seas, and from many considerable rivers, into the Mediterranean, yet no visible: discharge, some philosophers have supposed that it has a subterraneous communication with the ocean , but no sign of any such communication has been discovered. Without having recourse, with Buffon, to the sinking of immense caverns, into which the sea has from time to time retired, other causes of the diminution of the Mediterranean may be assigned. It is highly probable that there is a reflux, or lower ourrent, in the straits of Gibraltar: a considerable quantity of water is incessantly raised into the atmosphere by evaporation : and, though there are no perceptible tides in this sea, except in the gulf of Venice, and on the coast of Tripoli, yet part of it may be converted into earth by the action of the winds as well as by heat.

No direct communication between the Mediter. ranean and the Red Sea, by a canal, appears to be practicable, because the opposite coasts of both are low and marshy, and the intermediate barren plain of 19 leagues is destitute of water. A communication, however, between these seas, might be opened by the medium of the Nile. A canal was anciently formed in this direction, and repeatedly restored. From Cairo it extended to Kolzoun, in the vicinity of Suez; and some vestiges of it still remain. There are several considerable islands in the Mediterranean, viz. Sicily, Sardinia, Corsica, Cyprus, Majorca, &c.

The strait of Gibraltar is about 14 leagues in length, and 8 in breadth. In navigating this strait three capes occur on either coast. Cape Trafalgar is opposite to Cape Spartel ; Tariffa to Alcazar ; and Gibraltar to Ceuta. The two last mentioned capes, anciently called Calpe and Abila, were the pillars of Hercules; and the strait was distinguished by the appellation, Fretum Herculeum, and Fretum Gaditanum, or strait of Cadiz, from a small island on the coast of Spain.