as

the Morea; and, holding a middle place between the two extremes, is a peninsula with one large island lying to the south; in one case Hindostan with Ceylon, in the other Italy with Sicily. The members of the eastern coast of Asia display a very peculiar arrangement; peninsulas stretching southward, and prolonged by semi-circular ranges of islands, encircle bays, and enclose them separate seas. A similar arrangement occurs in North West, and Central America. On the contrary, the inland seas of Europe are for the most part accessible by only one narrow passage, or by a few straits, and thus have the true character of gulfs. It is easy to recognise the circular enclosures of the arctic Icy Sea; the extremely narrow strait which connects it with the Pacific; the wider passage by which it joins the Atlantic, and its possession of the greatest number of islands in proportion to its area.

There is a striking resemblance between the opposite shores of the Atlantic ocean, in the manifold divisions of the North American and European coasts, and in the simple forms and few islands of Africa and South America. Europe, indeed, is the most accessible of all the great divisions of the globe, and presents the greatest facilities of intercourse. The Pacific and Indian oceans are both nearly enclosed on the northern side; the coasts of both retreat further from each other as they run southwards, till at length they open into the southern Icy Sea, and unite with the Atlantic ocean-the one south of America, the other south of Africa. America separates the Pacific, Africa the Indian ocean, from the Atlantic; both, however, flow very near to it at the isthmuses of Panama and Suez. The Indian ocean abounds in archipelagos of some magnitude near the coasts, but is almost without islands in its central part; the Pacific has only in the north important islands, not far from the shores of the continents, its central part is occupied with the island-world of Australasia. The existence of a great archipelago in the southern polar sea is no longer a subject of doubt, but who can tell whether its outlines will ever be ascertained?

The striking parallelism of the western continent to the meridians is of nearly the same extent as that of the prevailing

correspondence of the eastern continent with the parallels of latitude; the maximum of the former amounting to 9,206, that of the latter 10,580 English miles. The greatest breadth of the western continent is 3,682 miles, not half the extent from south to north of the eastern continent, which is 7,820 miles. America, therefore, by its elongated and narrow figure, lies more open to the influences of the ocean than the more massive forms of the great eastern continent; but the latter has two tracts within which the maritime influence greatly modifies the continental, in consequence of the disintegration of the solid mass of land into peninsular and insular members. These two tracts

are Further India with the East Indian Archipelago, and South and Western Europe.

The pelagic element, therefore, amounts to 1-24th, the peninsular to 1-10th of the land, or, taking the whole area as represented by 100, the proportions will be-for the islands,4; for the peninsulas, 10; and for the continents proper, 86. The superficial area of the land, previously given as 52,963,961 square miles, is here somewhat diminished, because the polar island-worlds are not taken into account. If we add them, with an area of 1,610,104 square miles, to the islands of the globe, it will give 7 for the islands, and 83 for the continents.

In total area, the ratios of Australia, Europe, Africa, America, and Asia, are to each other as 1:1:31:4:5. It is otherwise with their continentality, which, following the order of the foregoing series, are as 6: 34:50 : 16 : 44.

Hence Africa possesses the continental character in the highest degree, Europe in the lowest. The diversity of form in the coasts will be still more evident, by a comparison of the ratios of the coast lines, and the areas of the respective continents. In this calculation the islands are disregarded, and the two Americas are considered separately.

In this comparison, Europe and Africa are most strongly contrasted, while Europe and North America exhibit the nearest resemblance.

From the foregoing statement, the Atlantic appears to possess the fewest islands, and the Indian ocean has three times as great an insular area. The area of the Pacific ocean alone exceeds that of all the land on the globe; the Indian ocean is ten times larger than the Australian continent; and the Atlantic is ten times larger than Europe. The superficial area of the sea, previously stated at 143,638,262 square miles, is here somewhat exceeded; the antarctic Polar Sea may, perhaps, be reduced to 6,355,675 square miles, and the arctic to 3,686,292 square miles, bv allowing a larger proportion of island area to these seas.

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The mean circumference of the globe is taken at 24857-55. This + 3.14159 7912-41, the mean diameter; and 7912:412 × 3.14159 196,683 111, the superficial area given. Bessel gives the mean diameter of the earth 7912-359, and Mr. Airy 7912-409.

LESSON VII.-TUESDAY.

THE PRESSURE OF FLUIDS.

Fluids, as well as solids, are subject to the action of gravity. The latter, in consequence of the great cohesion of their parts, obey as a whole the attraction of gravitation; whereas each individual atom of a fluid is separately affected by this force, its yielding thereto being in no degree impeded by the operation of cohesion. Thus, a solid body may be balanced as a whole in any position around its centre of gravity, but a fluid body can only be supported when contained in a vessel. Whatever be the form of a solid, we may suspend it by a string, or rest it on a level surface, without any danger of its falling to pieces; but to do this with a fluid is obviously impossible. From this it follows that the surface of every fluid is level, and that the fluid in two vessels communicating with each other stands at the same level in each. To this latter fact we trust in conveying water by means of conduits for the supply of cities; the reservoirs being formed in elevated situations, the water in other parts rises to the same height. Our artificial fountains, as well as the artesian wells and natural springs, are illustrations of this property. The greater part of the crust of the earth consists of various strata ranged one above another. Many of these layers are of a loose and porous nature, and are penetrated with clefts, whilst others are dense and free from flaws. Through the former the atmospheric moisture, rain, and melting snow find their way, till they arrive at some solid stratum or subterranean channel, where further progress is stayed. These impervious strata are generally arranged between those of an opposite character; so, lying below some more porous, they catch the water that permeates through the upper strata; when this has accumulated it finds vent, sometimes on the side of hills, forming springs, which not unfrequently trickle down from a considerable elevation. If, on the contrary, the harder and impervious rocks form the outer coating of the hill side, they will retain the water that has already found its way below the surface, and compel it to take an underground course through the

porous strata which form the mass of the hill. Such subterranean bodies of water are often of considerable extent. If the fluid makes its escape at some low point of these cisterns, wherever an opening or flaw presents itself, a natural fountain will be formed. If a channel be provided for it by means of boring, care being taken that the course thus made be continuous, so that the water may not escape laterally, but be led to a point below its original level, and then brought to the surface, the water will spout forth in a jet; this constitutes an artesian well; and the height to which the fluid rises will equal that of its source. Another conclusion deducible from the property under consideration is, that fluids always assume the figure of the vessel that contains them. On this depends the form of ponds, seas, and lakes; the founding of different metals, as cannon and musket balls; glass moulding, and other arts.

When pressure is applied to the surface of a fluid, it is transmitted equally in all directions. The pressure on any point in a fluid is proportional to its depth below the surface. Of these propositions, a variety of illustrations may be drawn from machines in ordinary use. If fluids did not transmit pressure equally in all directions, the determination of the position of the safety-valve in a steam boiler would involve insurmountable difficulties; whereas the object to be attained is effectually secured wherever it is placed above the water-mark. In furnaces it is immaterial, so far as the effect of the blast is concerned, where the aperture in the bellows is situated which conveys the air to the fire. The efficacy of Bramah's hydrostatic press, a machine by which a man can raise a weight of one thousand tons, depends on the same principle. Similarly, if the steam pipe of a boiler has an area of one square inch, and the steam issues from it with a force of twenty pounds, the same pressure will be exerted on every square inch of the piston, whatever may be its total area. A strong hogshead filled with water has been burst by inserting a long vertical tube of small diameter, filled with the same fluid; thus showing that the pressure on every part of the hogshead is proportional to the depth of water from the top of the tube, and that this pressure is transmitted in all directions. The pressure on