« PreviousContinue »
how far Greenwich is north of the equator, we must measure the proper distance along the circumference of this circle from where it cuts the equator, and we shall have determined where Greenwich must be. If we also know the distance of the meridian of Cape Verde from that of Greenwich, we can measure that distance along the equator, and the meridian passing through that point will pass through Cape Verde; and so Cape Verde must be the point where this meridian cuts the parallel circle on which Cape Verde was to be. In this way we can determine other places, and so map out our globe.
The equator is about 24,000 miles round; so that, if our globe be one foot in diameter, the circumference of a great circle will be about 3 feet, and a quarter of this circumference (9 inches) will represent 6000 miles. If our globe be two feet in diameter, the circumference of a great circle will be about 6 feet, and a quarter of that circumference (18 inches) will represent 6000 miles.
We have only then to agree upon the manner of subdividing the equator, and whatever size the globe may be, we can measure upon it the distances required. We divide the equator into 360 parts, called degrees. Each degree is about 69 common English miles. And in the same way the circumference of every great circle is divided.
The latitude of a place means how far it is north or south of the equator.
If we draw a meridian through the place, and measure along the circumference from where it cuts the equator, we shall know its latitude.
London is 51 degrees north latitude-i. e. if we
draw a meridian through London, and measure upwards from the equator, we should have 51 such parts, as there are 360 in the whole circumference.
Botany Bay is 34 degrees south latitude-i. e. if we draw a meridian through Botany Bay, and measure downwards from the equator, we shall have 34 such parts of the circumference.
The poles are the points most distant from the equator. So no place can have a greater latitude, north or south, than 90 degrees.
Longitude means the distance measured upon the equator from some fixed meridian. In England we take Greenwich. To the right of this circle it is called east longitude, and to the left west longitude.
Thus Achil, in Ireland, is 10 degrees west longitude— i. e. if I measure to the left along the equator from the Greenwich to the Achil meridian, there are 10 such parts as there are 360 in the whole circumference. Lyons, in France, is 5 degrees east longitude-i. e. the Lyons meridian cuts the equator at a distance of 5 degrees east of the Greenwich meridian.
As we measure longitude along the equator to right and left, the east longitude and west longitude meet just half-way round, and no place can have a greater longitude east or west then 180 degrees.
The meridians drawn upon our globes are called circles of longitude, and the parallel circles are called circles of latitude.
We might draw as many meridians as we please. It is convenient to draw twelve such circles, all of which pass through the poles, and divide the equator into 24 equal parts of 15 degrees each.
Since the earth turns round its axis from west to
east once in twenty-four hours, it turns through 15 degrees in one hour. So, when it is noon at Greenwich, it is eleven in the morning 15 degrees west of Greenwich, and one in the afternoon 15 degrees east of Greenwich.
So the circles of longitude, thus drawn on the globe, tell us the different times of day at different parts of the earth. They are therefore called hour circles.
LAND AND SEA.
ONE of the first things that will probably strike us when we look upon a terrestrial globe, is the manner in which land and water are distributed over the earth.
The ocean covers nearly three out of four parts of the surface. Land appears in the form either of Continents or Islands.
There are two vast continents: the one comprising Europe, Asia, and Africa,-for all these are closely united together; the other, North and South America. The former is often called the Old, the latter the New World.
These continents are both surrounded by water, and so might be called immense islands.
But it is convenient to distinguish them as conti
The rest of the land on the earth is distributed in islands of different shapes and sizes. The largest is Australia. Its area is not very much smaller than that of the whole of Europe.
The Pacific Ocean is studded with numerous clusters of islands, most of them very small; but some
are larger. The area of New Zealand, for instance, is greater than that of Great Britain, and that of Borneo more than three times as great.
The land is distributed in different ways in the continents of the Old and the New World. In the Old World its greatest extent is from east to west; in the New, from north to south.
In both cases this corresponds with the principal ranges of mountains. In the Old World, mountains stretch, in one vast chain, with some interruptions, from west to east. These are the Pyrenees, the Alps, and the Carpathian Mountains, in Europe; the Caucasus, separating Europe from Asia; the Hindoo Kush and the Himalaya, in Asia; and further north, but in the same direction, the Altai and the Jablonoi Mountains.
The mountains in Africa are the Atlas range, which is a kind of continuation of the range just mentioned; and it is supposed that there is a great range of mountains across the centre of Africa, from Cape Verde to Abyssinia.
In the New World, there are the vast chain of the Rocky Mountains in North America, and that of the Andes in South America.
There are also other ranges of mountains, which determine the shape of various countries. For, as a general rule, the outline of the land follows the direction of the mountains. The Ghauts of India and the Apennines of Italy are instances of such mountains.
This is just what would follow from an uneven surface, once covered with water, being by some mighty agency raised above the water, or from the water being drawn off from such a surface. "And God
said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so. And God called the dry land Earth; and the gathering together of the waters called he Seas and God saw that it was good."
There are also mountains standing apart from others, and not belonging to a range, as Mount Ætna in Sicily. These have generally been produced by volcanoes.
The grand central ridge of a range of mountains is called the stem or axis; the lesser ridges that branch from it are the lateral or side branches; and the smaller ridges which branch from these, the spurs.
The names are taken from a tree, because ridges branch out from a range of mountains, as branches from the trunk of a tree.
Valleys lie between mountains or hills. Principal valleys separate extensive parallel ranges of mountains, such as the Valais, or Valley of the Rhone. Lateral valleys are valleys formed by the lateral branches; and subordinate valleys, those formed by spurs or minor branches.
Plains are level tracts, of which there are many of great extent. The Netherlands, Denmark, the northern districts of France and Germany, a considerable portion of Poland, and nearly the whole of European Russia, form one vast plain, which reaches up to the Ural Mountains. In all this tract of country, there are no hills of any height except a low range of mountains, the Valdai, between Smolensk and Moscow. On the other side of the Ural mountains, there is a still vaster plain, the Siberian lowland, which occupies nearly the whole of northern Asia.