CHAPTER 2
THE PLANET EARTH
5.The Shape of the Earth. -Men who have in different ways made careful measurements of the shape of the earth tell us that it is an oblate spheroid (Fig. 6), that is, a spherewhich is somewhat flattened at two opposite
points. An ordinary orange has this shape. The earth has been so little flattened, however, that its shape is very much nearer that of a perfect sphere than is that of an orange. Its polar di- ameter is only 27 miles shorter than its equato-rial diameter, so when we consider that each ofFig. 6.
its diameters is nearly 8000 miles, a shortening of only 27 miles in one of these would not change its shape from that of a sphere enough to be noticed except by the most careful measurements.
Experiment 6. -Attach a centrifugal hoop to a rotator apparatus and revolve. The hoop bulges at the center or point of greatest motion and flattens at the top and bottom or points of least motion. The earth revolves in a way similar to the hoop and is very slightly flattened at the poles.
Although some of the mountains of the earth rise above sea level to a height of over five miles, and there are depths in the sea which are somewhat deeper than this below sea level, yet these distances are so little in comparison to the size of the earth that the surface is comparatively less irregular than that of an orange.
In these days many men have sailed around the earth, but valiantindeed was that little company which in 1522 first proved that it wasMOUNT EVEREST.
As it would appear if placed in the deepest part of the sea.
possible to sail continually in one direction and yet reach the home port, thus demonstrating that the earth was probably round. Long before, wise men had come to believe that the earth was a sphere, for it had been noted as far back as the time of Aristotle, the famous Greek philosopher,that when the shadow of the earth fell upon the moon, causing an eclipse of the moon, the boundaries of the shadow were curved lines. It was also later noticed that when ships are seen approaching at sea the masts appear first and then gradually the lower parts of the ship. The reverse was seen to be true when ships sailed away.
Experiment 7. -Add alcohol to water until a solution is obtained in which common lubricating oil will float at any depth. Insert with a glass tube a large drop of oil below the surface of the solution. The oil will float in the solution in the shape of a sphere. This illustrates the fact that if a liquid is relieved from the action of outside forces, it will take the form of a perfect sphere.
A spherical surface is the smallest surface by which a solid can be bounded, so the maximum distance which can separate places located on a given solid will be least when its surface is spherical. Thus the inhabitants of the earth, considering the surface over which they may scatter themselves, are brought into the closest possible relation to one another. One of the most noteworthy consequences of the earth"s shape is the ease with which knowledge, news, and the products of both agriculture and manufacture are carried between its most distant parts.
6.The Size of the Earth. -It is easy to say that the polar diameterof the earth is 7900 miles, its equatorial diameter 7927 miles, and its equatorial circumference 24,902 miles, but a true conception of these distances is not so easy. There are, however, distances on the surfaceof the earth over which we have passed and about which we have real knowledge; and if we can translate other distances into terms of these, then the unfamiliar distances will become appreciable.
One of the best ways to do this is to draw a line which shall represent our known distance and then with this as a measure draw other lines which shall represent the distances of which we wish to getFig. 7.
an appreciation. Using as our standard any distance with which we are really acquainted, we shall find that the lines representing the different dimensions of the earth are very long. How vastly greater, then, must be the distances which were mentioned when treating of the stars.
7.Effect on Life of the Irregularities of the Earth"s Exterior. -Although the irregularities of the surface of the earth, when consid- ered in relation to its size, are insignificant, yet in relation to the size of the men and animals thatdwell upon the earth they are very great. Some of the mountains rise to heights that are inaccessible, and the oceans in some places sink to depths which until recently were immeasur- able. The lofty mountains and broad oceans are bar- riers which plants and animals have tried in vain to cross and which manhimself has had difficulty
in surmounting.
THE HIMALAYA MOUNTAINS.
An insurmountable barrier in central Asia.
These regions have furnished protected spaces to different species of plants, groups of animals, communities of men, and unmolested in these protected places they have developed theirA COTTAGE IN THE SCOTCH HIGHLANDS.
peculiar characteristics. Where man has not succeeded in thoroughly overcoming the barrier, secluded and unprogressive peoples are still to be found. Such are the Highlanders of Scotland and of our own Appalachian plateau. The gentler inequalities of the earth"s surface have by their lakes and rivers afforded easy means of transportation from one community to another and have opened paths into hitherto unexplored regions. These waterways have been from the earliest times arteries of commerce and progress, and largely through them have the interior lands been colonized and developed.
Where great plains and plateaus stretch their broad and level surfaces over vast areas, plants, animals, and men for ages, unimpeded by natural barriers, have been thrown together and have striven unrelentingly for mastery. They have here developed into great aggregations, not into small distinct communities. Here there has been no shelter for the weaker group except as its identity was lost in merging with the mass.