CHAPTER V.

OUR HOME-THE EARTH.

Difficulty of Observation-Form of the Earth-Proofs of Sphericity-An Oblate Spheroid - Equatorial and Polar Diameter - Area, Bulk, and Density-Rotation upon an Axis-Proof-Cause of the Spheroidal Form -Sidereal Day - Solar Day - Grand Mechanical Expedient - Annual Revolution - Aberration of Light-Solar Year-Reform of the CalendarPrecession of the Equinoxes-Its Rate, Cause, and Effects-Eccentricity of the Earth's Orbit-Position of its Axis-Succession of Seasons-Superficial Physical Characteristics-Division into Land and Water-Habitable parts of the Earth-Mountains and Hills-The Ocean-Its Saline Constitution-Its Stability-The Atmosphere-Its Extent and Composition - Reflection of Light-Twilight - Refraction of Light-Regularity and Grandeur of Operation.

THE third member of the planetary system in order from the sun-our own world-has only in very modern times been admitted to its due rank as a celestial body, for the intimate connexion of mankind with their terrestrial habitation, so apparently auxiliary to knowledge, has been an actual obstacle in the way of acquiring correct views respecting it. If we suppose the inmates of a dwelling of comparatively enormous magnitude never to have stepped across its threshold, or to have held intercourse with any who have seen it from an exterior position, it is obvious that they will labour under great difficulties in rightly conceiving of it as a whole, though they may be well acquainted with the outline and aspect of objects within the range of vision from its windows. In a precisely similar manner, yet very feebly expressed by the illustration, confinement o the surface of the earth, and to a very limited horizon at any one time, has operated to hinder the

acquisition of accurate knowledge of its form, dimensions, and general character. For ages men gazed upon the lights of the firmament, and admired their brilliance. They recognised the spherical shape of the sun and moon; and were conscious of movements going on upon a great scale in the outer universe. But not the remotest suspicion was entertained that any of these features belonged to their mother earth. Yet nothing is more susceptible of demonstration, than the convexity of its surface, the incessant motion in space of its entire volume, the smallness of its apparent magnitude viewed from a neighbouring orb, and the luminosity of the opaque mass, reflecting the solar light. Could we visit the surface of Venus, the earth would be seen as a very resplendent object in her night, while as an interior planet to Mars, from his surface, with a telescope, we should witness terrestrial phases, and, at long intervals, see this apparently huge world as a small dark spot transiting the solar disk.

Various fanciful opinions were entertained in early times respecting the figure of the earth. The one most common is supported by the first impression made upon the eye, namely, that it is a continued plane stretching out to an indefinite extent, only interrupted by the inequalities of the land. But its spherical form has long been a received truth; and the evidence of this fact appeals so obviously to our senses, as to render a statement of the arguments which substantiate it almost superfluous. Still, for the information of youthful readers, the principal of them may be briefly adduced.

It is common for vessels on the open ocean to telegraph to each other, while only the upper parts of their masts are visible, the hulls being hid from view by the

convexity of the intervening water. Standing on the shore, on a clear day, watching a ship leave the coast in any direction, the body first disappears, then the rigging, and finally the top-masts, while those on board first lose sight of the strand, then of the ordinary buildings, and lastly of the lofty spires of the port they are quitting. Now circumstances would be precisely opposite if the surface were a dead level, for the largest objects, as the body of the vessel and the buildings of the port, would then be visible longer than the smaller, such as the masts and the spires. In a lunar eclipse also, which is occasioned by the moon passing through the earth's shadow, the form of the shadow projected on the lunar disk is invariably found to be such as one globe would cast upon the surface of another. But navigators steering continually in the same direction, east or west, have come round to the point from which they started, thus decisively proving the fact of the earth's rotundity, east and west. Its convexity north and south is demonstrated by the gradual declination and rise of the north and south circumpolar stars as the equator is approached or receded from. It may be objected, that such inequalities of the surface as the Alps, Andes, and Himalaya, are incompatible with a globular figure. But in reality the greatest mountain heights and oceanic depths are reduced to absolute insignificance, and become insensible, in the view of the earth's comparatively stupendous volume.

Though properly described as spherical, the earth is not a perfect sphere, with its diameters of equal length, and all parts of the circumference at the same distance from the centre. Its true form is that known in geometry as an oblate spheroid, or a globe flattened at the poles. This has been determined with great

nicety and wonderful labour by the measurement of arcs and degrees of the meridian in different latitudes. If the earth were an exact sphere, it is obvious that a meridian, or imaginary great curve passing through the poles and intersecting the equator, would be an exact circle; and that a degree of latitude measured on it would be everywhere of equal length. But instead of this being the case, it has been found, that the length of a degree gradually increases as we proceed from the equator towards the poles, indicating very plainly the greater convexity of the equatorial surface and the polar flattening. The distance through the earth is therefore greater at the equator than from pole to pole, the former diameter exceeding the latter by the both of its length. The following are the determinations of the most eminent and recent authorities, using round numbers :

The other members of the system have the same spheroidal shape, the cause of which will be speedily stated. But the departure from perfect sphericity is much more considerable in the instance of the larger planets. The equatorial diameter of Jupiter exceeds the polar by about six thousand miles; and the disproportion is still greater in the case of Saturn.

Thus our Home-Earth, which seems as a boundless plain to the naked eye, is really a huge slightly-flattened globe, smoother than the rind of an orange, for all the asperities it presents may be regarded as insensible in comparison to its own magnitude. The mean