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It may perhaps be objected to this system of the universe, that it is directly in opposition to the evidence of our senses, to which, it is plain and obvious, that the earth is motionless, and that the sun and stars revolve round it. But our senses sometimes deceive us. When sailing on the water with a very steady breeze, the houses, trees, and every object appear to move, whilst we are insensible of the motion of the vessel in which we sail. It is only when some obstacle impedes our motion, that we are conscious of moving; and were you to close your eyes while sailing on calm water, with a steady wind, you would not be sensible of your motion; for you could not feel it, and you could see it only by observing the change of place of objects on shore. So it is with the motion of the earth every thing on its surface, and the air that surrounds it, accompanies it in its revolution--it meets with no resistance, therefore we are insensible of motion. The apparent motion of the sun and stars affords us the same proof of the earth's motion, that the crew of a vessel have of their motion, from the apparent motion of the objects on shore. Imagine the earth to be sailing round its axis, and successively passing by every star, which, like objects on land, we suppose to be moving, instead of ourselves. Persons who have ascended in balloons, tell us that the earth appears to sink beneath the balloon, instead of the balloon rising above the earth. What an immense circuit the sun and stars would make daily, were their apparent motions real! Why should these enormous globes traverse such an immensity of space, merely to prevent the necessity of our earth revolving on its axis? The motion produced by the revolution of the earth on its axis is about thirteen miles and a half in a minute to an inhabitant of London. A person at the equator moves much quicker, and one situated near the poles much slower, since they each perform a revolution in twenty four hours. But in performing its revolution round the sun, every part of the earth moves with an equal velocity; and this velocity is no less than a thousand miles a minute.
In ancient times, the earth was supposed to occupy the centre of the universe; and the sun, moon, and stars to revolve round it. This was the system of Ptolemy; but since the beginning of the sixteenth century, that system has been discarded, and the solar system, such as we have described, was established by the celebrated astronomer Copernicus, and his followers, and is thence called the Copernican system. But the theory of gravitation, the discovery of the source whence this beautiful and harmonious arrangement flows, we owe to the genius of Newton, who lived at a much later period.
During the prevalence of the plague, in the year. 1665, Newton retired into the country to avoid the contagion. When sitting one day in his orchard, he observed an apple fall from a tree, which it is said led to that train of thought, whence his grand theory of universal gravitation was ultimately developed. His first reflection was, whether the apple would fall to the earth if removed to a great distance from it; then how far it would require to be removed from the earth, before it would cease to be attracted; would it retain its tendency to fall at the distance of a thousand miles, or ten thousand, or at the distance of the moon-and here the idea occurred to him, that it was not impossible that the moon herself might have a similar tendency, and gravitate to the earth in the same manner as the bodies on or near its surface, and that this gravity might possibly be the power which balanced the centrifugal force implied in her motion in her orbit. It was then natural to extend this idea to the other planets, and considered them as gravitating towards the sun, in the same manner as the moon gravitates towards the earth. Who would imagine that the simple circumstance of the fall of an apple would have led to such magnificent results? It is the mark of superior genius to find matter for observation and research in circumstances which, to ordinary minds, appear trivial, because they are common, and with which they are satisfied, because they are natural, without reflecting that nature is our grand field of
observation-that within it is contained our whole store of knowledge: in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of God.
THE TERRESTRIAL GLOBE.
As the earth is the planet in which we are most particularly interested, we shall explain the effects resulting from its annual and diurnal motions; but for this purpose it will first be necessary to make you acquainted with the artificial terrestrial globe. This globe, or sphere, represents the earth. The line A B which passes through its s centre, and on which it turns, is called its axis ; and the two extremities of the axis are the poles, distinguished by the names of the north and the south pole. The circle CD, which divides the globe into two equal parts between the poles, is called the equator, or equinoxial line; that part of the globe to the north of the equator is the northern hemisphere; that part to the south of the equator, the southern hemisphere. The small circle EF which surrounds the north pole, is called the arctic circle; GH surrounding the south pole, the antarctic circle. There are two intermediate circles, between the polar circles and the equator, that to the north, IK, called the tropic of Cancer; that to the south, LM,
called the tropic of Capricorn. Lastly, the circle LK, which divides the globe into two equal parts, crossing the equator, and extending northward as far as the tropic of Cancer, and southwards as far as the tropic of Capricorn, is called the Ecliptic. The delineation of the ecliptic on the terrestrial globe may convey false ideas; for the ecliptic is an imaginary circle in the heavens, passing through the middle of the Zodiac, and situated in the plane of the earth's orbit. In order to understand the meaning of the earth's orbit, let us suppose a smooth, thin, solid plane cutting the sun through the centre, extending out as far as the fixed stars, and terminating in a circle which passes through the middle of the zodiac. In this plane the earth moves in its revolution round the sun; it is therefore called the plane of the earth's orbit; and the circle in which this plane cuts the signs of the zodiac is the ecliptic.
The spaces between the several parallel circles on the terrestrial globe are called zones; that which is comprehended between the tropics is distinguished by the name of the torrid zone; the spaces, which extend from the tropics to the polar circles, the north and south temperate zones; and the spaces, contained within the polar circles, the frigid zones.
The several lines which are drawn from one pole to the other, cutting the equator at right angles, are called meridians. When any one of these meridians is exactly opposite the sun, it is mid-day, with all places situated on that meridian; and with the places situated on the opposite meridian, it is consequently midnight. To places situated equally distant from these two meridians, it is six o'clock. If they are to the east of the sun's meridian, it is six o'clock in the afternoon, because the sun will have previously passed over them; if to the west, it is six o'clock in the morning, and the sun will be proceeding towards that meridian.
Those circles which divide the globe into two equal parts, such as the equator and the ecliptic, are called great circles-to distinguish them from those which divide it into two unequal parts, as the tropic and polar
circles, which are called small circles. All circles are divided into 360 equal parts, called degrees; and these degrees into 60 equal parts, called minutes. The diameter of a circle is a right line drawn across it, and passing through the centre; the diameter is equal to a little less than one-third of the circumference, and consequently contains a length equal to nearly 120 degrees. À meridian, reaching from one pole to the other, is half a circle, and therefore contains 180 degrees; and the distance from the equator to the pole is half of a meridian, or a quarter of the circumference of a circle, and contains 90 degrees.
Besides the usual division of circles into degrees, the ecliptic is divided into twelve equal parts, called signs, which bear the names of the constellations through which this circle passes in the heavens. The degrees, measured on the meridians from north to south, or from south to north, are called degrees of latitude; those measured from east to west on the equator, or any of the lesser circles parallel to it, are called degrees of longitude. These lesser circles are called parallels of latitude; because being every where at the same distance from the equator, the latitude of every point contained in any one of them is the same.
The degrees of longitude must necessarily vary in length according to the dimensions of the circle on which they are reckoned: those, for instance at the polar circle, will be considerably smaller than those at the equator. The degrees of latitude, on the contrary, never vary in length; the meridians, on which they are reckoned, being all of the same dimensions. The length of a degree of latitude is 60 geographical miles, which is equal to 69 Fnglish statute miles. The degrees of longitude at the equator would be of the same dimensions as the degrees of latitude, were the earth a perfect sphere; but its form is not exactly spherical, being somewhat protuberant about the equator, and flattened towards the poles. This form proceeds from the superior action of the centrifugal power at the equator. The revolution of the earth on its axis gives every particle a tendency to fly off from