תמונות בעמוד

and horizontal, because the circle, of which this small portion is a part, is of so great a diameter, that the line joining the eye and the object, and the circumference, may be re. garded as entirely coinciding ; but when the portion of this circle is enlarged, the curvature of the surface will depart so sensibly from, and rise so much above the lines between the eye and the object, as entirely to intercept the view. (See this subject farther explained in Practical Geometry, page 437, vol. I.)

Again, when a ship leaves the land, the observers on shore first lose siglot of her hull, and then of the lower sails, until, by increasing her distance, the tops of the masts themselves disappear; and, on the contrary, when a ship approaches the land, or another ship at sea, observers at a distance first perceive the top of her rigging, but as their distance lessens, more and more of the sails is discovered, and last of all the hull; which phenomenon can only be occasioned as before, by the round swelling surface of the water between the ship and the observers; and the effect is not confined to the sur, face of the sea, for those whu have traversed the vast plains of Flanders, Lombardy, &c, must have observed similar appearances in the gradual discovery of steeples, towers, &c, in proportion as the intervening space was diminished.

Another proof that the earth is not a plane surface, but spherical, is drawn from the voyages repeatedly performed by those who have sailed round the world, who have, in a generul sense, proceeded on in the same direction, some going always westward, others always eastward, until they returned to the port where the yoyage began.

An observer on board a vessel bound for India in the middle of winter, perceives that, as he quits the coast of England, and advances towards the Cape of Good Hope, standing in general in a southerly direction, the sun comes daily to approach more and more to be directly over hin at noon, until at last this actually happens; and as he advances

[ocr errors]

still farther on his course, the sun which at first appeared to bear to the southward of him, now bears to the northward : after he has doubled the Cape, however, and steers a course tending in general northerly, he again brings the sun directly over him at noon, before his arrival at Calcutta, when that luminary again appears always to the southward, as was the case before the traveller left England,

This appearance is not, however, confined to the sun, for as the observer proceeds to the southward from England, he will gradually discover stars appearing in the southern parts of the horizon, which had not before been seen, whilst others in the northern parts of the horizon, and even the north pole star itself, will gradually cease to appear, and those situated about the south pole will constantly be visible. If no interruptions from either land or bodies of ice presented themselves to the navigator, in steering his course still forward in the direction from north to south, he would pass under the south pole, and soon after discover the north pole, which, as he advanced in his voyage, would by degrees seem to rise higher and higher in the heavens, until he. passed under it, and arrived at the place from which he sailed, when that pole would have regained the position it originally occupied in the hemisphere.

These several appearances, it must be evident, can only be explained on the supposition that the earth is not a flat circular plane, but a spherical body, totally unconnected with any other part of the universe.

In an eclipse of the moon, which, as shall be explained when we come to treat of astronomy, is occasioned by the earth coming in between her and the sun, and so intercept. ing his light, the boundary of the shadow of the earth, as it

appears on the moon's body, is invariably of a circular form, which could not be the case if the body producing this shadow were not itself circular, and circular in all di. rections, or, in other words, spherical ; for the eclipses of


[ocr errors]

the moon happening in very various positions of the body of the earth, both with respect to her and to the sun, if that body were not a globe instead of a round flat surface like a table, the shadow cast on ibe moon would, at one time or other, assume the appearance of an ellipse, of a straight line, or of some other form different from that wbich it has constantly been found to present.

When the earth was understood to be a spherical body, attempts were naturally made to ascertain its dimensions. Eratosthenes, a celebrated geographer of Cyrene in Africa, and keeper of the Alexandrian Library, who was born 276 years before our era, by means of observations of the sun's meridian altitude at Alexandria and Syene, a town of Upper Egypt, nearly due south from Alexandria, calculated the circumference of the earth, supposing it to be a perfect sphere, to be 250,000 stadia, each stadium containing 547,4 English feet : consequently one degree, or the 360th part of the circumference would be 694 stadia, equal to about 71,24 English miles, and the whole circumference equal to about 25646,4 English miles, which exceeds the truth, but which is a wonderful approach to it, considering the very imperfect state of the science of geography in his time, as well as the defectiveness of the instruments he must have employed in his operations.

Ptolemy mentions other attempts made to ascertain the dimensions of the earth, and gives it as his own opinion, that the circumference was only 180,000 slalia ; but when the various measures known by the ancients under the common name of stadium are considered, it appears that the dimensions here assigned were equal to about 24873 English miles, the 360th part of which, or i degree, would be 69, 1 miles, which is but a little less than that now agreed upon since the latest observations.

When measurements for the purpose of determining the diniensions of the earth were made in later times on remote.


[ocr errors]

parts of its surface, the results were so far from agreeing one with another, that philosophers began to suspect the earth not to be a perfect sphere, all whose axes were equal; and the difficulty was to ascertain in what direction the longest and shortest axes were situated. From considerations arising from the nature of the earth itself, and the mutions to which, it is subjected, Sir Isaac Newton proved, that the shortest axis must be that which passes through the centre from Dorth to south, and the longest that which passes through from east to west : this opinion was, however, combated by other learned men on the continent; but the repeated measurements of degrees on various and distant parts of the earth's surface, performed with the greatest care, and by means of the highly improved instruments of modern times, have fully established the general truth of Sir Isaac's theory. It has even been supposed that the axis passing through the centre of the earth, from west to east, is about 34 English miles longer than that which passes from north to south.

Upon taking a medium of all the different dimensions of the earth, it has been found that if it were a perfect sphere, the axis would be about 7,930 English miles, and consequently the circumference about 24,913 miles. Hence the superficial area of the globe would be about 197,560,000 square English miles, and the length of a degree on a great circle of such a sphere would be equal to 69,02 English miles.

When we observe the sun, the moon, and the stars begin regularly to appear or rise in the east, advance to their

greatest elevation, and then disappear or set in the west, it is natural for us to imagine that these bodies are actually in motion from east to west, whilst we on the earth are perfectly fixed and motionless: but when our views of the magnitudes and distances of these bodies come to be more enlarged, we will be convinced that the same appearance's may be produced, with respect to us, if we suppose that the whole


[ocr errors]

heavenly host remains unmoved, while the globe we inhabit is equably turned round upon itself; just as a person who climbs a hill to enjoy an extensive prospect, obtains his wish by gradually turning himself round until he return to the point where he began, equally well as if, while he stood still, the whole surrounding landscape was rapidly moved about him, and thus made to bring each object in succession before his eyes. In the same manner, by a very simple and natural motion of the globe of the earth, turning round upon itself, the same appearances of the heavens are produced with respect to us, as if they were carried round the earth, with a motion rapid in proportion to the respective distances of the several bodies we observe.

If we pass a long needle through the centres of the flattened sides of an orange, and turn the orange round like a wheel upon this needle as an axis, we may have an idea of the manner in which the earth turns round upon itself: not that there is in the globe any thing corresponding to the axis or needle in the orange, any more than there is in the bowl when it rolls along the green ; but for the sake of mutual comprehension it has been agreed on, amongst geographers and astronomers, to give the name axis to the imaginary line on which the earth revolves. The points where this imaginary line terminates on the surface of the globe, are called poles, from a Greek term, signifying to revolve. If this imaginary axis or line be supposed to be produced in both directions, until it apparently touch the surrounding - heavens in two opposite points, these points will become the poles of the world, as far as regard our earth, for round these two points will the whole heavenly bodies seem to revolve, while the points themselves will appear to be at rest. To the point which is visible from our part of the globe we give the name of North Pole of the world, and the opposite, which cannot be perceived by us, 'is sermed the South Pole.

[ocr errors]
« הקודםהמשך »