Page images
PDF

at (be summer solstice, was vertical to (he inhabitants of Syene, a town ou the confines of Ethiopia, under the tropic of Cancer, where they hail a ivell sunk for the purpose of ascertaining the time of the solstice, which would be on the day when the rays of the sun fell perpendicularly on the bottom of the well. He observed by the shadow of a wire sot perpendicularly in a hemispherical bason, how far the sun was distant from the zenith of Alexandria at the noon of the same day; and found that distance to be one fiftieth part of a great circle in the heavens. Then Syene and Alexandria being supposed to be under the same meridian, he concluded the distance between them to be the fiftieth part of a great circle upon the earth; and this distance being by measure 5000 stadia, he coneluded the circumference of the earth to be 250,000 stadia.

The map of Eratosthenes appears to have contained little more than the states of Greece, and the dominions of the successors of Alexander, digested from the surveys that had been made.

Timocharh and Aristiuxs, who flourished about 300 years before the Christian era, seem to have been the first who attempted to fix the longitudes and latitudes of the fixed stars, by considering their situation with respect to the equator.* One of their observations gave rise to the discovery of the precession of the equinoxes, which was made-by Hipparchus about 150 years afterward; and he made use of their method in order to delineate the parallels of bttitude and the meridians on the surface of the earth; thus laying the first sohd foundation of the science of geography, as we hare it at the present time, and uniting it more closely to astronomy.

Although latitudes and longitudes were thus introduced by Hipparchus, it does not appear that any subsequent writers on (he subject attended to them before the time of Ptolemy. At the beg-ining of the teeojui Punic war, according to Polybius, when Hannibal was preparing for his expedition against Rome, by crossing from Africa into Spain, and so through Gaul into Italy, the Romans measured or surveyed all these places with the greatest care. Julius Caesar caused a general survey to be made of the whole Roman empire, by a decree of the senate. Three surveyors, who were said to have been very wise mea and accomplished philosophers, were appointed to this business, and to each was assigned a different division of the empire. Zenodoxus completed his survey of the eastern part of the empire in 14 years, 5 months, and 9 days; Tbeedotus finished the northern part in 20 years, 8 mouths, and 10 days; and Polyclitus, the southern part in 25 years, 1 month, and 10 days. This survey was begun in the year 44, and finished in the year 19, before Christ.

Sikabo and Ptolkhv were (he most eminent of (he ancient geographers. Strabo rela(es very little more than he saw himself; he made a vast number of voyages to obtain the information that

• The longitudes and latitude* of the »tar» were referred to the equator both by Timochoris and Hi|i|iarchu«; and never uniformly to tlic elliptic, till after the precession of the equinoxes was folly established by I'tolemy.

was necessary, in order to give the requisite certainty to his accounts, and is very short in what he relates from others. He was a philosopher, as well as a geographer. Good sense, perspicuity, accuracy, and solidity of judgment, are visible in every part of his works. The geography of Ptolemy is more extensive; it takes in a greater part of the earth, while it seems to be equally circumstantial every where; but this extent renders it liable to more errors. He had the merit of-carrying into full execution and practice the invention of Hipparchus, for designating the situation of places on the earth by latitude and longitude, after it had Iain dormant upward of 250 years; and thus he greatly advanced the state of the science.

The Roman empire had been enlarged to its greatest extent, and all its provinces well known and surveyed, when Ptolemy, about 150 years after Christ, composed his system of geographyThe principal materials used in composing this work were, the proportions ofthe gnomon to its shadow, taken by different astronomers at the times of the equinoxes and solstices; calculations founded upon the lengths of the longest days; the measures or computed distances of the principal roads contained in the Roman surveys and itineraries; and the various reports of travellers and navigators, who often determined the distances of places by hearsay and conjecture. All these were compared together, and digested into one uniform bcly or system ; and were afterwards translated by him, as far as was necessary in adopting the plan of Hipparchus, into the new mathematical language of degrees and minutes of longitude and Intitude.

The degree of accuracy in the latitudes and longitudes, given by Ptolemy, depended upon the veracity of the facts or suggestions communicated to him, from which they were afterward deduced. We must not therefore be surprised at the multitude of errors to be found there, when his original materials were so imperfect for executing so large a work, as the fixing of the longitudes and latitudes of all the places, coasts, bays, and rivers of the then known world. Ihs system, with all its imperfections, continued in vogue (ill the beginning of the 17th century; and the capital error* of Ptolemy's work kept their place in all maps, by a sort of unquestioned prescription, down even to that time.

Little was done in geography from the days of Ptolemy to the restoration of learning in Europe; for the Arabian geographers copied and retailed all his principal errors. They observed indeed, under their C.iliph Almnnon, in the beginning of the ninth century, a degree of latitude on the plains of Shinar near Babylon, and found it equal to 5t!| Arabian miles, each of which is 4000 cubit*, or t'lOOO feet; hence they determined the circumference of the earth.

The ancients were acquainted with but a small portion of the earth's surface. Ou the west, the Atlantic oiean nud British isl«-« limited their knowledge. The Fortunate islands, now called the Canaries, were the remotest knoivn lands towards the south. Their notions with regnrd to the northern countries were very imperfect. Though Scandinavia was known, yet that and gome otherconntries on the same continent were considered its large islands. It is not easy to determine what place (he ancients understood hy Ultima Thule; many take it for Iceland, but Procopius thinks it wax u part of Scandinavia. Their knowledge of Sarmatin and Sovthia was far from extending to the sea, which bounds Russia and Great Tartary on the north-east. Their discoveries went no farther than the Kiphean mountains, which now divide Russia from Siberia. The western frontier of China seems to have bounded their knowledge on the east Ptolemy indeed had a very imperfect notion of the southern parts of that extensive empire. How far the ancients extended their discoveries with regard to Africa cannot be certainly known. Some are of opinion, that they were acquainted with the whole coast, having sailed round the southern extremity, now called the Cape of Good Hope, and extended their voyages from the Red Sea to the Mediterranean. Ptolemy, however, supposed that Africa was not surrounded by the sea, but extended in its breadth eastwardly till it joined to India.

lu the fiftecuth century the Portuguese, animated with the desire of finding a passage t<> the East-Indies, pushed their enquiries along the western coast of Africa, till tbey found the Cape of Good Hope, 10 I486. In 1497, Vasquez de Gama doubled the Cape, and the next year rmide a voyage to India, and thus completed the discovery of that country by the east. The passage being thus opened, several European nations, desirous of sharing in the rich commerce of the east, sent their ships to the Indian Sea, where they discovered the Asiatic islands, and penetrated to the empire of Japan. The voyages of the Russians have completed our knowledge of the eastern parts of the continent of Asia.

The Poituguese had just crossed the equator, when CiwstoFHF.R Colikbcs, a native of Genoa, conceived the idea of lidding India by a western course. In 1492, he crossed the Atlantic ocean: but, instead of the Indies he discovered the .vr.w Worlp.

The improvements in geography at the time of the revival of learning in Europe, and since, have been very much owing to the great progress of astronomy. More correct methods and instruments for observing the latitude have been invented; and the discovery of Jupiter's satellites afford a much easier method of rinding the longitude, than was formerly known. Solar and lunar eclipses, transits of Mercury and Venus over the suit's disc, and occullatious of the lixed stars by the moon, also furnish means lor determining longitudes. And since the luiuir (ablet were improved by Professor Mayer, and time keepert by Mr. Harrison and others, this important object has been obtainable by measuring ilitlancet of' the moon from the tun and from certain fised stars, and by keeping time. The voyages of different nations brought lo our knowledge a vast number of countries utterly unknown before. The late voyages of Cup t. Cook and other navigators, together with the travels of Messrs. Bruce, Park, Mackenzie, and many others, contributed greatly to the improvement of geography during the 18th century; to that now the geography of lite utmost cxtrcouties of the earth is in a fair way of being much better known to the moderns, than that of the adjacent countries was to the ancients. This science, however, is yet very far from perfection; and our best maps ought to be considered only as unfinished works, which are to be altered and corrected by farther observations and discov

[ocr errors]

ASTRONOMY,

AS CONNECTED WITH THE SCIENCE OF GEOGRAPHY.

Astronomy is the science, which treats of the heavenly bodies. By it we learn the figure and dimensions of the earth, and the relative situation of places upon its surface. Hence the propriety of giving a short account of this science in an Introduction to Geography.

EXPLANATION OF TERMS.

Angle. An angle is the space included between two lines, which meet each other.

Circle. A circle is a regular figure, bounded by a curve line, every part of which is equally distant from a point within it, called the centre. The circumference of a circle is the curve line, which bounds it. The radius of a circle is a straight line drawn from the centre to the circumference; and the diameter is a straight line drawn through the centre from one side of the circumference to the other. The circumference of every circle is supposed to be divided into 360 equal parts, called degrees; each degree into 60 minutes; each minute into 60 seconds. An arc of a circle is part of its circumference. All angles are measured by arcs of circles, or by the number of degrees they contain.

Sphere. A sphere is literally a ball, or globe. By the celestial sphere is meant, the apparently concave orb, which surrounds the earth, and in which the heavenly bodies appear to be situated at equal distances from the eye. In order to facilitate the knowledge of the places of these bodies in the sphere, several circles are supposed to be described on its surface, and are denominated circles of the sphere. The circles of the celestial sphere are supposed to have their centres coincident with the centre of the earth, and to mark correspondent circles on the earth's surface, where their planes cut it; so that the celestial and terrestial spheres or globes are considered as concentric, and as having concentric circles on their surfaces.

Great Circles. Great circles are those, whose planes pass through the centre of the sphere, and, of course, divide it into two equal parts. Of these there are four, the Equator, the Ecliptic, the Meridian, and the Horizon.

Small Circles. Those circles, whose planes divide the sphere unequally, are called small circles. Their planes do not pass through its centre. The two Tropics, and the two Volar Circles, are small circles.

Axii. The axis of the earth, or any heavenly body, is an imaginary straight line passing through the centre, around which it performs its diumal rotation.

Pules. The poles arc the extremities of the axis.

Equator. The Equator is a great circle, whose plane divides the earth and the heavens into northern and southern hemispheres. The nxis of the earth makes a right tingle with its plane. It is often called the Equinoctial; because, when the sun is directly over it, the days and oights arc of equal lengths in all parts of the world.

Meridian. The Meridian is a great circle, whose plane divides the earth and the heavens into eastern and western hemispheres. There is an indefinite number of meridians ; for all places, that lie east or west of each other, have different meridians. They all pass through the poles of the earth, and cut the equator at right angles. The word meridian is derived from rneridies, mid-day; because, when the sun is on the meridian of any place, it is noon at that place. Geographers usually assume the meridian, which passes through the metropolis of their own country, as the first meridian. But as great inconvenience and confusion result from this practice, the first meridian, throughout the following work, will be that of the Royal Observatory at Greenwich, near London.

Ecliptic. The Ecliptic is a great circle whose plnnc make* an angle of 23 28 with the plane of the equator. Considered as a circle in the heavens, its circumference is the path, which the earth describes annually in its revolution round the sun. The points in which the ecliptic intersects the equator arc called the equinoctial points; because, when the sun is in either of those points, it shines on both poles, and the day is then equal to the night throughout the earth. The meridian, which passes through these points, is called the equinoctial colure. The two points in the ecliptic, which are 00 degrees distant from these, are called the solstitial points. The meridian passing through these points is called the solstitial colure, and is the only meridian which cuts the ecliptic at right angles. The sun passes through the equinoctial points on the 20th March, and the 23d of September. The former is called the vernal; the latter, the autumnal equinox. The sun is in the solstitial poinis on the 21st of June, and the 21st of December. The former is Called the summer; the latter the winter solstice.

The ecliptic is divided into 12 equal parts of 30 degrees each, called signs. These begin at the vernal intersection of the ecliptic with the equator, and are numbered from west to east. The names and characters of the signs, with tho month* in which the sun enters them, are a« follows:

« PreviousContinue »