cisely similar to those below it, and obviously once extended like them.

The irregularity of the summit of Fairhead, plainly shews that its gigantic columns once reached higher.

And in the façade of Magilligan, the highest of all, a few desultory patches of an upper stratum (no doubt once perfect and continuous) are to be traced along its summit.

Our mountains themselves seem to shew clearly that they were once higher; the top of Magilligan mountain is an extensive plain, over which a wandering stratum is interrupted and resumed at intervals for a great way.

At the highest part of Donald's Hill, over the valley of Glenuller, we find a hummock; also at the termination of the ridge, at its highest part over the valley of Mayola, similar hummocks.

[Phil. Trans. Vol. XCVIII. 1808.]

CHAP. V.

GEOGRAPHY, OR THE DOCTRINE OF THE GENERAL FACE OF THE EARTH.

In the sense in which we have already stated it is our intention to use the term GEOLOGY, Geography holds the same relation to it as Geognosy; the former constituting that department of Geology which contemplates the actual surface of the Earth, as the latter constitutes that department which describes its apparent origin and chemical structure. We shall briefly contemplate the science under the two sections of its history and its principles.

SECT. I. History of Geography.

The study of geography being of so much practical importance in life, must have commenced in the early ages of the word. It was regarded as a science by the Babylonians and Egyptians, from whom it passed to the Greeks, and from these to the Romans, the Arabians, and the western nations of Europe. Thales of Miletus, in the 6th

century before Christ, first made observations on the apparent progress of the sun from tropic to tropic; and is said to have written two treatises, the one on the tropic, and the other on the equinox, whence he was led to the discovery of the four seasons, which are determined by the equinoxes and solstices. We are assured this knowledge was obtained by means of the gnomon. Thales, it is also said, constructed a globe, and represented the land and sea upon a table of brass.

Meton and Euctemon observed the summer solstice at Athens, on the 27th of June, 432 years before Christ, by watching narrowly the shadow of the gnomon, with the design of fixing the beginning of their cycle of 19 years.

Timocharis and Aristillus, who began their observations about 295 B. C., first attempted to fix the latitudes and longitudes of the fixed stars, by considering their distances from the equator, &c. One of their observations gave rise to the discovery of the precession of the equinoxes, which was first remarked by Hipparchus about 150 years after; who also made use of their method for delineating the parallels of latitude and the meridians, on the surface of the earth; thus laying the foundation of this science as it now appears.

The latitudes and longitudes, thus introduced by Hipparchus, were not however much attended to till Ptolemy's time. Strabo, Vitruvius, and Pliny, have all of them entered into a minute geographical description of the situation of places, according to the length of the shadows of the gnomon, without noticing the longitudes and latitudes.

Maps at first were little more than rude outlines, and topographical sketches of different countries. The earliest on record were those of Sesostris, mentioned by Eustathius, who says, that "this Egyptian king, having traversed great part of the earth, recorded his march in maps, and gave copies of them not only to the Egyptians, but to the Scythians, to their great astonishment." Some have imagined, with much probability, that the Jews made a map of the Holy Land when they gave the different portions to the nine tribes at Shiloh; for Joshua tells us that they were sent to walk through the land, and that they described it in seven parts in a book; and Josephus relates that when Joshua sent out people from the different tribes to measure the land, he gave them as companions

persons well skilled in geometry, who could not be mistaken in the truth.

The first Grecian map on record was that of Anaximander, men• tioned by Strabo, supposed to be that referred to by Hipparchus under the designation of the ancient map. Herodotus minutely describes a map made by Aristagoras, tyrant of Miletus, which will serve to give some idea of the maps of those times. He relates, that Aristagoras shewed it to Cleomenes, king of Sparta, to induce him to attack the king of Persia at Susa, in order to restore the Jonians to their ancient liberty. It was traced upon brass or copper, and seems to have been a mere itinerary, containing the route through the intermediate countries which were to be traversed in that march, with the rivers Halys, the Euphrates, and Tigris, which Herodotus mentions as necessary to be crossed in that expedition, It contained one straight line called the royal road, or highway, which took in all the stations or places of encampment from Sardis to Susa; being 111 in the whole journey, and containing 13,500 stadia, or 1687 Roman miles of 5000 feet each.

Eratosthenes first attempted to reduce geography to a regular system, and introduced a regular parallel of latitude, which began at the Streights of Gibraltar, passed eastwards through the isle of Rhodes, and so on to the mountains of India, noting all the intermediate places through which it passed. In drawing this line, be was not regulated by the same latitude, but by observing where the longest day was 14 hours and a half, which Hipparchus afterwards determined was the latitude of 36 degrees.

This first parallel through Rhodes was ever after considered with a degree of preference, in constructing all the ancient maps; and the longitude of the then known world was often attempted to be mea sured in stadia and miles, according to the extent of that line, by many succeeding geographers.

Eratosthenes soon after attempted not only to draw other parallels of latitude, but also to trace a meridian at right angles to these, passing through Rhodes and Alexandria down to Syene and Meroe ; and at length he undertook the arduous task of determining the circumference of the globe, by an actual measurement of a segment of one of its great circles. To find the magnitude of the earth is indeed a problem which has engaged the attention of astronomers and geographers ever since the spherical figure of it was known.

It seems Anaximander was the first among the Greeks who wrote upon this subject. Archytas of Tarentum, a Pythagorean, famous for his skill in mathematics and mechanics, also made some attempts in this way; and Dr. Long conjectures that these are the authors of the most ancient opinion that the circumference of the earth is 400,000 stadia; and Archimedes makes mention of the ancients who estimated the circumference of the earth at only 30,000 stadia.

As to the methods of measuring the circumference of the earth, it would seem, from what Aristotle says in his treatise de Cœlo, that they were much the same as those used by the moderns, defi cient only in the accuracy of the instruments. That philosopher there says, that different stars pass through our zenith, according as our situation is more or less northerly; and that in the southern parts of the earth stars come above our horizon, which are no longer visible if we go northward. Hence it appears that there are two ways of measuring the circumference of the earth; one by observing stars which pass through the zenith of one place, and do not pass through that of another; the other, by observing some stars which come above the horizon of one place, and are observed at the same time to be in the horizon of another. The former of these methods, which is the best, was followed by Eratosthenes at Alexandria in Egypt, 250 years before Christ. He knew that at the summer solstice, the sun was vertical to the inhabitants of Syene, a town on the confines of Ethiopia, under the tropic of Cancer, where they had a well made to observe it, at the bottom of which the rays of the sun fell perpendicularly the day of the summer solstice: he observed by the shadow of a wire set perpendicularly in an bemispherical bason, how far the sun was on that day at noon distant from the zenith of Alexandria; when he found that distance was equal to the 50th part of a great circle in the heavens. Then supposing Syene and Alexandria under the same meridian, he inferred that the distance between them was the 50th part of a great circle upon the earth; and this distance being by measure 5000 stadia, he concluded that the whole circumference of the earth was 250,000 stadia. But as this number divided by 360 would give 6943 stadia to a degree, either Eratosthenes himself, or some of his followers, assigned the round number 700 stadia to a degree, which multiplied by 360, makes the circumference of the earth 252,000 stadia; whence

both these measures are given by different authors as that of Eratosthenes.

In the time of Pompey the Great, Posidonius determined the measure of the circumference of the earth by the 2d method above hinted by Aristotle, viz. the horizontal observations. Knowing that the star called Canopus was but just visible in the horizon of Rhodes, and at Alexandria finding its meridian height was the 48th part of a great circle in the heavens, or 7 degrees, answering to the like quantity of a circle on the earth; then supposing these two places under the same meridian, and the distance between them 5000 stadia, the circumference of the earth will be 240,000 stadia; which is the first measure of Posidonius. But according to Strabo, Posidonius made the measure of the earth to be 180,000 stadia, at the rate of 500 stadia to a degree. The reason of this difference is thought to be, that Eratosthenes measured the distance between Rhodes and Alexandria, and found it only 3750 stadia; taking this for a 48th part of the earth's circumference, which is the measure of Posidonius, the whole circumference will be 180,000 stadia. This measure was received by Marinus of Tyre, and is usually ascribed to Ptolemy. But this measurement is subject to great uncertainty, both on account of the great refraction of the stars near the horizon, the difficulty of measuring the distance at sea between Rhodes and Alexandria, and by supposing those places under the same meridian, when they are really very different.

Several geographers afterwards made use of the different heights of the pole in distant places under the same meridian, to find the dimensions of the earth. About the year 800 the khalif Almemun had the distance measured between two places that were two degrees asunder, and under the same meridian, in the plains of Sinjar on the Red Sea; and the result was, that the degree at one time was found equal to 56 miles, and at another 56 or 563 miles.

The next attempt to find out the circumference of the earth was in 1525, by Fernelius, a learned philosopher of France. For this purpose he took the height of the pole at Paris, going thence directly northwards, till he came to the place where the height of the pole was one degree more than at that city. The length of the way was measured by the number of revolutions made by one of the wheels of his carriage; and after proper allowances for the declivities and