peditions of Alexander into Asia opened new countries in the east, and largely extended the geography of the Greeks. The Romans by their conquests added discoveries in the other direction; but these, while they removed further off, still served to fix the encircling ocean, the mare tenebrosum, as the impassable barrier and limit to the land. At a very early period the astronomers among the Egyptians, Chaldeans, and Greeks perceived that the heavenly bodies, while occupying the same positions, stood in different relations to different points upon the surface of the earth. In the school of Thales, Anaximander, Anaximenes, and Pythagoras, the sun dial was employed to mark the progress of the sun in its meridional range, and to determine the latitude of places, and the division of the year into 365 days. The length of the longest and shortest days at numerous places was determined by the Egyptians with this instrument, and they first added 54 days to the older division of the year into 360 days. Thales (born at Miletus, 640 B. C.) perceived the error of giving to the earth a plane surface, and ascribed to it a spherical figure and a position at the centre of the universe. Anaximander believed it was cylindrical; and in the Pythagorean cosmography the extraordinary advance was made of placing the sun in the centre of the system with the earth moving about it. But this step was soon lost, and the knowledge of the extent and form of the earth made but slow progress as the limited observations of travellers were gradually accumulated. A latitude observation is recorded of Meton and Euctemon at Athens, 432 B. C. As commercial intercourse was extended among the nations and navigation became an important art, the spherical figure of the earth must have become apparent by the same phenomena which are now commonly appealed to in proof of it, viz.: the sinking of distant objects seen upon a level plain, as the sea below the horizon; the greater or less elevation of the circumpolar stars, as the observer is further toward the north or the south; the different angles under which the sun is seen at noon of the same day at different points on the same meridian; and other appearances of the same character. This form being recognized, it was natural to seek the measure of its circumference, and it is extremely probable that attempts of this kind were made before any of those of which we have account. Some of the measures of the most remote antiquity appear to have relation to the terrestrial circumference; and, as stated by Laplace, they seem "to indicate not only that this length was very exactly known at a very ancient period, but that it has also served as the base of a complete system of measures, the vestiges of which have been found in Asia and Egypt." Aristotle states that before his time the circumference had been determined by mathematicians at 400,000 stadia. Eratosthenes, who lived the next century after Aristotle, appears to have been the first to clearly perceive the true method of applying astronomical observations to the measurement

of a degree upon the surface of the earth, and from this to calculate the whole circumference. At Syene, in upper Egypt, was a well, at the bottom of which the full disk of the sun was seen at noon of the day of the summer solstice; at the same time from Alexandria, then taken to be on the same meridian, its angular distance from the zenith was 7° 12'. This was the measure of the celestial arc between the two zeniths, and bore the same relation to the whole circumference as the distance between the two points on the surface bore to the circumference of the earth. Presuming this distance to be 5,000 stadia, and 7° 12′ being of a circle, the total circumference was then 250,000 stadia. The world known by the reports of travellers extended only about 38,000 stadia in a N. and S. direction; and from the pillars of Hercules to the city of Thine upon the eastern ocean, along his base line drawn E. and W. across the Mediterranean, Eratosthenes reckoned a greatly exaggerated distance of 70,000 stadia, and yet less than of the whole circumference. He indulges only conjectures whether the remainder was occupied entirely by the ocean he called the Atlantic, or consisted in part of strange continents and islands. Posidonius next attempted a similar measurement by observations of the altitude of the star Canopus, when seen on the meridian at Rhodes, and again at Alexandria. Finding a difference of altitude of 7° 30', and assuming the meridional distance of the two points to be 5,000 stadia, he made the whole circumference 240,000 stadia. Of the real value of the stadium employed we are entirely ignorant; and it is certain that it was not, as employed at that time, a fixed determinate measure. The great astronomer Hipparchus of Rhodes, born at Nice, in Bithynia, 140 B. C., first determined the longitudes of places upon the earth by the eclipses of the moon, and produced maps upon which localities were designated by their latitudes and longitudes. Thus a means was furnished of determining the relative positions of places without the necessity of measurements upon the surface between them; and afterward, when suitable instruments should be contrived, of finding directly any spot beyond the sea, and returning to the starting point. Adopting these principles, Ptolemy, the astronomer and geographer, prepared the most complete map of the world so far as it was known, designating places by their latitudes and longitudes, and causing the meridians to approach each other toward the pole. For want of accurate measurement of the length of a degree, his map, however, was very imperfect. Still it continued for many centuries to be the great authority in geography; and it was not until 1635, when the difference of longitude between Marseilles and Aleppo was found to be only 30° in place of 45°, as represented upon the map, it became apparent that more perfect observations for longitudes must be adopted than those of the ancients. The uncertainty of the results obtained by observing eclipses of the moon was soon perceived, and at

last the suggestion of Galileo was adopted of observing the eclipses of the satellites of Jupiter. In the 9th century an attempt was made by direction of the caliph Al Mamun, who reigned at Bagdad from 813 to 833, to determine the length of a degree of latitude. His mathematicians assembled on the plain of Shinar, and, taking the altitude of the polar star, separated in two parties, travelling in opposite directions till they found a difference of altitude of one degree. They made the distance upon the surface the same as that given by Ptolemy, probably adopting his conclusion, which they were set to verify. From this time to the middle of the 16th century no further attention was given to ascertaining the dimensions and true figure of the earth by astronomical observations; but vast accessions of geographical knowledge were made by the enterprise of the navigators of this period. They at last solved the mystery of the mare tenebrosum. The next attempt to determine the circumference was made by Fernel, a French physician, who died in 1558. In the want of exact surveys, by which the true distance between places might be known, he measured the space between Paris and Amiens by the number of revolutions of his carriage wheel, and making his observations for latitude he made the length of a degree 57,070 French toises; a remarkably close approximation to the actual length. Willebrord Snell, a mathematical teacher of Holland, made in 1617 a similar attempt between Alkmaar and Bergen-op-Zoom; and he was the first to apply a systein of triangulation to expedite his geodetic measurements. His instrument for observing angles was a quadrant of 5 feet radius. As afterward corrected by Muschenbroek, the length was 57,033 toises. In 1635 Norwood in England repeated the experiment, measuring along the road the distance between London and York, making the degree 367,176 feet, or 57,800 toises. Toward the close of the same century Picard first applied the telescope attached to a quadrant, and furnished with cross wires, to observe the angles for his triangulation, and twice measured between Amiens and Malvoisine with wooden perches a base of 5,663 toises, or nearly 7 m. in length, employing also at the other extremity a base of verification of 3,902 toises. The celestial arc of 1° 22′ 55′′ was measured by a sector of 10 feet radius. He made the degree 57,060 toises, a result very nearly accurate, attained by a fortunate compensation of errors in his method and in his standard of measure. In 1718 the second Cassini published a work upon the magnitude and figure of the earth, with an account of measurements further north and south on Picard's line made by La Hire and himself. About the time of Picard's observations the question began to be agitated, whether the form of the earth was really that of a true sphere. The tendency of the centrifugal force of bodies revolving upon their axis, established by Huyghens and Newton, must evidently be to throw their movable particles from the poles toward

the equator and there accumulate them in a belt, increasing the equatorial diameter. Newton calculated that to maintain the hydrostatic equilibrium the proportion of the polar to the equatorial diameter must be as 230 to 231. Richer, who was sent by the academy of sciences of Paris to Cayenne in 1672, observed that the pendulum which vibrated seconds in Paris lost about 2 minutes daily at Cayenne. This fact, as Newton explained in his Principia, must be a consequence of the reduction of the force of gravity, either by effect of the centrifugal force or of increased distance from the centre. The deductions of Newton and Huyghens that the earth was a spheroid like that already observed of Jupiter, flattened at the poles, conflicting with the opposite conclusions of the first Cassini, induced the academy of sciences to cause exact measurements of meridional arcs to be made both near the equator and the polar circle. The celebrated commission of their members left Paris in 1735, Bouguer, La Condamine, and Godin to join in Peru the officers appointed by Spain, Antonio d'Ulloa and Jorge Juan; and Maupertuis with 4 others to proceed to the gulf of Bothnia, where they were joined by the Swedish astronomer Celsius. Ten years were spent by the party in Peru in the measurement of an arc of over 3° in length, extending from lat. 2' 3" N. to 3° 4' 32" S. In 2 measurements of the original base the difference was hardly 24 inches; and a second base of 5,259 toises differed when measured less than a toise from its length as calculated from the triangles. The length of the degree at the equator, reduced to the level of the sea, was calculated by Bouguer at 56,758 toises, or 362,912 feet; by La Condamine, at 56,749 toises; and by Ulloa, at 56,768 toises. The northern party found a place for their operations between Tornea in Lapland and the mountain of Kittis, 57′ 29.6" further north, in lat. 66° 48′ 22′′. The difference of latitude being determined, they measured a base line upon the frozen rivers, 2 measurements giving a difference of only about 4 inches. The arc being then determined, it was found to give 57,422 toises to the degree. With this result they returned to France, being absent only 16 months. The greater length of the degrees as they approach the poles was thus established, and consequently the greater equatorial than polar diameter of the earth. Multiplied measurements in different parts of the earth now became important to determine its true figure. They have been made in various countries, and confirm the general conclusions of Huyghens and Newton. La Caille's measurement at the cape of Good Hope in 1751, the only one in the southern hemisphere, presented anomalies, or showed great irregularity in the figure of the earth, which were not explained till, nearly a century afterward, the arc was remeasured with great care under the auspices of the British government, and it was shown that the discrepancy was owing principally to the deviation of the plumb

line of La Caille by attraction of the mass of the mountain near by. In North America the first measurement of this character was by Mason and Dixon in 1764-5, on the peninsula between Delaware and Chesapeake bays. The arc was measured throughout with wooden rods, and the degree in mean lat. 39° 12' was found to be 363,771 feet, or 68.896 English miles. It has never been supposed that this was a very exact measurement, but its accuracy has not been disproved. In 1784 measurements were commenced larger than any ever before undertaken for the purpose of accurately determining the difference of longitude between the observatories at Paris and Greenwich. Instruments of great size and improved construction were prepared expressly for this work, and the base line of 27,404 feet upon Hounslow heath was measured once with wooden rods of 20 feet length, and once with glass rods of the same length in frames. The junction of the triangles on the two sides was completed in 1788; but the operations on the English side were regarded only as a portion of the full survey of the island to be afterward carried out. Still more extensive surveys were commenced in France in 1791, with the object of obtaining the exact length of the quadrant of the meridian, in order to make use of a definite part of this natural and permanent quantity as a standard for all linear measures. The pendulum vibrating seconds in some determined latitude had been proposed as a means of furnishing an unchangeable measure, but it was given up because of its dependence upon the element of time, the measure of which is arbitrary, and its sexagesimal divisions are inadmissible as the foundation of a system of decimal measures. Local causes also, as the geological structure of the locality, affect the rate of its vibrations. The length of the quadrant of the meridian, not being liable to these objections, was adopted instead, and a new measurement was carried out on the meridian of Paris under the distinguished astronomers Delambre and Mechain, and the work was not interrupted by the political disorganizations of the years 1792, 1793, and 1794. The line was extended across France from Dunkirk to Barcelona, making an arc of about 9°, and every precaution was taken to insure the most perfect accuracy in the measurements. The base line near Paris was more than 7 m. in length (6,075.9 toises), and another of verification of 6,006.25 toises near the southern extremity of the arc differed by measurement less than a foot in length from its extent calculated from the triangles extending from the first base more than 436 m. distant. Though this arc thus determined was sufficient for the purpose required, the French astronomers in 1805, after an interval of 3 years, began to carry the measurement still further south, Biot and Arago directing the work after the death of Mechain. The island of Ivica in the Mediterranean was connected with the system by a triangle, one side of which exceeded 100 m. in length; and

by means of another the line was made to reach Formentara, distant 12° 22' 13.39" from Dunkirk, its northern extremity. The result of this extension affected the quadrantal arc before obtained so little, that the standard unit, the mètre, equal to the 10.000.000 of the quadrant, would differ scarcely 250.00 of the value before given it. A singular anomaly was noticed upon some portions of this arc, and the same was observed in the English surveys, that where these portions were considered separately, the length of the degrees appears to increase toward the equator. This is supposed to be owing to some disturbing cause, as, possibly, inequalities in the density of the strata which affected the instruments in use upon them. The effect is to produce a slight uncertainty in the exactness of the result obtained, and in the calculated proportion of the polar to the equatorial axis of the earth. The length of the quarter of the meridian was found to be 5,130,740 toises. Of the other measurements which have been made of an are of the meridian, the most important are those conducted in Hindostan by Col. Everest, in continuation of the work commenced by Col. Lambton in the early part of the present century; and those by Struve and Tenner in Russia (the latter commenced in 1817 and completed in 1853). A small arc of 1° 35' was measured near Madras by Col. Lambton; and another was commenced from Punna in the southern extremity of the peninsula, in lat. 8° 9′ 32.51′′, and extended to Damargida, lat. 18° 3' 15". After Lambton's death in 1823, Col. Everest carried the work on further north for some time. In 1882, after an interruption, it was resumed and continued till 1840, when it reached Kaliana, lat. 29° 30′ 48", thus including 21° 21' (1,477 m.). Every precaution was taken, and the most perfect instruments were provided, to insure the utmost accuracy; and notwithstanding the natural obstacles of the climate, the heat, rains, and thick atmosphere, the malaria of the plains, and the impenetrability of the jungles, the results obtained from the bases of verification indicate as great exactness as has been attained in the best European measurements. The whole extent of the Russo-Scandinavian arc is from Ismail near the mouth of the Danube, in lat. 45° 20', to Fugeloe in Finmark, lat. 70° 40'. The portion extending N. from Tornea (4° 49') was measured by the Swedish and Norwegian engineers. The ground throughout the whole extent of the line is remarkably favorable for the execution of this work, on account of its freedom from great irregularities of surface; but in the southern part forests spreading over a level country have rendered it necessary to raise many temporary elevated stations; and in the north the extraordinary refractions of that region have added to the difficulties of the work. This arc, and that of Hindostan, give the measure of a large portion of the quadrant of the meridian, leaving only the degrees between 29° 30′ and 45° 20′ unmeasured from lat. 8° 9′ to 70° 40'.

meridional arc of 42.94". But by 337 observations the difference of latitude appeared to be 54.6", giving 11.6" as the double attraction. By complicated calculations, devised by Cavendish and carried out by Dr. Hutton, the density of the earth was computed to be to that of the hill as 17,804 9,933. Dr. Playfair, after carefully examining the geological structure of the hill, made the probable mean specific gravity of the earth to be between 4.56 and 4.87. By a similar experiment made by Col. James, superintendent of the ordnance survey, at Arthur's Seat, the mean density of the earth has been found to be 5.316.-A second method of estimating the density of the earth is by an experiment exceedingly delicate and beautiful, in which the attractive power of small spheres of known weight is weighed and compared with that of the earth. The principle of this method has also been recognized by Newton, in his observation that the attraction at the surface of any sphere is directly as its radius, but incomparably less than its tendency toward the earth, or in other words, its weight. The experiment was devised by the Rev. Mr. Michell, who also prepared the apparatus with which it was first conducted by Cavendish ("Philosophical Transactions," 1798). Two balls of lead of about 2 inches diameter were fixed one at each end of a slender wooden rod 6 feet long, which was suspended by a fine wire 40 inches long attached to the centre of the rod. At each extremity of a support of the length of the rod was placed a leaden sphere of 174 lbs. weight; and the support was adjusted upon a centre exactly beneath the centre of the rod suspended above it, so that the great balls could be swung around and present their opposite sides in turn to opposite sides of the smaller balls. When brought near to the latter as they swung at rest, protected by a glass case from currents of air, they turned toward the large balls, slightly twisting the wire till its torsion equalled the attractive force. This observation being made through a telescope at a little distance off to avoid disturbing influences, the large balls were then moved round, and a similar measure of the movement was made on the other side. Cavendish after a long series of trials found the attractive force equal to of a grain weight, the centres of the balls being 8.85 inches apart, and he computed from this the density of the earth to be 5.48 times that of water. The experiment has been repeated by Reich of Freiberg and Baily of London, the latter making more than 2,000 observations. Reich made the density 5.44, and by a still later trial ("Philosophical Magazine," March, 1853), 5.58. Baily found it 5.66. It is remarkable that Newton should have stated in his Principia (iii. prop. 10) that the quantity of matter in the earth is probably 5 or 6 times what it would be if all were water. Another method of determining the density is by comparison of the different rates of vibration of the same pendulum at different distances from

the centre; either at the summit and base of a mountain, or on the surface and at a considerable depth below it. The Italian astronomers Plans and Carlini, from their experiments on Mont Cenis, in Savoy, obtained the figures 4.950 as the result. Professor Airy made a similar experiment at the Harton coal pit, near South Shields, in 1854. He found that a pendulum vibrating seconds at the surface gained 24 seconds per day at the depth of 1,200 feet; and he hence computed the density of the earth to be 6.565. Sir John Herschel ("Outlines of Astronomy," 5th ed., p. 559) thus presents the final result of the whole inquiry: "The densities concluded being arranged in the order of magnitude:

Schehallien experiment, by Maskelyne, calculated by
Playfair....

.D=4718

Carlini, from pendulum on Mont Cenis (corrected by
Giulio)...
Col. James, from attraction of Arthur's Seat...
Reich, repetition of Cavendish experiment..
Cavendish, result 5.48, corrected by Mr. Baily's recom-
Baily's repetition of Cavendish experiment..
Airy, from pendulum in Harton coal pit..
General mean..

putation

4.950 5.314 5.488

5.449

5.660

6.565

5.441

5.639

Mean of greatest and least....... calculating on 5 as a result sufficiently approximative and convenient for memory; taking the mean diameter of the earth, considered as a sphere, at 7,912.41 m., and the weight of a cubic foot of water at 62.3211 lbs.; we find for its solid content in cubic miles, 259,373 millions, and for its weight in tons of 2,240 lbs. avoird. each, 5,842 trillions (=5842 x 1018)." All these experiments give a less density to the earth than would appear to be required by the somewhat compressible nature of its materials, and to explain this the theory of the existence of a high degree of temperature in the interior is appealed to by some as presenting a sufficient counteracting influence. The probabilities of the existence of such conditions have been considered in the article CENTRAL HEAT.-The various divisions of the earth's surface are described in the article GEOGRAPHY; its structure is treated in GEOLOGY. See also PHYSICAL GEOGRAPHY. The subject may be further studied in the following works: Steffens, Beiträge zur innern Naturgeschichte der Erde (Berlin, 1801); Ritter, Die Erdkunde im Verhältnisse zur Natur und Geschichte des Menschen (Berlin, 17 vols., 1832-52; not yet complete), and other writings of the same author; Steinhuser, Neue Berechnung der Dimensionen des Erdsphäroids (Vienna, 1858); Burmeister, Geschichte der Schöpfung (Leipsic, 6th ed. 1856); Sandberger, Der Erdkörper (Hanover, 1856); Berghans, Was man von der Erde weiss (Berlin, 1857, parts 19-23); Newton's Principia; Laplace, System of the World," Harte's translation; Humboldt, "Cosmos" (5 vols., 1844-58); Guyot, "Earth and Man" (revised edition, Boston, 1858); Sir John F. W. Herschel, "Outlines of Astronomy" (5th ed., 1858).

EARTH WORM (lumbricus terrestris, Linn.), an articulate animal belonging to the abranchi