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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 system 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 56,753 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 are 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 T0.000.000 of the quadrant, would differ scarcely 3 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 arc 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 Punnæ 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 1832, 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′.

The French arc, extending from lat. 38° 40′ to 51°, fills up a portion of this gap, and they all together afford abundant data for an exact computation of the curvature of the meridian; and this is rendered the more certain from the standards of length used in India and Russia having been directly compared. Other arcs have been measured by Bessel and Bayer in Prussia; Schumacher in Denmark; Gauss in Hanover; beside a few others of less import. The longest arc measured in the progress of the U. S. coast survey is one of 34°, extending from Nantucket to Mount Blue in Maine. Great confidence is felt in the accuracy of this measurement, from the extreme care with which the triangulation is conducted. The work is not yet quite completed. An arc of parallel will also be measured along the Mexican gulf.-From the various measurements that have been already made, different values have been calculated for the ellipticity of the earth, or the proportions between the polar and equatorial diameters. Prof. Airy, before the completion of the recent surveys, found the ellipticity, and Bessel afterward made it. The French and Indian arcs give a smaller ellipticity, as, but the Russian, it is thought, will be about. The following statement presents the average of several of the measurements: Equatorial diameter, 41,843,330 feet, or 7,924.873 miles; polar diameter, 41,704,788 feet, or 7,898.634 miles; difference of diameters, or polar compression, 138,542 feet, or 26.239 miles; ratio of diameters, 302.026 301,026; ellipticity, ready estimated that a hemispherical mountain length of degree at equator, 362,732 feet; length of degree at lat. 45°, 364,543.5 feet. Profs. Airy and Bessel, calculating from different sets of measurements, obtained the following results:

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1

Bessel. 7,925,604 7,899.114 26.471 299.15 to 298.10

The ellipticity of the earth is always expressed by a larger fraction than the above when computed from observations upon the vibrations of the pendulum in different latitudes. It is variously given from to. These observations have been made at so large a number of places, that the effects of local causes of irregularity would be expected to disappear; yet there is an unexplained discrepancy with the results of the geodetic method. This is perhaps owing in part to the variable resistance opposed by air of different densities, the effect of which can be obviated by conducting the experiments in a vacuum. The ellipticity has also been calculated from some irregularities in the motions of the moon, caused by the equatorial protuberance; and it may well be remarked as an extraordinary fact that from this source a strong confirmation should be afforded of the correctness of the results obtained from the measures of the meridional arcs. The ellipti

city thus obtained is generally or, different values being allowed for the rate of increase in the density of the earth from the surface toward the centre. Degrees of longitude might be measured instead of latitude for determining the figure of the earth; but the difficulty would be in the precise estimation of differences of longitude in the celestial arc. The close approach of the earth in its general form to the figure of hydrostatic equilibrium forcibly suggests the probability of the particles which compose its mass having been in condition to move freely together under the influence of the centrifugal force and their mutual attractions. The conditions that now obtain upon the outer portion of the earth in the mobility and transporting power of its waters, which cover of its surface, may be regarded as sufficient to give, in long periods of time, the observed external form; but the indications afforded by the pendulum of regularly increasing gravity from the equator toward the poles, and hence of symmetrical arrangement of the layers throughout, imply the existence of similar conditions during the entire period of the construction of the earth.-The form and dimensions of the earth being obtained, calculations respecting its density or weight may be made by several distinct methods. The one first applied was originally suggested by Bouguer-a comparison of the attractive power of a mountain of known dimensions and density with that of the earth of known dimensions, whence its density might be computed. Newton had al3 m. high and with a base of 6 m. diameter would cause a plummet to be deflected 1' 18" from the vertical. In making the trial the plummet is attached to a delicate astronomical instrument, with which observations are made to determine the meridian altitudes of stars near the mountain, and on the same parallel at a distance accurately determined and sufficiently far off to be beyond its influence. The difference in the 2 altitudes shows the power of attraction. Observations are sometimes made from stations on opposite sides of the mountain, and the result is then obtained by a different plan from the above. Bouguer, in 1738, observed the influence of Chimborazo in deflecting the plummet, and unsuccessfully endeavored to compute its amount from observations made at 2 stations on the S. side only. In 1772 Dr. Maskelyne proposed to the royal society to try the experiment upon some mountain in Great Britain; and the society thereupon appointed a committee of attraction," including in it, with Maskelyne, Cavendish, Franklin, and Horsley. Mr. Charles Mason was intrusted with the selection of a proper hill, and finally Schehallien in Perthshire, Scotland, was fixed upon. The primary measurements were made by Mason in 1774, to determine the distance apart of the stations to be used, one on the N. and the other on the S. side of the hill, under similar slopes. By triangulating, Dr. Maskelyne found this distance to be 4,364.4 feet, corresponding in that latitude to a

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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 Plana 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=4713

4.950 5.816

......... 5.438

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..

putation

General mean..

5.448

5.660

6.565

5.441

5.689

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 × 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).

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

ate division of the class of annelids. (See ANNELIDA, for the characters of the class.) This wellknown worm has a long, cylindrical, contractile body, divided into many apparent rings (sometimes 150) by transverse wrinklings; the internal surface of the muscular envelope sends off annular septa, dividing the cavity of the body into as many chambers as there are segments, the partitions having openings which allow the passage of the contents of the general cavity from one chamber to the others. Each segment is pro. vided with sete or bristles, beginning at the 14th ring from the head, 4 on each side, united in pairs, forming 8 longitudinal rows, of which 4 are lateral and 4 inferior; they are short and rough, and are used as fulcra during creeping or climbing in the ground. The sense of touch is very acute, as is shown by the quickness with which they retire into the ground when touched, or at the jar produced by an approaching footstep; the sense is believed to be most acute toward the head, especially in the 1st segment. The eyes are wanting. The mouth is near the anterior extremity of the body, without teeth, with 2 somewhat prominent lips; the pharynx is simple, short, and muscular, the oesophagus narrow, the stomach very muscular, and the intestine short, straight, constricted by the muscular septa, and opening at the posterior extremity of the body. The blood is red, and the circulation is complete and closed; the several pairs of simple transverse canals, situated above the stomach, whose pulsations may be distinctly seen, may be considered the heart. The dorsal vessel lies upon the intestinal canal enveloped in the hepatic tissue. The blood, though red, is quite different from that of the vertebrates; according to Siebold, it contains colorless, spherical, unequal-sized granular globules; these, Quatrefages says, are not part of the blood, but belong to the fluid of the general cavity; the latter maintains that the coloring matter is in simple solution. There is no apparent external organ of respiration, and the peculiar canals in the abdominal cavity are regarded by some as internal branchiæ or aquiferous vessels. The structure of these organs is little understood; but in all genera of the division there are at the commencement of the intestine very tortuous canals, opening generally on the ventral surface; these canals are lined with ciliæ, which have an undulatory movement always in one direction; they never contain air, according to Siebold, but circulate an aqueous respiratory fluid by means of the cilia; even the terrestrial earth worms can live only in damp earth, from which they obtain the necessary aqueous fluid. In the lumbricus these canals are surrounded by a distinct vascular net-work; they appear to end in loops, and their external orifices have not been satisfactorily ascertained. The most probable opinion is that the respiration is carried on principally by the general integument, and partly by the vascular system on the walls of the intestine; the ciliated canals described by Siebold are believed by Quatrefages to be organs for the secretion of the mucus which invests the body;

but Dr. Williams (in his "Report on the British Annelida" to the British association, in 1851) considers them as utero-ovaria. The lumbrici reproduce by sexual organs; their eggs are spherical and present nothing remarkable; both sexes are united in the same individual. During the breeding season, from 6 to 9 of the segments (from the 26th to the 37th, as generally described) are developed into a kind of collar, nearly surrounding the body, by which these animals seize each other during coition; its component glandular follicles secrete a whitish viscid fluid, probably used for the formation of their cocoons or egg-cases. According to Dufour, these cocoons have a long narrow neck, each, in the large species, containing from 1 to 6 eggs; the statement of Montègre that the young are born alive seems to be confirmed by the observations of Dr. Williams (op. cit.), who says that they escape from the egg before leaving the body of the parent; these conflicting opinions have been reconciled by some authors by calling these animals ovo-viviparous, producing their young sometimes completely formed, and at others surrounded by their egg-like envelope; it is probable that, like the leech, most lumbrici lay oviferous capsules, fringed at the ends, in which the young are developed without undergoing metamorphosis. It seems certain from the experiments of Dufour (Annales des sciences naturelles, t. v. p. 17, and t. xiv. p. 216, 1st series) that the earth worm reproduces by means of eggs; he describes them as an inch in length, of a corneo-membranous consistence, deposited in the earth at a depth of from 6 inches to 6 feet, in localities where the soil is neither inundated nor too dry, isolated, and each egg containing 1 or 2 young. In this case the eggs cannot properly be called cocoons, as the young undergo no metamorphosis in them; this would be the mode of reproduction usually noticed in the class; in the branchiate annelids it is stated by good observers that some are born alive and mature, and others of the same species are developed from eggs deposited in a gelatinous covering; so that there is no anomaly in the mode of reproduction described by Dr. Williams, and there would seem no necessity for maintaining that the viviparous mode of reproduction rested on mistaken observations, or that the excluded worms in these cases are entozoa, which, it is well known, are very common in the earth worm. Still, the subject is much in need of a thorough revision. Earth worms live in moist earth, in which they make galleries in all directions, swallowing the earth as they proceed; their food is principally soft and decaying vegetables, as may be proved by any one who chooses to watch a garden walk by the light of a lantern on a damp evening, when they may be seen creeping out of their holes, elongating their first tactile segment, feeling in all directions for food, and, seizing any suitable substance with their projected proboscis, retiring backward into the ground; their constant presence wherever there is decaying vegetable matter proves that their food is principally

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