Earth, the globe on which we live, and the third planet in order of distance from the sun. The earth is a rotating globe, somewhat compressed or flattened at the poles. Its mean diameter is 7,912 m., its polar diameter 7,898 m., and its equatorial diameter 7,926 m. It travels in a nearly circular orbit around the sun, at a mean distance of about 91,500,000 m. When nearest to the sun the earth is about 90,000,000 m. from him, and when at her greatest distance about 93,000,000 m. She completes her circuit around him in 365.2564 days, rotating on her axis in 23h. 56m. 4s. of mean solar time. - It was supposed by the first astronomers and geographers that the earth was a vast fixed plane, probably circular, and that the heavenly bodies were carried around this fixed earth, passing alternately above and below its level. The discovery that the earth is not a plane has been ascribed to Thales of Miletus (born about 640 B. C), and it is said that he ascribed to it a spherical figure, but Anaximander judged it to be cylindrical. Gradually, as more and more of the earth became known through travels and explorations, its real figure and dimensions became more clearly recognized.

Voyages northward and southward were found to lead to a steady rising or sinking of the north pole of the heavens, a circumstance which showed that there must exist a curvature in that direction; while voyages eastward and westward, being found by careful observation to lead to a shortening or lengthening of the intervals between noon and noon, proved that in that direction also the earth is curved. We pass over the history of such researches, in order to give more space to the exact investigations of modern times. The measurements made on the assumption that the earth is a true sphere had been conducted to a sufficiently satisfactory issue exactly at the time when Newton's discovery of gravitation was about to lead him to the inference that the earth must be somewhat oblate. Picard showed in 1679 that each degree of a great circle of the earth contains rather more than G9 m., instead of 60 as had been supposed. Soon after Newton pointed out that if the earth were regarded as originally a homogeneous fluid rotating mass, its shape would not he globular, but so far compressed that the polar diameter would bear to an equatorial diameter the ratio 229 to 230. By a singular misapprehension the elder Cassini was led to imagine that if the earth were thus compressed the degrees of longitude must diminish as either pole is approached; an obvious mistake, seeing that the polar flattening implies diminished polar curvature.

But Cassini's actual measurements seemed to indicate a diminution of the degrees of longitude toward higher latitudes; and when it was pointed out to him that his results were the reverse of what Newton's theory required, he maintained their accuracy, and the inference that the earth is a prolate instead of an oblate spheroid; in other words, he maintained that the polar diameter exceeds the equatorial. The controversy hence arising led to the famous earth-measuring expedition of 1735-'45. Bouguer, La Condamine, and Godin left Paris for Peru, where they were joined by Antonio d'Ulloa and Jorge Juan from Spain. Haupertuis with four others sailed to Bothnia, where they were joined by the Swedish astronomer Celsius. The measurements made in both places were most satisfactory, repeated observations leading to results differing only a few feet per mile. The length of the degree in Peru was found to be 362,790 ft,, while the estimated length of a degree in Sweden amounted to 365,744 ft. The difference was far too great to be ascribed to errors of measurement, and it was justly regarded as demonstrative of the general accuracy of Newton's reasoning.

However, the value actually ascribed to the compression by Newton (on a certain hypothesis as to the earth's structure) was not confirmed by these observations; on the contrary, it appeared that the compression was 1 in about 300, instead of 1 in about 230. Subsequent observations, as well as considerations founded on the attraction exerted by the moon upon the bulging equatorial parts of the earth, have shown that the compression has not so great a value as Newton's hypothesis required; nor need we wonder at this when we remember that under the influence of attraction the interior parts of an originally fluid earth would necessarily be much denser than the outer parts, and that Newton himself only introduced the hypothesis of homogeneity to simplify his calculations. The following list of measurements of degrees in different latitudes indicates what has been done since such labors were first undertaken, and serves to show how satisfactorily all observations agree in pointing to an increase of the length of a degree with increase of latitude, whether north or south of the equator:

These measurements are due to the following mathematicians and observers: The two Swedish to Svanberg and Maupertuis; the two Russian to Struve and Tenner; the Prussian to Bessel and Bayer; the Danish to Schumacher; the Hanoverian to Gauss; the two English measurements to Roy and Kater; the two French to Lacaille, Cassini, Delambre, and Mechain; the Roman to Boscovich; the American to Mason and Dixon; the Indian to Lambton and Everest; the Peruvian to La Condamine and Bouguer; and lastly, the two measurements at the Cape of Good Hope to Lacaille and Maclear. Later measurements made in India by Sir George Everest give 363,606 ft. for the length of a meridional degree in lat. 26° 49', and 363,-187 ft. for the length in lat. 21° 5'. Combining all the observations, and attributing minor irregularities either to errors of observation or to local peculiarities of the earth's surface, we deduce the following table of the lengths of degrees of latitude in feet for every tenth degree:

It follows from the measurements that the compression of the earth is very nearly 1/300. But it is believed that the compression is different for different longitudes; in other words, that the earth is not a figure of revolution. It will be interesting for the reader to compare the following three sets of results: First, as the result of comparing the best measurements before the recent Indian and Russian observations, we have - Earth's equatorial diameter 41,843,330 ft., or 7,924.873 m.; polar diameter, 41,704,788 ft., or 7,898.634 m.; difference of diameters or polar compression, 138,542 ft., or 26.239 m.; ratio of diameters, 302.026 : 301.026; compression, 1/301.026; length of degree at equator, 362,732 ft.; length of degree in lat. 45°, 364,543.5 ft. Secondly, Sir John Herschel thus states the results obtained by Capt. A. R. Clark, R. E., from a combination of all the results which have been obtained, and especially those resulting from the recent extension of the great arcs surveyed in India and Russia: "The earth is not exactly an ellipsoid of revolution. The equator itself is slightly elliptic, the longer and shorter diameters being respectively 41,852,864 and 41,843,-096 ft.

The ellipticity of the equatorial circumference is therefore 1/4283; and the excess of its longer over its shorter diameter about two miles. The vertices of the longer diameter are situated in Ion. 14° 23' E. and 194° 23' E. of Greenwich, and of its shorter in 104° 23' E. and 284° 23' E. The polar axis of the earth is 41,707,796 ft. in length; and consequently the most elliptic meridian (that of Ion. 14° 23' and 194° 23' E. of Greenwich) has for its ellipticity 1/287.5, and the least elliptic (that of Ion. 104° 23' and 284° 23' E. of Greenwich) an ellipticity of 1/308.3" Thirdly, Gen. Schubert, in the memoirs of the imperial academy of St. Petersburg, arrives (by a mode of reasoning which Sir John Herschel regards as less exact) to a similar but not identical conclusion. " He makes the ellipticity of the equator 1/8885," says Herschel, " and places the vertices of the longer axis 26° 41' to the eastward of Capt. Clark's. His polar axis, as deduced from each of the three great meridian arcs, the Russian, Indian, and French respectively is 41,711,019 ft., 41,-712,534 ft., and 41,697,496 ft., the mean of which, giving to each a weight proportional to the length of the arc from which it is deduced, is 41,708,710 ft." It may be added that the figure of the earth as thus determined accords well with the observed change of rate in a pendulum set swinging in different latitudes, as also with the observed values of precession and nutation (the motions of the earth's globe caused by the attraction of the sun and moon on the protuberant mass round the equator). - Next to the determination of the earth's figure, that of her density may be regarded as the most important terrestrial problem which men of science have attempted to solve.

Newton was the first to show how the problem might be attacked, since he first showed that a plummet would be deflected from the vertical by the attraction of a mountain. But Bouguer (born 1698) was the first to suggest that the method should be put in practice with direct reference to the problem of determining the earth's mass. The method is applied by means of the instrument called a zenith sector, a telescope with a graduated arc attached to its lower extremity and a plumb line to the upper. This telescope, pointed to the same star successively at two stations separated by a known distance, serves to show how much the centre of gravity changes in passing from one to the other; and it is known that for each 100 ft. of horizontal distance on a north and south line, the change of direction is very nearly one second of angle. But if one of the stations be at the foot of a mountain, the same change of direction is not observed, because the attraction of the mountain deflects the plumb line; and the effect is even greater if both the stations lie at the foot of a mountain, one on the northern side and the other on the southern.

Thus, let us suppose that the two stations are separated by 4,000 ft.; then the difference in the direction of gravity would be about 40" if the stations were on a plain; but if a mountain separates them, this difference will be increased, because the positions of the lower ends of the plumb line, already tending to convergence in consequence of the fact that the earth's gravity is directed always toward the centre of the earth, are brought yet nearer together by the mountain's attraction. If this difference is carefully determined, and if the geological structure of the mountain is known, as well as its general shape and dimensions, it becomes possible to compare the density of the earth with the known mean density of the mountain. This method was first applied by Bouguer in 1738, on the flanks of Chimborazo; but as both his stations were on the southern side, he was unable to determine the direction of the plumb line by means of such an instrument as the zenith sector; he failed accordingly to obtain any trustworthy results.

But in 1772 Dr. Maskelyne proposed to the royal society to renew the experiment on some mountain in Great Britain. Schehallien was selected, and after a careful series of measurements and observations it was found that at stations separated by 4,364.4 ft. the difference in the direction of gravity was 54.6" instead of 42.94", the difference due to gravity; so that the double attraction exerted by the mountain was found to be 11.6". By a series of calculations devised by Cavendish and carried out by Dr. Hutton, the density of the earth was computed to be to that of the mountain as 17,804 to 9,933; and after carefully examining the geological structure of Schehallien, Dr. Playfair inferred the probable mean specific gravity of the earth to lie between 4.56 and 4.87, that of water being unity. More recently Col. James, superintendent of the ordnance survey in Great Britain, has deduced a mean density of 5.316 from observations made on Arthur's Seat, near Edinburgh. - Another method of determining the earth's density is based on the circumstance that the earth's attraction is less on a body raised to a considerable height above its surface than on a body at the sea level. Hence a pendulum of given length swings more slowly the higher it is raised above the earth's surface.

Now if such a pendulum be taken to the summit of a mountain, it is clear that the reduction of the rate of swing will not be so great as the estimated reduction for the amount of elevation alone, simply because the attraction of the mountain will have an appreciable effect on the pendulum's rate. Thus as in the former case it becomes possible to compare the attraction exerted by the mountain with that exerted by the earth, and so to determine the density of the earth. From observations made on Mont Cenis by this method, Carlini and Plana deduced for the earth's mean density the value 4.950. - A third method may be described as the converse of that just mentioned. If a pendulum could be caused to swing at a great depth below the surface of the earth, it would swing more slowly than at the sea level. For, considering a spherical surface concentric with the earth's to pass through the place of the pendulum, the attraction of all the mass of the spherical shell outside that surface would produce no effect on the pendulum; while the sphere within that surface would exert a less influence on the pendulum than is exerted by the earth's mass on a pendulum at the sea level; for though the pendulum would be nearer to its centre, yet this cause could only be effective to increase attraction inversely as the squares of the radii of the two spheres, whereas the volume of the inner sphere would be less than the volume of the earth in the direct proportion of the cubes of the radii.

But a pendulum placed at the bottom of a cavity, like a mine, would not have its rate reduced to the same extent, or might even have its rate increased. For, since the whole of the spherical shell just referred to exerts no attraction on the pendulum, the space dug out to form the mine should be occupied in order that the attraction of the shell should be nil; and when it is vacant there is wanting a portion of that matter whose attraction outward (or diminishing the earth's) is necessary to produce the equilibrium referred to. Failing, then, this attraction outward, the remainder of the shell in question must exert a certain amount of attraction inward, and according to the shape and extent of the cavity this attraction inward may cause the resultant attraction on the pendulum to be nearly or quite equal to, or even greater than, that at the sea level. And it is clear that if the geological structure of the region surrounding the mine is known, as well as the exact shape and extent of the mine, this method, like the last, supplies a means of determining the mean density of the earth.

Prof.

Airy applied this method at the Harton colliery in 1854; but it is to be noted that he first tried the experiment, though unsuccessfully, in 1826 and 1828. "The two stations selected were exactly in the same vertical, excellently walled, floored, and ceiled. Every care was taken to secure solidity of foundation and steadiness of temperature. At each station (upper and lower) was mounted an invariable brass pendulum, vibrating by means of a steel knife edge upon plates of agate, carried by a very firm iron stand. Close behind this was a clock, and before it a telescope mounted so that coincidences of the pendulum of the clock might be accurately observed through a slit in front of the telescope. By this means the proportion of invariable pendulum swings to clock pendulum swings was found; and then, as the clock pendulum swings in any required time are denoted by the clock dial, the corresponding numbers of invariable pendulum swings at the two stations were determined. In order, however, to do this, the clock rates had to be frequently compared; this was done by means of an electrical apparatus.

In this manner the pendulums were observed, with 104 hours of incessant observations, simultaneous at both stations, one pendulum (A) being above and the other (B) below; then with 104 hours, B above and A below; then with 60 hours, A above and B below; then with 60 hours, B above and A below. No less than 2,454 effective signals were observed at each station. The results showed that the pendulums suffered no injury in their changes, and that the acceleration of the pendulum on being carried down 1,260 feet was 214 seconds per day, or that gravity is increased by 1/19.160 part. It does not appear likely that this determination can be sensibly in error." From it, taking into account the structure of the region and the figure of the mine, Prof. Airy deduced for the earth's density the value 6.565. - The remaining method of determining the earth's density is that of comparing the earth's attraction directly with the attraction of large spheres of lead or other heavy metal. It was devised by Michell, who also prepared the apparatus by means of which it was first applied by Cavendish in 1789. Two globes of lead, / and g in the engraving, are attached to the extremities of a strong horizontal bar, movable in a horizontal plane around its centre.

Above this centre a light horizontal rod, a b, is supported by a fine wire. Two equal balls of lead, a and b, about two inches in diameter, are attached to the ends of this rod; and the proportions of the instrument are so adjusted that the distance between these two small spheres (about 6 ft.) is equal to the distance between the two large ones. When the rod bearing the two small spheres is in as nearly perfect equilibrium as possible, the bar bearing the globes of lead is rotated on its vertical axis until these globes are brought nearly into perfect contact with the small balls on opposite sides, as at f and g. Their attraction on these balls being thus called into play, they tend to draw the light rod from its position of rest. The amount of the torsion thus produced in the supporting wire is observed through a telescope placed some distance off, so as to avoid disturbing influences. Then the bar is turned round in a contrary direction until the large balls are again nearly in contact with the small ones, as at h and k, so that the tine rod is swayed in a contrary direction from its position of rest, and the torsion thus caused is observed as before. The mean of the two results indicates the actual amount of torsion which the attraction of the two large globes is capable of producing.

The experiments of Cavendish gave the attractive force exerted by two leaden spheres, each 174 lbs. in weight, as equivalent to 1/4300 of a grain weight, and he thence computed the density of the earth to be 5.48 times that of water. Reich of Freiberg, in two series of experiments, made the density 5.438 and 5.582. The late Francis Baily made more than 2,000 experiments by this method, and deduced from them a density equal to 5.660. It is worthy of notice how closely the results obtained by Micheirs method (or, as it is called, the Cavendish experiment) agree together. The inference appears to be that this method is more trustworthy than any of those before described. It is also remarkable that Newton had stated in his Principia that probably the mean density of the earth is five or six times that of water ( Verisimile est quod copia material totius in terra, quasi quintuplo vel sextuplo sit quam si tota ex aqua constaret). - Sir John Herschel ("Outlines of Astronomy," 11th ed., p. 559) thus sums up the evidence hitherto obtained respecting the earth's density and mass: "The final result of the whole inquiry will stand as below, the densities concluded being arranged in order of magnitude:

Michell's Torsion Balance.

Calculating on 5 1/2 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. avoirdupois each 5,842trillions(=5,842 x 10'8)." The low specific gravity of the earth, compared with that which might be expected from the enormous pressure to which her interior parts are subjected and the compressible nature of their materials, has led some men of science to the conclusion that the temperature of the interior is sufficiently high to exert an important counteracting influence. - The principal motions of the earth are her rotation on her axis and her revolution around the sun. The main proofs of both these motions are found in the results of astronomical observation. The revolution of the earth around the sun is in particular placed beyond all possibility of question by the phenomenon called the aberration of light, which affects every star in the heavens, and is at once explained on the hypothesis of the earth's motion; while it remains absolutely without explanation, and even without the possibility of explanation, if the earth be regarded as at rest.

And when the revolution of the earth is once admitted, her rotation must be admitted at the same time; for it would obviously be absurd to regard the sun's apparent annual motion around the heavens as due to a real motion of the earth around the sun, while at the same time the sun's apparent diurnal motion around the heavens was regarded as due to a real motion of the sun around the earth. But there are certain proofs of the earth's rotation which may be regarded as in a sense terrestrial, since they have no reference to the celestial bodies. Among these we may mention Foucault's experiments with the gyroscope and pendulum. The gyroscope as applied by him to this problem is a heavy disk or ring rapidly rotating in its medial plane, and so suspended as to be free to turn in any direction. Such a disk or ring tends to preserve the plane of rotation unchanged in position, and if the earth were not rotating would remain unchanged under the closest scrutiny. But as the rotation of the earth tends to change the position of the medial plane of the rotating body, the tendency before alluded to is called into action and causes the medial plane apparently to shift, while in reality it is only maintaining its position against the effects of the earth's rotation.

The result is, that when carefully examined through a telescope the rotating disk or ring is seen to shift steadily in a direction opposed to that of the earth's rotation. Foucault's pendulum experiment depends on similar principles : the "plane of swing" of a pendulum tends to resist any motion by which it would be caused to take up a position intersecting its former one. Now, if a pendulum be set swinging in a north and south plane, a tangent to the arc of swing at its lowest point passes when produced through a point on the produced axis of the earth; and if the pendulum continued to swing in a north and south plane while the earth rotated through any considerable angle, this tangent would continue to pass through the same point on the axis; in other words, the plane of swing would have taken up a position intersecting its former position. This at least would be the case for all points on the earth's surface except those lying on the equator. But the plane of pendulum swing resists the influence, thus causing it really to shift its plane, and therefore this plane apparently shifts from the north and south position; this apparent shifting being obviously greater or less according as the pendulum is further from or nearer to the equator.

The experiments on this plan, when carefully conducted, have been found to accord perfectly with theoretical anticipations based on the theory of the earth's rotation. Another proof of the earth's rotation is founded on the fact that bodies let fall from a considerable height fall slightly to the east of the point which lies directly below the point whence they had been dropped. Newton showed that this should be the case, because the point of suspension, being further from the earth's centre than the point directly beneath it, has a greater velocity on account of the earth's rotation. The experiment is not an easy one, for many reasons; and it is only on the average of a great number of experiments that the tendency to easting can be established. Benzenberg in 1820 made a series of such experiments, letting 31 balls fall from a height of 235 ft., within a tower, upon a sheet of soft wax. He found the sum of deviation toward the north to be 46.4 Paris lines, toward the south 92.6 lines, toward the east 174*5 lines, and toward the west 50.5 lines. He repeated these experiments in an abandoned coal pit at Schlebusch in Rhenish Prussia, where the fall was 262 ft.

Twenty-nine balls gave the following sums of deviation : toward the north 124 lines, toward the south 103 lines, toward the east 189 lines, and toward the west 42 lines. Lastly, a long and most convincing series of experiments was carried out on the same plan by Prof. Reich in the mines of Freiberg. He dropped 106 balls to a depth of no less than 488 ft. There was a balance of southerly deviations amounting to 48'76 lines, and a balance of easterly deviations amounting to 1,093.92 lines; so that the mean deviation toward the south was but 0.46 lines, or practically inappreciable, while the mean deviation toward the east was 10.32 lines. Thus we may regard the rotation of the earth as abundantly demonstrated, independently of any evidence afforded by the celestial bodies. - 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: Stef-fens, Beitrage zur innern Naturgeschichte der Erde (Berlin, 1801); Bitter, Die Erdkunde im Verhaltnisse zur Natur und Geschichte des Menschen (2d ed., 19 vols., Berlin, 1822-'59), and other writings of the same author; Stein-hauser, Neue Berechnung der Dimensionen des Erdspharoids (Vienna, 1858); Burmeister, Ge-scltichte der Schopfung (Leipsic, 7th ed., 1867); Sandberger, Der Erdkorper (Hanover, 1856); Berghaus, Was man von der Erde weiss (4 vols., Berlin, 18G0); Figuier, La terre avant le deluge (8vo, Paris, 1862); Haekel, Naturliche Schopfungsgeschichte (8vo, Berlin, 1808); Re-clus, La terre (2 vols., Paris, 1867-'8; Woodward's translation, 1871); Newton's Principia,; Laplace, "System of the World," Harte's translation; Humboldt, "Cosmos" (5 vols., 1844-'58); Guyot, "Earth and Man" (revised ed., Boston, 1858); Sir John F. W. Herschel, "Outlines of Astronomy " (11th ed., 1872).

Earths #1

Earths, the oxides of the metals aluminum, glucinum, thorinum, zirconium, lanthanum, erbium, and yttrium, called alumina, glucina, etc. They are often called the earths proper, to distinguish them from the alkaline earths, baryta, strontia, lime, and magnesia, the oxides of the metals barium, strontium, calcium, and magnesium. Before the decomposition of some of them by Sir Humphry Davy they were thought to be elements. Silica was formerly regarded as an earth; but on account of its forming definite compounds with the earths in which they act the part of bases, it must be classed as an acid. The earths generally exist in nature in combination with silica; although in the varieties of corundum, such as the gems sapphire, ruby, oriental topaz, and oriental amethyst, alumina exists principally as oxide, the Indian sapphire having the composition A1403, 97.5 per cent.; magnesia, 1.9; silica, .8 per cent.; and the Indian ruby containing barely more than 1 per cent. of silica. In feldspar alumina is found combined with silica, sometimes with silicate of soda, but more frequently with silicate of potash. The gem hyacinth is composed of silicate of zircon. The silicate of glucina is found in the beryl and erbium.

Associated yttrium is found, combined with silica, in the mineral gadolinite.