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....

Giulio)..

.D=4.718

Carlini, from pendulum on Mont Cenis (corrected by
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

Mean of greatest and least.

4.950 5.316 5.489

5.448

5.660

6.565

5.441

5.639

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 anthor; 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

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derived from such substances; they also, as Montègre observed, will feed upon animal matters; it seems more reasonable to believe, with De Blainville, that they swallow earth for the purpose of making progress in their galleries, than that they do this to extract humus or any other nutritious substance from it. They seek each other chiefly at night and in the latter part of spring, though some species have been noticed together at all times of the day, and during all the warm months; it is well known that they are most abundant on the surface of the ground during and after nocturnal rains. It has long been believed that this animal possesses a remarkable power of reproducing parts lost by accident or design, even to the extent of forming perfect individuals from separated portions; the experiments of Dugès prove that very important parts may be reproduced, and it may easily be believed that in a worm divided into two, the anterior portion might produce an anus by the simple contraction of the wound; but that the posterior portion should be able to reproduce cerebral ganglia, mouth, stomach, cardiac and sexual organs, cannot be admitted; the anterior may survive a long time, but the posterior division gradually dries up and dies. Dr. Williams, after attending to the experiments of Bonnet, Spallanzani, and others, to which Prof. Owen gives assent, says: "On the authority of hundreds of observations, laboriously repeated at every season of the year, the author of this report can declare with de liberate firmness, that there is not one word of truth in the above statement." In 1853 Mr. Newport exhibited before the Linnæan society of London 3 specimens of earth worms, one of which was living, in which more than of the anterior part of the body had been restored, smaller in diameter and with shorter segments than the anterior portion; and he says that it is not uncommon to find specimens with parts similarly restored. Though generally a despised creature, and occasionally marring the beauty of the garden walks by little hillocks of earth, they not only do not injure vegetation, but are really useful in permitting air and water to penetrate the ground through the channels which they pierce in every direction, manuring the fields, and throwing up fine dirt around the roots of grass; a field in which no worms exist can be safely put down as of little value to the agriculturist; they are most active in spring, when most needed, and retire during winter deep into the ground; according to Mr. Darwin, they perform under ground that which the plough and the spade do on the surface, and have covered a field manured with marl, in the course of 80 years, with a bed of earth 18 inches thick. Worms also furnish food for birds, moles, frogs, and other small animals, and are used as bait for many kinds of fish. The rapid ascent and descent of worms in the ground are easily understood from the action of their numerous setæ; they have often been seen high up on perpendicular surfaces, and in situations

which they could not have reached without climbing perpendicularly. In their movements they display great muscular force, each seta being moved by its appropriate system of muscles; Dr. Williams says that these seta, with their fine hair-like appendages, will actually penetrate a deal board, and that the path of a worm on such a polished surface will show under the microscope 4 series of minute perforations. This would hardly explain their ascending perpendicular surfaces, especially when of glass or similar impenetrable material; in such cases, which are incontestable, they must retain their hold by means of the tenacious mucus with which their skin is covered. There is no question that many species have been confounded under L. terrestris (Linn.). The largest European species is called L. gigas, and is 18 inches long, and as large as the little finger; other common and smaller species are L. anatomicus and L. trapezoides. Whether all the American species are distinct or not has not been sufficiently demonstrated; there are certainly some species described peculiar to this country, but proba bly the L. terrestris is common to both hemispheres. The history and habits of this commonest of animals, trodden under foot by everybody, show how creatures apparently the best known may give rise to the most contradictory opinions among naturalists, and how a com plete study of the most insignificant worm may illustrate some of the highest problems of animal physiology.-Those who wish to pursue this subject into its details are referred to the writings of Dufour, Dugès, Milne-Edwards, Blanchard, and especially Quatrefages in the Annales des sciences naturelles since 1828; to the article Annelids," ," in the "Cyclopædia of Anatomy and Physiology;" to the report of Dr. Williams, above quoted; and to Siebold's "Comparative Anatomy," with its ample references to the best works.

66

EARTHENWARE. See POTTERY.

EARTHQUAKE. In every part of the world the surface is subject to be shaken at times by movements taking place in the interior and transmitted somewhat like a wave to distant regions. No country escapes these visitations, but in volcanic districts they occur more frequently than elsewhere; and commencing in these, they have been known to pass beneath sea and land, from one hemisphere to another, till full of the entire surface of the globe has been more or less disturbed by the movement. Such was the great earthquake of the year 1755, known as that of Lisbon, which will be described below. Some countries are so subject to these disturbances that the habitations of the people are built low, with broad bases and substantial walls, with particular reference to their stability against the shocks. This is the case throughout Central America, and in Chili, Peru, &c. Taking into account the whole surface of the earth, there is probably not a day that passes without the occurrence somewhere of a sensible disturbance of this kind, and hardly a month

without one or more worthy of note. The same countries have continued to be frequently visited by earthquakes from remote periods. Calabria, the southern extremity of Italy, has been repeatedly devastated since its early settlement by the Greek colonists, and, together with the neighboring island of Sicily, has been the scene of some of the most terrible earthquakes on record. From Feb. 1783, to the end of the year 1786, a period of nearly 4 years, this country was almost constantly disturbed. No fewer than 949 shocks were experienced in the first of these years, of which 501 were shocks of the first degree of force. Lyell observes that these convulsions were not remarkable above many others for their duration, violence, or extent, but great importance is given to them from the minuteness of the observations of men competent to collect and describe with accuracy the physical facts which throw light on geological questions. The details that have been recorded of the earthquakes in the countries bordering on the Mediterranean would make this region appear more subject to them than any other part of the globe; but had any other volcanic region been settled during the same periods by a population of the same degree of civilization, it is probable that the records of the two would not have materially differed in this respect. Among the earliest accounts of earthquakes of particular interest is that which resulted in the destruction of Herculaneum and Pompeii in the year 63, which was 16 years previous to the time when those cities were buried in the ashes from Vesuvius. The ancient city of Antioch in Syria was almost destroyed in the year 115, at the time of the visit of the emperor Trajan, who was himself hurt. In 458 it was again visited by an earthquake, and in 526 occurred the most disastrous one of which any record has been preserved. Gibbon states that 250,000 persons are said to have perished at this time, a conflux of strangers to the festival of the Ascension swelling the multitudes belonging to the city. "History," he remarks, will distinguish the periods in which these calamitous events have been rare or frequent, and will observe that this fever of the earth raged with uncommon violence during the reign of Justinian. Each year is marked by the repetition of earthquakes of such duration that Constantinople has been shaken above 40 days; of such extent that the shock has been communicated to the whole surface of the globe, or at least of the Roman empire."-The approach of earthquakes is heralded by several premonitory symptoms of an unmistakable character. The air appears to be affected in some respect, perhaps in its electric condition, and the brute animals show a sensitiveness to this by uttering cries of distress and running wildly about. Men sometimes are affected with dizziness, and a sensation like seasickness. The atmosphere is often hazy for months, and the sun seen through it appears red and fiery. The weather suddenly changes from fierce gusts of wind to dead calms, and rains VOL. VI.-46

721

pour down in torrents at times, or in places in which they are usually of rare occurrence. Immediately before the shocks occur, the air is generally very still, while the surface of the ocean or lakes is unusually disturbed. A sound then breaks upon the stillness like distant thunder, or like a carriage rumbling afar off upon a rough pavement; or it may break at once with an awful explosion, as when the peal and the flash come together from every part of the cloud in which one is enveloped; at the same time the ground is shaken and lifted upward, or thrown forward, as by the passage of an irresistible wave beneath it. The shocks may be repeated several times in quick succession, or recur after long intervals; the movements may be so great as to rend the surface into chasms, and these may open and shut again, or remain in fissures of the width of a few feet or yards, and extending to unknown depths; smoke and flames are occasionally sent forth from them during the continuance of the earthquake, even if the region be not volcanic. Torrents of water are ejected from these chasms, and springs of water are often forced by the convulsion into new outets and directions. Objects upon the surface, as dwellings, trees, and animals, are engulfed in the chasms; and by subsidence of the surface, large trees, mountains even, and whole cities are swallowed up. Occurring as they most frequently do along the seaboard, the water is observed commonly to retire to some distance, leaving the harbors dry, and then to return in a great wave of many feet in height, which sweeps every thing before it. This may occur by the progress of the great wave, the recession being occasioned in the same way as the similar movement upon a small scale noticed along the shore as a steamboat approaches it, the water first receding, and then returning in a great wave; or it may be owing to a tract being uplifted in the sea at some distance, toward which the waters would first be drawn from every direction, and immediately after be propelled back with redoubled force. Of all the calamities to which man is exposed, there are none of so fearful a character as earthquakes; none involve such terrible and devastating destruction to life and property. There are none of the approach of which he is less forewarned, and none against which he can take fewer precautions. The very mysteriousness of the danger oppresses him with terror. He is ignorant in what form it is most imminent, or in what direction to seek a way of escape. Of modern earthquakes, that of Lisbon, in 1755, and that of New Madrid, Mo., in 1811, present some of the most interesting details. That of Chili, in 1822, is interesting for the permanent elevation of the country between the Andes and the coast which attended it. The area thus raised has been estimated to equal fully 100,000 square miles, and the height of the elevation to vary from 2 to 7 feet. Lines of sea beaches at higher levels and further inland indicate the previous lifting up of the same region at different times along the same lines.