in many instances, and in both hemispheres, that there is an appreciable influence exerted by surrrounding denser rocks, (basalt, greenstone, diorite, and melaphyre, in opposition to specifically lighter secondary and tertiary formations,) in the same manner as volcanic islands influence gravity and augment its intensity. Many of the anomalies which presented themselves in these observations do not, however, admit of being explained by any visible geological characters of the soil.

For the southern hemisphere we possess a small number of admirable, but very widely diffused observations made by Freycinet, Duperrey, Fallows, Lütke, Brisbane and Rümker. These observations have confirmed a fact which had been strikingly demonstrated in the northern hemisphere, namely, that the intensity of gravity is not the same for all places having the same latitude, and that the increase of gravity from the equator towards the poles appears to be subjected to different laws under different meridians. Although the pendulum measurements made by Lacaille at the Cape of Good Hope, and those conducted in the Spanish circumnavigating expedition by Malaspina, may have led to the belief that the southern hemisphere is in general much more compressed than the northern, comparisons made between the Falkland Islands and New Holland on the one hand, led to an error in the calculation, and had rendered a correction necessary as early as 1786, (when a somewhat obscure one was given by the Chevalier de Buat,) on account of the difference in the loss of weight of solid bodies, when they are either at rest in a fluid, or impelled in a vibratory motion, Bessel with his usual analytical clearness laid down the following axiom in his Untersuchungen über die Länge des einfachen Secundenpendels, s. 32, 63, 126-129. "When a body is moving in a fluid (the atmosphere), the latter belongs with it to the moved system, and the moving force must be distributed not only over the particles of the solid moved body, but also over all the moved particles of the fluid." On the experiments of Sabine and Baily, which originated in Bessel's practically important pendulum correction (reduction to a vacuum), see John Herschel in the Memoir of Francis Baily, 1845, pp. 17-21.

21 Cosmos, vol. i, p. 159. Compare, for the phenomena occurring in islands, Sabine Pend. Exper. 1825, p. 237, and Lütke, Obs. du Pendule invariable, exécutées de 1826-1829, p. 241. This work contains a remarkable table, p. 239, on the nature of the rocks occurring at 16 pendulum stations, from Melville Island (79° 50′ N. Lat.) to Valparaiso (32° 2′ S. Lat.).

and New York, Dunkirk, and Barcelona on the other, have, however, by their more exact results shown that the contrary is the case, as I have already elsewhere indicated. 22

From the above data, it follows that the pendulum (although it is by no means an unimportant instrument`in geognostic observations, being as it were a sort of plummet cast into the deep and unseen strata of the earth) does not determine the form of our planet with the same exactitude as the measurement of a degree, or the movements of our satellite. The concentric, elliptical, and individually homogeneous strata, which increase in density according to certain functions of distance from the surface towards the centre of the earth, may give rise to local fluctuations in the intensity of gravity at individual points of the earth's surface, which differ according to the character, position, and density of the several points. If the conditions which produce these deviations are much more recent than the consolidation of the

22 Cosmos, vol. i, p. 161. Eduard Schmidt (Mathem. und Phys. Geographie, Th. i, s. 394), has separated from a large number of the pendulum observations which were made on board the corvettes Descubierta and Atrevida, under the command of Malaspina, those thirteen stations which belong to the southern hemisphere, from which he obtained a mean compression of 28.3. Mathieu obtained from a comparison of Lacaille's observations at the Cape of Good Hope and the Isle of France with Paris, but the iustruments of measurement used at that day did not afford the same certainty as we now obtain by the appliances of Borda and Kater, and the more modern methods of observation. The present would seem a fitting place to notice the beautiful experiments of Foucault, which afford so high a proof of the ingenuity of the inventor, and by which we obtain ocular evidence of the rotation of the earth on its axis by means of the pendulum, whose plane of vibration slowly rotates from east to west. (Comptes rendus de l'Acad. des Sc., Séance du 3 Février, 1851, t. xxxii, p. 135). Experiments for noticing the deviation towards the east in observations of falling bodies, dropped from church towers or into mines, as suggested by Benzenberg and Reich, require a very great height, whilst Foucault's apparatus makes the effects of the earth's rotation perceptible with a pendulum only six feet long. We must not confound the phenomena which may be explained by rotation (as, for instance, Richer's clock experiments at Cayenne, diurnal aberration, the deviation of projectiles, trade winds, etc.), with those that may at any time be produced by Foucault's apparatus, and of which the members of the Academia del Cimento appear to have had some idea, although they did not farther develope it (Antinori, in the Comptes rendus, t. xxxii, p. 635).

outer crust, the figure of the surface cannot be assumed to be locally modified by the internal motion of the fused masses. The difference of the results of pendulum measurements is however much too great to be ascribed at the present day to errors of observation. Even where a coincidence in the results, or an obvious regularity has been discovered by the various grouping and combination of the points of observation, the pendulum always gives a greater ellipticity (varying between the limits and) than could have been deduced from the measurements of a degree.

If we take the ellipticity which, in accordance with Bessel's last determination, is now generally adopted, namely, 299157, we shall find that the bulging23 at the

23 In Grecian antiquity two regions of the earth were designated as being characterised, in accordance with the prevalent opinions of the time, by remarkable protuberances of the surface, namely, the high north of Asia and the land lying under the equator. "The high and naked Scythian plains," says Hippocrates (de Aëre et Aquis § xix, p. 72, Littré), "without being crowned by mountains stretch far upward to the meridian of the Bear." A similar opinion had previously been ascribed to Empedocles (Plut. de Plac. Philos. ii, 3). Aristotle (Meteor. i, 1 a 15, p. 66, Ideler) says that the older meteorologists, according to whose opinions the sun "did not go under the earth, but passed round it," considered that the protuberances of the earth towards the north were the cause of the disappearance of the sun, or of the production of night. And in the compilation of the Problems (xxvi, 15, page 941, Bekker), the cold of the north wind was ascribed to the elevation of the soil in this region of the earth, and in all these passages there is no reference to mountains, but merely to a bulging of the earth into elevated plateaux. I have already elsewhere shown (Asie Centrale, t. i, p. 58) that Strabo, who alone makes use of the very characteristic word oporέdia, says that the difference of climate which arises from geographical position must everywhere be distinguished from that which we ascribe to elevation above the sea, in Armenia (xi, p. 522, Casaub.), in Lycaonia, which is inhabited by wild asses (xii, p. 568), and in Upper India, in the auriferous country of the Derdi (xv, p. 706). "Even in southern parts of the world," says the geographer of Amasia, "every high district, if it be also a plain, is cold" (ii, p. 73). Eratosthenes and Polybius ascribe the very moderate temperature which prevails under the equator not only to the more rapid transit of the sun (Geminus, Elem. Astron. c. 13, Cleom. Cycl. Theor., 1, 6), but more especially to the bulging of the earth (See my Examen Crit. de la Géogr. t. iii, pp. 150-152). Both maintain, according to the testimony of Strabo (ii, p. 97), "that the district lying immediately below the equator is the highest, on which account much rain falls there, in consequence of the very large accumulation of northern clouds at the period when

equator amounts to about 645,457 feet; about 11, or more accurately, 11.492 geographical miles. As a comparison has

those winds prevail, which change with the season of the year." Of these two opinions regarding the elevation of the land in Northern Asia (the Scythian Europe of Herodotus) and in the equatorial zone, the former of the two, with the pertinacity characteristic of error, has kept its ground for nearly two thousand years, and has given occasion to the geological myth of an uninterrupted plateau in the Tartar district lying to the north of the Himalayas, whilst the other opinion could only be justified in reference to a portion of Asia, lying beyond the tropical zone, and consequently applies only to the colossal, "elevated or mountain plateau, Meru," which is celebrated in the most ancient and noblest memorials of Indian poetry. (See Wilson's Dict. Sanscrit and English, 1832, p. 674, where the word Meru is explained to signify an elevated plateau). I have thought it necessary to enter thus circumstantially into this question, in order that I might refute the hypothesis of the intellectual Fréret, who, without indicating any passages from Greek writers, and merely alluding to one which seemed to treat of tropical rain, interprets the opinion advanced regarding bulgings of the soil as having reference to compression or elongation at the poles. In the Mém. de l'Acad. des Inscriptions, t. xviii, 1753, p. 112, Fréret expresses himself as follows:- "To explain the rains which prevailed in those equinoctial regions, which the conquests of Alexander first made known, it was supposed that there were currents which drove the clouds from the poles towards the equator, where, in default of mountains to stop their progress, they were arrested by the general elevation of the soil, whose surface at the equator is farther removed from the centre than under the poles. Some physicists have ascribed to the globe the figure of a spheroid, which bulges at the equator and is flattened towards the poles, while on the contrary, in the opinion of those of the ancients who believed that the earth was elongated towards the poles, the polar regions are farther removed than the equatorial zone from the centre of the earth." I can find no evidence in the works of the ancients to justify these assertions. In the third section of the first book of Strabo (page 48, Casaub.), it is expressly stated that, "after Eratosthenes has observed that the whole earth is spherical, although not like a sphere that has been made by a turning-lathe (an expression that is borrowed from Herodotus, iv. 36), and exhibits many deviations from this form, he adduces numerous modifications of shape which have been produced by the action of water and fire, by earthquakes, subterranean currents of wind (elastic vapours?), and other causes of the same kind, which, however, are not given in the order of their occurrence, for the rotundity of the entire earth results from the co-ordination of the whole, such modifications in no degree affecting the general form of our earth, the lesser vanishing in the greater." Subsequently we read, also in Groskurd's admirable translation, "that the earth, together with the sea, is spherical, the two constituting one and the same surface. The projection of the land, which is inconsiderable and may remain unnoticed is

very frequently been made from the earliest times of astronomical inquiry between this swelling or convex elevation of the earth's surface and carefully measured mountain masses, I will select as objects of comparison the highest of the known peaks of the Himalayas, namely, that of Kintschindjinga, which was fixed by Colonel Waugh at 28,174 feet, and that portion of the elevated plateau of Thibet which is nearest to the sacred lakes of Rakas-Tal and Manassarova, and which, according to Lieutenant Henry Strachey, is situated at the mean height of 15,347 feet. The bulging of our planet at the equatorial zone is therefore not lost in such magnitudes, so that in these cases we are unable to determine its spherical form with the same accuracy as in the case of a sphere made by a turning-lathe, or as well as the sculptor, who judges from his conceptions of form, for here we are obliged to determine by physical and less delicate perception." (Strabo, ii, p. 112). "The world is at once a work of nature and of providence,-a work of nature inasmuch as all things tend towards one point, the centre of the whole, round which they group themselves, the less dense element (water) containing the denser (earth)." (Strabo, xvii, p. 809). Wherever we find the figure of the earth described by the Greeks, it is compared (Cleom. Cycl. Theor. i, 8, p.51) with a flat or centrally depressed disc, a cylinder (Anaximander), a cube or pyramid, and lastly we find it generally held to be a sphere notwithstanding the long contest of the Epicureans, who denied the tendency of attraction towards the centre. The idea of compression does not seem to have presented itself to their imagination. The elongated earth of Democritus was only the disc of Thales lengthened in one direction. The drum-like form, тò σxñμа тvμπavоedés, which seems more especially to have emanated from Leucippus (Plut. de Plac. Philos. iii, 10; Galen. Hist. Phil., cap. 21; Aristotle, de Cœlo, ii, 13 page, 293 Bekker), appears to have been founded upon the idea of a hemisphere with a flat basis, which probably represented the equator, whilst the curvature was regarded as the οἰκουμένη. A passage in Pliny, regarding Pearls (xi, 54), elucidates this form, whilst Aristotle merely compares the segments of the sphere with the drum (Meteorol. ii, 5, a 10, Ideler, t. i, p. 563), as we also find from the commentary of Olympiodorus (Ideler, t. i, p. 301). I have here purposely avoided referring to two passages which are well known to me in Agathemerus (de Geographia, lib. i, cap. 1, p. 2, Hudson) and in Eusebius (Evangel. Præparat. t. iv, p. 125, ed. Gaisford, 1843), because they prove with what inaccuracy later writers have often ascribed to the ancients views which were totally foreign to them. According to these versions, "Eudoxus gave for the length and breadth of the earth's disc values which stood in relation to one another as 1 to 2; the same is said in reference to Dicæarchus, the pupil of Aristotle, who, however, advanced his own special proofs of the spherical form of the earth (Marcian, Capella, lib. vi, p. 192). Hipparchus regarded the earth as rрarεloεidns, and Thales held it to be a sphere!"