angular variation would be the whole 360° of a circle, in the time 24 hours, being the duration of the sidereal day. At the Equator there will be no visible effect, for the point of suspension will be carried round the Earth's axis equally with the ground beneath the weight; on the other hand, because the point of suspension at the Pole was at the Pole it would have no motion at all and the plane of vibration would be telling its own tale every instant. For a station intermediate between the Pole and the Equator the effect will be, so to speak, of an intermediate character; the ground will shift to a certain extent, but not through the angle of 360° in 24 hours. The extent of the shifting will vary with the latitude, so that it will not always be easy to obtain a covered building free from currents of air, and with an available point of suspension sufficiently elevated above the ground to insure the vibration going on long enough to enable the experiment to be readily visible to an audience. This experiment was first tried by Foucault at the Pantheon in Paris, and subsequently in London at The Russell, London, Polytechnic, and Royal Institutions and King's College, and at York, Bristol, Dublin, Aberdeen, New York, Ceylon, and other places. The angular deviation for 1 hour was found to be at Paris 11°; at Bristol 11°; at Dublin nearly 12°; and at Aberdeen about 12, whilst at New York (Lat. 40°) it was only 9° and in Ceylon (Lat. 7°) only 1·8°. · Binet calculated that the time required for one revolution of the pendulum in the latitude of Paris would be 32h 8m. At Dublin a complete revolution was watched and observed to occupy 28h 26m. In the engraving the figures 1, 2, 3, 4, 5, 6, are supposed to indicate the hours of the duration of the experiment after the pendulum has been set in motion by the severance by the candleflame of the cord which held the weight at rest. The following table of the greatest possible length of the day in different latitudes I cite from Mädlerm: The 8646 hours which make up a year, are, according to Among the ancients, Aristarchus of Samos, and Philolaus, maintained that not only did our globe rotate on its own axis, but that it revolved round the Sun in 12 months". Nicetas of Syracuse is also mentioned as a supporter of this doctrine. The Egyptians taught the revolution of Mercury around the Sun'; and Apollonius Pergæus assigned a similar motion to Mars, Jupiter, and Saturn-but I am digressing. Hesiod states that the Earth is situated exactly half-way between Heaven and Tartarus :: "From the high heaven a brazen anvil cast, Nine days and nights in rapid whirls would last, And reach the Earth the tenth; whence strongly hurl'd, Theogonia, ver. 721. Our ancestors 3c0 or 4c0 years ago termed the ecliptic the "thwart circle"; the meridian, the "noonsteede circle"; the equinoxial, “the girdle of the sky"; the Zodiac, "the Bestiary,” n Archimedes, In Arenario; Plutarch, De Placit. Philos., lib. ii. cap. 24; Diog. Laert. In Philolao. • Cicero, Acad. Quæst., lib. ii. cap. 39. P Macrobius, Comment. in Somn. Scip., lib. i. cap. 19, and others. and "our Lady's waye." The origin of the division of the zodiac into constellations is lost in obscurity. Though often attributed to the Greeks, it now seems certain that the custom is of much earlier date; and is possibly due to the Egyptians or even to the ancient Hindùs or the Chinese, in whose behalf, however, a claim to prior knowledge is always put in, whenever we Europeans fancy that we have made a discovery. 1 The following are recent values of the mass of the Earth compared with that of the Sun:-Encke 551, Littrow 353000 Mädler, and Le Verrier 300 Le Verrier, however, once seemed to consider that these values were all too small, but that in our state of uncertainty as to the Sun's parallax it was not possible to assign with confidence a definitive value. Newcomb taking the Earth and the Moon together gives for their combined mass the fraction 37, or for the Earth alone 332260. See Month. Not., vol. xxxii. pp. 302 and 323. 1872. |