Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/678

 626

ECHUCA — ECLIPSE

Eclipse.—The general subject of eclipses, and the conditions under which they may occur, were fully treated in the article Astronomy (Ency. Brit. vol. ii.). To that article nothing need be added on the subject of eclipses of the moon, and the present article is therefore confined to eclipses of the sun, especially those which are total. The complete computation of the circumstances of an eclipse ab initio requires three distinct pro Elemeats cesses. The geocentric position of the sun and o/ec//pses> moon have first to be computed from the tables of the motions of those bodies. The second step is to compute certain elements of the eclipse from these geocentric positions. The third step is from these elements to compute the circumstances of the eclipse for the earth generally, or for any given place on its surface. The national Astronomical Ephemerides, or “ Nautical Almanacs,” give in full the geocentric positions of the sun and moon from at least the early part of the 19th century to an epoch three years in advance of the date of publication. It is therefore unnecessary to undertake the first part of the computation except for dates outside the limits of the published ephemerides, and for many years to come even this computation will be unnecessary, because tables giving the elements of eclipses from the earliest historic periods up to the 22nd century have been published by Oppolzer and Newcomb. We shall therefore confine ourselves to a statement of the eclipse problem and of the principles on which such tables rest. Two systems of eclipse elements are now adopted in the ephemerides and tables; the one, that of Bessel, is used in the English, American, and French ephemerides, the other Hansen’s—in the German and in the eclipse tables of Oppolzer. The two have in common certain fundamental geometric constructions. A fundamental axis of reference in both systems is the line passing through the centres of the sun and moon; this is the common axis of the shadow cones, which envelop simultaneously the sun and moon (see Fig. 37, Ency. Brit. vol. ii. p. 803). The surface of one of these cones, that of the umbra, is tangent to both bodies externally. This cone comes to a point at a distance from the moon nearly equal to that of the earth. M ithin it the sun is wholly hidden by the moon. Outside the Fig. 3.—Adult male, Bonellia viridis, Rol. The original was 15 mm. long. The nervous system is not shown. (Ajter belenm.) a, umbral cone is that of the penumbra, within which the sun generative pore with spermatozoa coming out; o, anterior Dima is partially hidden by the moon. The condition that the end of intestine attached to the parenchymatous tissue hy muscular strands ; c, green wandering cells containing chlorophj II, two bodies shall appear in contact, or that the eclipse shall d, parenchymatous connective-tissue ; e, epidermis ; t, intestine , begin or end at a certain moment, is that the surface of j, vas deferens; l, internal opening of vas deferens; m, the left anal vesicle ; n, spermatozoa in the body-cavity. one of these cones shall pass through the place of the semblances to the genus Sternaspis, which in one species, observer at that moment. Let a plane, which we call the S. spinosa, is said to carry a bifid proboscis resembling fundamental plane, pass through the centre of the earth that of the Echiuroids. The relationship with the Sipuncu- perpendicular to the shadow axis. On this plane the loidea and the Priapuloidea are discussed in the article centre of the earth is taken as an origin of rectangular coordinates. The axis of Z is perpendicular to the plane SlPUNCULOIDEA. (A- E- S') and therefore parallel to the shadow axis; that of Y andZ Echuca, a borough of Victoria, Australia, in the lie in the plane. In these fundamental constructions the county of Rodney, on the river Murray, across which it is two methods coincide. They differ in the direction of the connected by bridge with Moama, which has railway con- axis of Y and Z in the fundamental plane. In Bessels nexion with Deniliquin, New South Wales. The town method, which we shall first describe, the intersection ol is the terminus of the Murray River Railway and the the plane of the earth’s equator with the fundamental plane entrepot of the overland intercolonial trade, and has large is taken as the axis of X. The axis of Y is perpendicular wool stores. The district, a rich agricultural one, is noted to it, the positive direction being towards the north, the for its vineyards. Altitude, 314 feet. Mean temperature Besselian elements of an eclipse are thenir, y, the cofor the year, 58°-6 F.; for January, 7l°-6; for July, 44°-7. ordinates of the shadow axis on the fundamental plane; a, Population (1881), 4789; (1901), 4083. the declination of that point in which the shadow axis Eckernforde, a town of Prussia, province of intersects the celestial sphere; y, the Greenwich hour ang e Schleswig-Holstein, on a bay of the Raltic, 20 miles, by of this point; l, the radius of the circle m which the penumbral or outer cone intersects the fundamental plane, rail north-west from Kiel. It has a good harbour, fishing, and V, the radius of the circle in which the inner or umbral trade in agricultural products, and production of tobacco, salt, and iron goods. There are an architects school and cone intersects this plane, taken positively when the vertex the cone does not reach the plane, so that the axis mus a teachers’ seminary. The place suffered from a, sea-flood of be produced, and negatively when the vertex is beyond tne in 1872. Population (1885), 5604; (1900), 6/19.

Affinities.—The occurrence of trochosphere larva and the temporary segmentation of the body have led to the belief that the Echiuroids are more nearly allied to the Annelids than to any other phylum. This view is strengthened by certain anatomical and histological re-