Page:The American Cyclopædia (1879) Volume XVI.djvu/862

 832 ZODIACAL LIGHT does not shade off uniformly to the borders, but has two distinct degrees of lustre a tri- angle within a triangle two different kinds of light as it were, as if the matter was more con- densed at its central parts and thinned out be- yond. The inner is termed by Mr. Jones the stronger light, and the outer the diffuse light. These are not bounded by sharp lines, but melt away by degrees ; still there is between the two a line of greater suddenness of transition, while the experienced eye has no difficulty in tracing the outer boundary of the diffuse light. The stronger he found to be approximately 60 in its greatest width, and the diffuse 90. The data furnished by these observations were as follows: 1, when his position was N. of the ecliptic, the main body of the zodiacal light was on the N. side of that line ; 2, when his position was 8. of the ecliptic, the main body of the zodiacal light was on the S. side of that line ; 3, when his position was on or near the ecliptic, the light was equally divided by the ecliptic, or nearly so ; 4, when by the earth's rotation on its axis he was during the night carried rapidly to or from the ecliptic, the change of the apex and of the direction of the boundary lines was equally great, and corre- sponded to his change of place ; 5, as the ecliptic changed its position as respects the horizon, the entire shape of the light became changed, which would result from new por- tions of the nebulous matter coming into posi- tion for giving him visible reflection, while portions lately visible were no longer giving him such a reflection. The first four of these results were not absolutely invariable, but the exceptions were few. Mr. Jones inferred from these observations that the zodiacal light is caused by a ring of matter surrounding the earth, not the sun ; for the changes resulting from the observer's change of position on the earth, as well as from the change of position caused by the earth's rotation, seemed to him much greater than could be explained if the ring were not relatively near to the earth. These changes of appearance also seemed to correspond in character with the theory thus advanced. But it is certain that no ring sur- rounding the earth could possibly explain the phenomena of the zodiacal light when these are all considered together, however competent to explain the particular phenomena observed by Mr. Jones. It is to be noted in particular that the phenomena observed in high latitudes, though not so striking as those observed in low latitudes, are in reality even more instructive. It will be manifest that if there were a ring surrounding the earth at a distance so moder- ate that a traveller in tropical regions could recognize the zodiacal's change of position as he passed from the northern to the southern side of the equator, it would be invisible from places in high latitudes. This is clearly shown in the writer's treatise on Saturn, where the configu- ration of the rings viewed from different Sa- turnian latitudes has been carefully calculated ZOEGA (not merely surmised from general considera- tions). Even in moderately high latitudes the zodiacal, if Mr. Jones's theory were sound, ought to be seen far toward the S. point of the horizon ; whereas, so far is this from being the case, thaj; in England the average position of the zodiacal's axis in the horizon is nearly identical with the ecliptic. The most probable interpretation of the zodiacal light is that which regards it as caused by multitudes of minute bodies travelling around the sun. At the same time two points must be carefully noted. In the first place, there are phenomena of the zodiacal which indicate some resem- blance between its structure and that of com- ets' tails, so that not meteoric matter alone, but cometic matter also, is probably present in it. Secondly, it is highly improbable that the greater portion of the matter forming the zodi- acal light travels on orbits of small eccentricity around the sun. Knowing that the orbita of meteors extend far out into space, even beyond the orbits of Uranus and Neptune, we must suppose the meteoric and cometic matter of the zodiacal to travel on paths similarly eccen- tric, so that the matter composing the zodiacal light at one time will at another be far beyond the bounds of its visible extent. This, indeed, so far from introducing a difficulty, helps to remove one. For it is manifest that according to this theory the zodiacal light should vary markedly in appearance from time to time, which is precisely what had been observed and had remained unexplained until the eccentric nature of meteoric orbits was recognized. The spectrum of the zodiacal appears not to be monochromatic aa Angstrom supposed, but continuous, indicating that the zodiacal light is reflected sun light. ZoEdA, Georg, a Danish antiquary, born in Jutland, Dec. 20, 1755, died in Rome, Feb. 10, 1809. He was the son of a Lutheran clergy- man, was educated at Gottingen, started in 1782 on a numismatic tour in Germany and Italy at the expense of the Danish govern- ment, and settled in Rome, where he joined the Catholic church in 1788. He was appointed interpreter of modern languages to the propa- ganda college, and published Nummi ^Egyptii Imperatorii prostantes in Museo Borgiano Ve- litri* (4to, Rome, 1787). Pope Pius VI. com- missioned him to explain the obelisks, and in 1800 appeared his great work, De Origine et Usu Obeliscorum, bearing the date of 1797. In 1798 he was made consul general for Denmark in the Papal States; and in 1802 he was ap- pointed professor in the university of Kiel, but never performed the duties of this office, though he received the salary. After this he published a catalogue of the Coptic manuscripts in the library of Cardinal Borgia, and also an account of the antique bass reliefs still remain- ing in Rome, under the title of / bassi-rilievi antichi di Roma, incisi da Tommaso Piroli (2 vols., Rome, 1808). This was translated into German by Welcker (Giessen, 1811-'12), who