period of 2,100 years, and travels 229,000,000,000 of miles from the sun. The comet of 1811 has gone on a journey of 3,000 years; the comet of 1680 is expected to be absent 8,814 years; while the comet of July, 1844, is under pledge to report at the headquarters of our system after an absence of. 100,000 years; During this journey it travels to the distance of 368,000,000,000 of miles from the sun, and yet throughout the utmost limits of their flight these mysterious wisps of luminous vapor acknowledge every hour their allegiance to the central authority of our system. There is not a moment when the gentle influence of our sun ceases to be felt. Across the silent and measureless void the subtle power of gravitation manifests its presence as distinctly as in the falling acorn in the forest. These comets all move, of course, in elliptic orbits. A large majority of them possess a direct motion, and conform approximately to the plane of the ecliptic. These are circumstances which intimate an alliance with our system, as well as a subjection to the general laws of matter.* But there are other comets which venture beyond the boundary line which separates the empire of our sun from some contiguous dominion. From time to time these unrecognized strangers plunge, unannounced, into the midst of our planetary system, hurry with excited haste to the neighborhood of our solar center, whirl round their perihelion, and dart forth again into the abyss of space, never to return. There can be little doubt that the non-periodic comets pass within the influence of other solar centers, around which they swing with similar haste, to dart off again to the neighborhood of other suns. These strange comets move, while within the limits of our system, in accordance with the same laws as the periodic comets, and their places can be similarly calculated from day to day, and from week to week; but as their paths are parabolic and hyperbolic curves, their very flight from the local government of our system is as much the subject of mathematical demonstration as is the return of comets moving in elliptic orbits. It cannot be doubted that if they visit other suns it is to acknowl * All comets moving in hyperbolic orbits also have a direct motion, and thus furnish other intimations of some connection between their history and that of the planetary bodies. Delaunay, however, (Op. Cit., p. 644,) is of the opinion that all comets are either strangers, or only naturalized denizens in the solar system. edge still their allegiance to the supreme force of gravitation, and to travel in paths which might become as strictly the subjects of computation as the paths described by them outside the limits of our system.* Thus these wanderers range from sun to sun, from system to system, impelled ever by one force, regulated every-where by one government, weaving the visible universe into a compact web of indissoluble relationships. † (3.) Stellar Phenomena. Still other intimations reach us. Viewed with the telescope, many of the fixed stars appear double. We know at least 6,000 of them. Not less than 650 of the double stars are demonstrated to be real systems, physically connected, revolving about a common center of gravity between them. The periods of their revolutions range from 14 to 1,200 years. The following are examples: A pair in Coma Berenices is thought to have a period as short as 14 years; Zeta Herculis revolves in 36 years;. Sirius, in 49; Zeta Cancri, in 59; Xi Ursa Majoris, in 61; Mu Coronæ Borealis, in 66; Alpha Centauri, in 81; Pi Ophiuchi, in 92; Gamma Virginis, in 150; 61 Cygni, in 520; and Gamma Leonis is thought to have a period of 1,200 years. These, also, are real orbital motions, manifested at such distances that even light, traveling 186,000 miles a second, would require years to reach us. Not only that; these orbits are also ellipses. Now there can be no orbital motion except under the action of centripetal and centrifugal forces: gravity and inertia are there. And since the velocity of rotation—to give a centrifugal force equal to the centripetal-must increase with the intensity of the centripetal, that is, the gravitating force, physicists have been enabled to calculate even the weight of distant suns revolving about their centers. Thus, we know that the mass of 61 Cygni is one third the mass of our own sun; and that Alpha Centauri is one tenth the sun's mass. A remarkable illustration of the truth of that generalization which extends the laws of gravity to these depths in space has recently been furnished. Certain minute movements of Sirius, *Watson: "A Popular Treatise on Comets," pp. 328, 358, 359, 361. + "Toutes les parties de notre monde planétaire auraient donc une origine commune, et le système tout entier serait en communication avec les systèmes étrangers par l'intermédiaire des comètes et des météores.”—Secchi: "Le Soleil,” p. 383. the brightest star in our heavens, led Bessel to suspect the existence of a small companion-star revolving about it, and exerting a perturbating influence. Such companion was afterward, in 1862, actually discovered by Mr. Alvan Clark, an American observer. Not only do we find stars thus swinging about in couples, but, in other instances, we find them grouped in threes, fours, and even higher systems, all rotating about a common center. Thus, Theta Orionis, a star scarcely visible to the naked eye, in the celebrated nebula of Orion, resolves itself, under the telescope, into seven mutually connected suns. Another stellar phenomenon of great interest, though not as yet demonstrably explained, is that of variable stars. Though numerous cases of irregular variability are known, we refer here to such stars as increase and decrease in brightness through regular periods, like the celebrated stars Mira in the whale and Algol in the head of Medusa. Two explanations have hitherto been offered of such phenomena. Perhaps the periodical variations are due to the rotation of these bodies upon their axes, combined with unequal luminosity on different sides. Perhaps they are caused by partial occultations by dark planetary bodies revolving about them. Both these explanations equally presume the existence of movements in the depths of space, which can only be regulated by the laws of central forces which hold such imperial sway within the limits of our own. system. More recently, variability has been attributed by M. Fayet to the effects of different phases of refrigeration. Father Secchi, and after him M. Stanislaus Meunier,§ connects variability with the phenomenon of "spots" so well known as marking the surface of our sun, and, by their regular increase and diminution through intervals of about ten years, imparting a variable character to the solar light. These physicists main * Zöllner, Stewart, Klinkerfues. This explanation requires regularity in the periodicity of the stars. Faye: "Révue des Cours Scientifiques," t. iii, p. 617. § Meunier: "Op. Cit.," p. 160. tain that spots, periodicity and total disappearance of stars, are but different degrees of one phenomenon, and that they are caused by a diffusion or eruption of heated currents from the central non-luminous and more highly heated portion, through the photospheric, partially cooled envelope. The plausibility of this explanation rests on the truth of Secchi's theory (adopted and elaborated by Faye) of the gaseous and nonluminous condition of the solar nucleus. Should this explanation prove true, the) phenomenon of variability still serves as a link of connection between our system and the remotest regions of space, since, by all admissions, our sun is a variable star.* (4.) Firmamental movements. Nearly all the stars called "fixed" are in actual motion. They do not all move with uniform apparent velocity, nor in a uniform direction. These movements, moreover, are, to our eyes, extremely slow. To travel across a space in the heavens equal to the apparent diameter of the moon's disc, would require from 300 to 1,500 years and upward. Of course, nothing can be known of the actual velocity of these motions, except in the case of stars whose distances have been determined. The following are examples of these: Arcturus moves 54 miles a second; 61 Cygni, 40 miles; Capella, 30; Sirius, 14; Alpha Centauri, 13; Vega, 13; Polaris, 1. For purposes of comparison, we may state that the earth moves in her orbit with a mean velocity of nineteen miles a second; the other planets, with velocities varying from three to thirty miles a second; and the sun has a proper motion in space (as we shall presently explain) of about four miles a second. Thus the stars called "fixed," and which are used as points. of comparison for all our observations on the motions of the planets, are themselves in perpetual motion. In consequence of their unequal motions, certain stars will travel, in the course of time, out of the constellations with which astronomy has identified them for three thousand years. Isolated and unconformable movements among the stars * Proctor: "The Sun," pp. 197-9; Secchi: "Le Soleil," pp. 113-117; Chambers: "Desc. Astron.," pp. 14, 15; Loomis: "Sun Spots," etc., in "Amer. Jour. Sci.," [2,] vol. 1, p. 153, et seq. We are indebted to the patient observations of Schwabe of Dessau for this determination. might not clearly appear referable to the same physical causes as we find acting within the limits of our system. Proctor,* * however, has clearly shown that groups of stars are characterized by a common "drift." Not all the stars within a circumscribed space can be reasonably imagined to sustain a physical connection with each other, since many of those which, to our eyes, are most closely approximated, may be at enormously unequal distances. Such, however, as manifest a common motion, may fairly be regarded as moving under uniform conditions. Even this, however, does not demonstrate that those conditions are reproduced in the solar system; but the suggestion is probable. Again, our own sun is one of the stars manifesting a proper motion. This motion is revealed by the slow opening of the ranks of the stars in one part of the heavens, and their gradual closing together in the opposite quarter. Such apparent motions of the stars remain, after making all allowance for their real motions. Thus our sun is traveling onward, with all his retinue of planets and satellites, toward a point in the constellation Hercules, and at a rate of 153,000,000 miles a year. Astronomers are of the opinion that this proper motion of the sun is directed in an orbit whose center is in the Pleiades, and whose circumference is so vast that 18,200,000 years are required for a single revolution. This probable revolution of the solar system about a center within our firmament renders it probable that the proper motions of the other stars are also due to orbital movements about the same center. Thus we shall be led to contemplate the starry firmament with its 77,000,000 of suns, as an example of a solar system on a more stupendous scale. There is still another order of stellar existences to which attention ought to be directed. More than five thousand nebulæ have already been laid down on the map of the heavens. Their ordinary appearance is that of a faint luminous cloud spread on the dark background of the sky. Very many of these, subjected to the scrutiny of the telescope, resolve themselves into stars and "star-dust," or minute points of light. Others defy all power of resolution. It seems quite certain that some of the nebula lie within * Proctor: "Other Worlds than Ours," pp. 277-281; "The Sun," pp. 428-9. FOURTH SERIES, VOL. XXV.-13 |