EVERY now and then the papers announee the discovery of a new planet. The occurrence, indeed, has beeome so frequent of late as to attract very little notice except from professed astronomers, and they, to speak the truth, are none too cordial in the welcome of the little strangers thus added from time to time to the already embarrassingly large family under their charge. For while these new planets are quite as troublesome to provide with ephemerides and orbits as their larger sisters, they are extremely insignifieant as regards their importance in the economy of the solar system, being seldom more than forty or fifty miles in diameter, and, as individuals, entirely without sensible influenee upon the motions of other heavenly bodies. We say as individuals, because they belong to a numerous group known as the Asteroids (so called beeause they look like little stars), and the united attraetion of the whole family does produce upon the orbit of Mars a perfectly sensible, though very minute, effeet. from whieh Leverrier has eomputed that the combined mass of the whole flock would suffice to make up a globe not exceeding one-third the size of the earth, and probably a good deal smaller. At present the number of these bodies known is 172: the whole number is probably to be reckoned by thousands, since it would take more than five hundred of the largest of them to make up the mass named. Ceres, the first of them, was discovered on the first day of the nineteenth century, that is, January 1, 1801. It had been notieed nearly two hundred years before, by Kepler, that the progression in the numbers representing the distances of the planets from the sun is such as to suggest the existence of an invisible body in the space between Mars and Jupiter, and he at one time went so far as to prediet its discovery. He abandoned the idea, however, supposing that in his celebrated, but fantastic, theory of the polyhedrons he had found the key to the mysteries of the planetary system. Ti- tius, in 1772, revived the original suggestion, and gave to the law of distanees nearly the same form as that in which it was stated a few years later by- Bode. * In 1781, Uranus was diseovered at a distance from the sun so closely eorresponding with the law as to satisfy astronomers that it could be no mere accidental coincidence; and to confirm them in the belief that there must be a missing planet outside of Mars. The impression was so strong that an organization of twenty-four astronomers was formed by the exertions of Baron Zaeh, to search for it. Curiously enough, however, the good fortune of discovery did not fall to any one of their number, but to Piazzi, the Sieilian astronomer, who, on the opening night of the eentury, in the eourse of observations for his famous star-eatalogue, came upon a star of the seventh magnitude, in a place where a short time before he was sure no such object existed. In a single day its motion was sufficient to prove its planetary ebaracter, and he continued to observe it, though mueh hindered by ill health and unfavorable weather, until it was lost in the rays of the sun. He was the only observer, however, for in those days communication was so slow that the planet had disappeared before the Continental astronomers could be notified of the discovery; and to find it again was hardly less difficult than at first. Gauss, then just beginning his eareer, came to the reseue with a new and entirely original method, by which * The law is generally, but unjustly, known as Bode's, and may be stated thus: set down a row of 4's, as many as there are planets. To the second 4 add 3; to the third 4, twice 3; to the fourth, four times 3; and bo on, each time doubling the number added. The result is as follows: s ffi t Asteroids. 2! 0 tp 4 4 4 4 (4) 4 4 4 4 3 6 13 28 28 J 96 193 as! A 4 7 10 16 52 100 196 87 B 4 7 10 15 53 85 102 30 The line A gives the numbers derived from the rule, and the line B the actual distances, correct to the nearest unit. While the agreement is generally close, it will be seen that in the case of Neptune the law breaks down completely; but Neptune was not discovered until 1846. LONTIN'S IMPROVEMENTS IN DYNAMO-ELECTRIC ENGINES.from Piazzi's six weeks of observation he deduced the planet's orbit and computed an ephemeris by means of which Zach rediscovered it on December 31st, and Olbers, independently, on January 1st. In searching for Ceres, Olbers had noted carefully the configuration of telescopic stars in that part of the sky where he expected to find her, and on re-examining the region a few weeks later he was so fortunate on March 28, 1802, as to discover another planet, Pallas. The existence of two of these little bodies suggested the hypothesis that they originated in the breaking up of a much larger body, of which, probably, numerous fragments must exist which might be found by careful search near the points where the orbits of Ceres and Pallas intersect. A search was instituted, and in 1804 Juno was discovered by Harding, and in 1807, Vesta, the only one ever visible with the naked eye, by Olbers. The hunt was kept up until 1816, but without result, as the observations did not include stars sufficiently faint. About 1830, Hencke, postmaster of the little village of Driessen, took up the subject, and after fifteen years of patient searching was rewarded by the discovery of Astrrna in December, 1845. The year 1846 was sufficiently signalized by the discovery of Neptune; but since then not a year has passed without adding to the roll of the Asteroids. In 1861 and 1876 each, 10 were discovered; in 1872, 11; in 1868, 12; and in 1875, 17. The list of discoverers includes 31 different names: 14 of them stand credited with a single planet each, and 10 with 5 or more apiece. Dr. Peters, of Clinton, New York, heads the list with 26; Luther, of Diisseldorf, comes next with 20; then follows Watson, of Ann Arbor, with 19, and Gold- of Paris, with 14. Fifty-two of these planets were discovered in France, 51 by Amcrican observers, 39 by Germans, 19 in England and its dependencies, and 11 in Italy and Sicily. The orbits of these bodies have an average radius of not far from 250 millions of miles, with a corresponding period of a little less than five years. But individual orbits differ widely from these figures. Thus Flora, the nearest to the sun, has a period of only 1,193 days—a trifle over three years and a quarter—and a mean distance of only 202 millions of miles; on the other hand Hilda, the most remote, has a period of 2,868 days, or very nearly 8 years, and the radius of her orbit is more than 360 millions of miles. The orbits of the large planets are all nearly circular; many of the asteroid orbits, on the contrary, are very eccentric, resembling those of comets. Thus 1Ethra has an eccentricity of 0'38, which amounts to saying that her least distance from the sun is considerably less than half her greatest. The inclinations of some of their orbits are no less remarkable, that of Pallas being more than 34°, while that of Mercury is only 7°, and cvcn that is altogether exceptional among the older planets. As has been said before, the asteroids are very minute, too small, indeed, to have their diameters determined with any certainty by direct measurement; we are limited to approximate results obtained by comparing their apparent brightness with that of planets whose size and distance from the sun are known. If we knew the reflecting power of their surface—their albedo as it is called—we could thus arrive at reliable conclusions; but wanting this element and being obliged to content ourselves with the mere assumption that this albedo does not differ much from that of the planet Mars, values inferred in this way must be accepted with a good deal of reserve. Littrow, Lespiault, and others have investigated the matter, and find that the diameters of the larger ones range from 300 to 150 miles, while the smaller ones lie between 15 and 30. They are so small that a good walker could easily make the circuit of one of these microscopic globes in a single day, and unless their density is much greater than that of any of the other planets, the force of gravity must be several hundred times less than that on the earth's surface. A stone thrown from a boy's sling would fly off into space, never to return. We have spoken of them as globes, but certain otherwise unexplained variations in the brightness of some of them, especially Pallas, have suggested the idea that they may be irregular pieces of rock rather than spheres. As to their origin two theories are held: one, that they are the fragments of an exploded planet, the other, that the ring of nebulous matter, which in different circumstances would, according to the nebular hypothesis, have formed a single planet like the -others in the system, was in this case broken up, mainly by the action of the great planet, Jupiter, just outside. If the first hypothesis be so modified from that proposed by Olbers as to introduce the idea of a number of disruptions, first of the original planet, and afterwards of its fragments, it becomes perhaps as tenable as the second, and there would seem to be at present no means of' deciding between them. The discovery of these bodies has hitherto been affected simply by patient and assiduous search. The asteroid hunter provides himself with charts of portions of the sky about 2° square near the ecliptic, so choosing his “preserves” as to have some one of them in convenient position for observation at all seasons of the year. On the chart he marks down all the stars visible with his instrument. The principal labor lies in preparing the charts; these once made, any interlopers are readily detected, and if planets (and not merely variable stars) their motion will reveal their character in a very few hours. The only remaining difficulty is to be sure that the object is really a new planet, and not one of the old ones, for it has happened more than once that a discovery announced with something of a flourish has had to be given up as a mistake for this reason. Hitherto there has been comparatively little difficulty in the matter, because the “Berlin Astronomical Year Book “has published each year ephemerides of all the planets whose opposition occurs during the year. But the labor and expense of the calculations has become so great on account of the increasing number, and the results are of so little importance to general astronomy, that it has been decided to give them up partially. and the ephemerides for 1877 contains the places of only 50 out of the whole 125 which come to opposition this year. This will often render it necessary, when a supposed new planet is found, to go through a long and laborious computation in order to make sure that it is not one of those already known. It is to be expected, therefore, that unless this difficulty is somehow met the number of annual discoveries will greatly diminish. The race between the planet hunters is frequently quite exciting. It has happened several times that the same planet has been discovered by two or more independent observers on the same evening, and both Goldschmidt and Peters have been so fortunate as to discover pairs of planets at a single sitting; the latter has done it twice. While these planets are personally, so to speak, of trifling account, very valuable results are obtainable from the study of their motions. An excellent determination of the solar parallax has been deduced by Galle from observations on the opposition of Flora. The most reliable value of the mass of Jupiteris that derived from the perturbations he produces upon the orbits of some of them. One or two cases of great prospective interest are presented, where the orbits of two of these bodies so closely coincide as to render it quite possible that some time they may, if they do not, actually collide, come to move around each other in an oval orbit like that of a double star. Minute as they are, they are not to be despised, and it is more than probable that in some way, though as yet beyond prediction, they will repay the labor spent upon them. Very few scientific facts remain forever barren.—Boston Journal of Chemistry.