In the year 1847, Dr. C. L. Gerling, a distinguished mathematician of the Marburgh University, suggested the importance of a new determination of the sun's parallax by observations upon Venus at and near her stationary periods. The determination of the dimensions of the solar system rests entirely upon the assumed value of the sun's parallax. The value now generally received, viz., 8′′.57, rests upon the observations of the transit of Venus in 1769. Transits of Venus over the sun's disc afford the best method of determining this parallax; but these phenomena are of very rare occurrence, there being not a single transit visible in any part of the world from 1769 to 1874. Now, although the observations of the transit of 1769 are believed to have afforded a very accurate value of the sun's parallax, yet it is much to be regretted that the results obtained by combining the observations at different stations two and two, differ among themselves by an entire second. It is therefore very desirable that this result should be verified by independent methods. Such methods are found in simultaneous observations of either Venus or Mars from two remote points of the globe. If an astronomer in a high northern latitude observes the position of one of these bodies when upon his meridian, and another astronomer in a high southern latitude does the same, a comparison of these two observations will give the parallax of the planet, from which we can compute its distance from the earth.

It has long been known that among the fixed stars are several which experience a periodical increase and diminution of brightness. The star Omicron, in the constellation Oetus, sometimes appears as a star of the second magnitude, but continues of this brightness only about a fortnight, when it decreases for about three months, till it becomes completely invisible to the naked eye, in which state it remains about five months, and then increases again to the second magnitude, the interval between its periods of greatest brightness being about eleven months. The star Algol varies from the second to the fourth magnitude, going through its changes in less than three days. More than forty such cases have been noticed, although in many of them the change of brightness is not very remarkable.

In the case of a few stars, remarkable changes of brightness have been observed, which have not been reduced to any law of periodicity. The star η (eta) Argûs is of this kind. This is a star of the southern hemisphere in right ascension 10h. 39m.; south declination 58° 54′. In Halley's catalogue, constructed in 1677, it is marked as of the fourth magnitude; yet in Lacaille's in 1751, and in subsequent catalogues, it is recorded as of the second magnitude. In the interval from 1811 to 1815, it was again of the fourth; and again from 1822 to 1826 of the second magnitude. In 1827, it increased to the first magnitude; it thence receded to the second, and so continued until the end of 1837.

This comet was discovered on the 1st of October, 1847, by Miss Maria Mitchell, of Nantucket. As a relaxation from the severer toil of a systematic course of observations, she had employed the intervals through the preceding year in sweeping for comets; but her labors had hitherto been only rewarded by a familiarity with comet-resembling nebulæ, which she had constantly and carefully recorded. The instrument employed on these occasions was a forty-six inch refractor, with an aperture of three inches, mounted on a tripod, and furnished with a terrestrial eye-piece of moderate power. On the evening of October 1st, a circular nebulous body appeared in the field of the telescope, a few degrees above Polaris. There was scarcely a doubt of the cometary character of this object, inasmuch as the region which it occupied had frequently been examined. Still, as the object was faint, and the weather uncommonly clear, a possibility existed that this too was a nebula not before observed. On the evening of the 2d, its change of place was manifest. No appearance of condensation of light toward its center, nor any indication of a train, could be detected. It is evident that its apparition, even to the telescope, was sudden. Its first apparent motion was inconsiderable, and the region of its discovery had been constantly swept over by the assistant observer at Cambridge, with his excellent comet-seeker, even as late as the previous evening. This idea is strengthened by the subsequent rapid increase of the brilliancy of the comet, and the acceleration of its apparent motion.

Various attempts have been made in this country to manufacture both reflecting and refracting telescopes. I shall speak of each of them in succession.

REFLECTING TELESCOPES.

A great many reflecting telescopes have been constructed by amateur astronomers in different parts of the country; but, for the most part, these attempts have been but moderately successful, and have contributed but little, if any thing, to the progress of science. The most important exception to this remark was in the case of a telescope manufactured in 1838, by Messrs. Smith, Mason and Bradley, the two former gentlemen being at that time students of Yale College. This telescope had an aperture of twelve inches, and a focal length of fourteen feet. The mirror was cast, ground, and polished by their own hands. Stars of less than one second's distance, were separated by this instrument; the faint star, “debilissima,” near ε Lyræ, was easily shown; and the nebula in Hercules, between η and ζ, was resolved into an immense number of small stars. With this instrument, Mr. Mason made some very accurate observations of three nebulas, of which an account is given in the Transactions of the American Philosophical Society. This paper affords but a foretaste of what might have been anticipated from the talents of Mr. Mason, had not his course been arrested by his premature death, which occurred Dec. 26th, 1840.

Several mechanics have undertaken the manufacture of reflecting telescopes for sale, but the only one who has pursued this business to any great extent is Mr. Amasa Holcomb, of Southwick, Massachusetts. Mr. Holcomb first attempted the grinding and polishing lenses about the year 1826.

Among astronomical publications in this country, the translation of La Place's Mecanique Celeste, by Bowditch, deservedly holds the first rank. Although in name merely a translation of a foreign book, with a commentary, it has many claims to the character of an original work.

The observations made by Lieutenant Gilliss at Washington from 1838 to 1842, have been published by order of Congress, and form an octavo volume of 672 pages. Three volumes of observations, made at the Naval observatory at Washington, have been published. The observations for 1845 constitute a quarto volume of 550 pages, with 13 plates; the observations for 1846 constitute a quarto of 676 pages; and the observations for 1847 constitute a volume of 480 pages, accompanied by 44 plates, showing a series of observations of solar spots by Professor Sestini, made at Georgetown observatory.

In 1852 was published No. 1 of the “Annals of the Georgetown Observatory,” being a quarto volume of 216 pages, chiefly occupied with a description of the building and instruments.

In 1855 was published Vol. I., Part II., of the “Annals of Harvard College Observatory,” being a quarto volume of 416 pages, containing a catalogue of 5,500 stars situated between the equator and 0° 20′ north declination.

With the preceding exceptions, the American contributions to astronomical science are to be found in periodicals and the transactions of scientific societies.

The Transactions of the Royal Society of London contain some observations by American astronomers before the Revolution.

Very extensive surveys have been undertaken, at the expense of the general government, and some by State governments, which have indirectly contributed very much to the science of astronomy. Of these, the survey of the coast of the United States is the most important.

The survey of the coast was proposed by Mr. Jefferson, and was authorized by Congress in 1807. Mr. Gallatin, then Secretary of the Treasury, sketched the plan of a magnificent geodetic work in which the principal headlands of the coast should be fixed by astronomical observations. Jn consequence of the unsettled state of the country, no active steps were taken toward carrying this plan into execution until 1811, when Mr. Hassler was placed in charge of this work, and was sent to Europe to procure the requisite instruments. He did not return with the instruments until the fall of 1815. In 1816 he commenced the survey; and in 1818, Congress not being satisfied with the progress of the work, it was stopped.

In 1832, the work was revived by an Act of Congress, and placed under the direction of Mr. Hassler, in whose hands it made steady progress until his death in 1844. Professor A. D. Bache was then appointed to take charge of the survey, and has continued it to the present time.

The astronomical part of this survey consists in determining the latitude and longitude of the stations, and the direction of the sides of the triangles with reference to a meridian.

Professor Bache has undertaken to determine the difference of longitude between Greenwich and the most important points upon our coast, with the greatest possible precision.

Uranus was discovered to be a planet by Sir William Herschel in 1781, and in 1787 he discovered two satellites, whose periods were satisfactorily determined by his subsequent observations. In 1797 he announced the discovery of four additional satellites, viz., one within the orbits of both the former two; one intermediate between the two; and two exterior to both of them, but the periods of these satellites he acknowledged to be very uncertain. In his last paper on this subject, communicated to the Eoyal Society in 1815, he says, “that there are additional satellites, besides the two principal larger ones, I can have no doubt; but to determine their number and situation, will probably require an increase of illuminating power in our telescopes.”

In 1834, Sir John Herschel published a paper containing a thorough discussion of his father's observations, together with his own, upon the two satellites first discovered; and he adds, “of other satellites than these two, I have no evidence.”

In the year 1838, Dr. Lamont, of Munich, published a few observations of the two brighter satellites of Uranus, and states that he had seen only one additional satellite, and that but in a single instance. This satellite he considered to be the most remote of the six enumerated by Herschel.

With the exception, therefore, of the solitary observation of Dr. Lamont, the only evidence we have had (until recently) of the existence of more than two satellites of Uranus was derived from the observations of Sir William Herschel; and he would not pronounce a decided opinion as to their number or their periods of revolution.

On the 22d of August, 1844, Father De Vico, director of the observatory at Rome, discovered a telescopic comet in the constellation of the Whale. He immediately announced the discovery to Professor Schumacher, of Altona, but his letter did not arrive till the 26th of September. Meanwhile, the comet had been discovered independently by several different observers. It was seen by Professor Encke at Berlin on the 5th of September, and on the 6th it was seen at Hamburg by M. Melhop, an amateur astronomer. On the 10th of September it was discovered by Mr. H. L. Smith of Cleveland, Ohio, who observed it every day for nearly a fortnight. About the third week in September, it was just discernible with the naked eye, and with slight optical aid had a very beautiful appearance, the nucleus being bright and star-like, and having a tail about one degree in length, extending in a direction opposite to the sun. At the Pulkova observatory, the comet was followed till the 31st of December.

It was soon found by M. Faye and others, that the comet deviated remarkably from a parabolic orbit; and it was ascertained that the curve described was an ellipse with a periodic time of about five and a half years. Dr. Brünnow (formerly of Berlin, but now director of the observatory at Ann Arbor, Mich.), undertook a thorough investigation of all the observations, embracing a period of more than four months, and took account of all the planets within the orbit of Uranus. He thus obtained an orbit which satisfied all the observations with extreme precision, and indicated that the length of the comet's revolution was 1996.5 days, or 5.4659 years.

Seventy-five years since, the only planets known to men of science were the same which were known to the Chaldean shepherds thousands of years ago. Between the orbit of Mars and that of Jupiter, there occurs an interval of no less than 350 millions of miles, in which no planet was known to exist before the commencement of the present century. Nearly three centuries ago, Kepler had pointed out something like a regular progression in the distances of the planets as far as Mars, which was broken in the case of Jupiter: Having despaired of reconciling the actual state of the planetary system with any theory he could form respecting it, he hazarded the conjecture that a planet really existed between the orbits of Mars and Jupiter, and that its smallness alone prevented it from being visible to astronomers. The remarkable passage containing this conjecture is found in his Prodromus, and is as follows: “When this plan, therefore, failed, I tried to reach my aim in another way, of, I must confess, singular boldness. Between Jupiter and Mars I interposed a new planet, and another also between Venus and Mercury, both which it is possible are not visible on account of their minuteness, and I assigned to them their respective periods. In this way I thought that I might in some degree equalize their ratios, which ratios regularly diminished toward the sun, and enlarged toward the fixed stars.”

But Kepler himself soon rejected this idea as improbable, and it does not appear to have received any favor from the astronomers of that time.