being purely hydrogen, have nothing to do with these solar spots. In consequence of this being a year of minimum of solar spots and protuberances, Father Secchi thinks he finds therein the explanation of Professor Langley's result, since the latter gentleman has observed that at present there is no difference of temperature between the sun's equator and its poles.-6 B, LXXX., 1273.

THE SOLAR OBSERVATORY AT POTSDAM.

Some account of the new German solar observatory estab lished at Potsdam, near Berlin, is given in Klein's Wochenschrift, from which we gather that this institution is situa ted upon a considerable elevation, and will consist of a group of several isolated observing domes, each to contain its own telescope. Of these domes, the southern one, having a telescope of eleven inches aperture, will be devoted to spectrum analysis, which observations will be applied also to the stars as well as the sun. In another dome the heliograph will be established, which will be devoted also to the photography of the sun, and possibly of the moon and stars. The western dome will contain a telescope of eight inches aperture for special investigations of sun spots and protuberances. The eastern dome will contain the telescope of seventeen feet focal length now employed by Professor Sporer for the study of the protuberances. North of these domes will be the principal building, devoted to optical, physical, and chemical researches, and north of it will be rooms for the assistants, micrometric apparatus, etc. Still north of this building will be one containing a large tank of water, in whose cellar magnetic apparatus will be established.

At the depth of one hundred feet under the ground a space will be provided for erecting a Zöllner's horizontal pendulum. Finally, the dwelling-houses are to be mentioned, which will accommodate quite a number of observers and officials. In connection with the prosecution of Professor Sporer's ob servations of sun spots and protuberances, it is stated that one of the most brilliant protuberances recently appeared in the polar region of the sun, where they are generally wanting. It shot forth in nineteen minutes, and indicated with certainty the presence of a violent storm at that point.Klein's Wochenschrift, 1875, 249.

SPECTROSCOPIC DIAMETER OF THE SUN.

At the recent meeting of Italian astronomers, experiments were made by Secchi, Tacchini, Rayet, and Dorna to determine the difference between the diameter of the sun as determined by the spectroscope and by the method of meridian transits. From the observations of ten days the spectroscopic semi-diameter was less than the transit semi-diameter by 0.124", and less than the Nautical Almanac semi-diameter by 0.104". Atmospheric conditions were found to be influential in these determinations.-2 G, November, 1875, 118.

IDENTITY OF COMET VII., 1873 (COGGIA), WITH COMET I., 1818 (PONS).

Dr. Weiss, of the Vienna Observatory, has investigated the orbit of the comet discovered by Coggia the 10th of November, 1873. As he has previously shown, there is scarcely a doubt that this comet is identical with the comet discovered by Pons in 1818, and in this case the determination of the period of revolution is quite important. Dr. Weiss finds that there are three possible periods, viz., 55.82 years, 18.61 years, and 6.20 years. Of these three, the second is the least probable; and of the first and third, Dr. Weiss considers the period of 6.20 years the most probable for the present, although his computations are to be carried farther as soon as he shall have received further observations of this body during its last appearance.-38 C, No. 2072.

A NEW MERIDIAN INSTRUMENT AT THE OBSERVATORY AT

RIO.

M. Liais, Director of the Observatory of Rio, describes a new arrangement of his mural circle (made by Dollond), which is intended to convert it into an instrument suitable for determining right ascensions. Within the tube of the mural-circle telescope, M, is fixed a plane mirror, which is perpendicular to the axis of rotation of the mural circle. At ten feet from the circle, and looking toward it, is a telescope with high magnifying power, A. Close to A is a mark (mire) which can be moved either vertically or horizontally by means of two micrometer screws of known value. If M is in any given position, the mark can be placed, by means of

its micrometer screws, so as to be seen on the cross-threads in the focus of A. If the mirror is truly perpendicular to the axis of the mural circle, and that axis is perfectly regular, the telescope, M, might be turned in any position, and the mark would still be on the cross-threads of A. If, however, in a given position of M the mark is not on these threads, the micrometer screws of the mark will serve to bring it back to these threads, and in this way it may be determined how far the normal to the surface of the mirror is displaced horizontally and vertically for each position of M. In this way the position of M may be referred to a vertical plane passing through the north and south collimators of the mural circle. The ordinary formulæ for the transit instrument will serve for this purpose if, first, the angle formed by the optical axis of M with the fixed vertical plane (if the mirror is put into that plane) be substituted for the error of collimation; second, if for the error of azimuth be substituted the angle between the vertical plane through the normal to the mirror and the vertical plane perpendicular to the plane of reference; third, if for the error of inclination be substituted the angle between the normal to the mirror and the horizon.

These quantities can be determined from the micrometers of the mark, first, when the telescope, M, is directed on the two collimators, and, second, when directed to the star observed. In this way the observations are reduced to the plane of reference, and this can be referred in the ordinary way to the meridian.

The south collimator at Rio is a mark in the focus of a long-focused objective. The north collimator is the axis of the prime vertical transit instrument, which is itself provided with collimators. Thus the meridian instruments of this observatory form one connected system. The device of Liais serves also to investigate the flexure of the telescope M.6 B, LXXXII., February 28, 1876, 495.

THE SOLAR ECLIPSE OF APRIL 16, 1874.

Mr.

Mr. E. J. Stone, Astronomer Royal at the Cape of Good Hope, observed the total solar eclipse of April 16, 1874, at Klipfontain, Africa, fifty-five miles from the coast. Stone's plan of campaign was necessarily simple, as the difficulties of travel in South Africa prevented the transpor

tation of delicate appliances, but the points to which he directed his attention were, although few, perfectly definite. By circulars distributed throughout the colony, instructions were communicated as to the proper methods of making drawings of the corona, and several important drawings were received, which are carefully studied by Mr. Stone. They exhibit the usual marked discrepancies in most cases, notably in two made by two expert draughtsmen seated side by side. at the same table; but Mr. Stone gives a satisfactory explanation of the differences here, which may serve to account for similar discrepancies in former drawings. Mr. Stone summarizes the results of his expedition as follows:

"1. A confirmation of Young's observation of the general, or nearly general, reversion of the Fraunhofer lines in the spectrum of the corona near the photosphere.

"2. A spectroscopic examination of the outer corona, in contradistinction to the inner corona, carried to the extent of rather more than a degree from the sun's centre, which has proved that the spectrum of the outer corona consists of a linear spectrum of one bright line, either exclusively or sensibly, whose wave length is 5.312, with a unit of one tenth of a meter, and of an ordinary sunlight spectrum with absorption lines. The spectrum of the outer corona has been shown to fade gradually away, as the extreme visible limit of the corona is approached, and not to disappear sharply, as if the extreme limit of the corona had been reached.

"3. The spectroscopic examination of the outer corona, combined with the unchanged character of its principal features, as seen at [three different observing stations] at intervals of absolute time extending to ten minutes, and at distances of more than 500 miles, proves, I venture to think, the solar origin and cosmical character of the outer corona.

...

"4. A comparison of the drawing of Mr. Henry Hall and the photographs obtained in 1869 and 1871, shows the permanent character of the contraction of the inner corona in a direction parallel or nearly parallel to the sun's axis of rotation. The strongly marked character of the general contraction of the outer corona in the same direction may not improbably ultimately lead to a similar inference in the case of the outer corona also."-Memoirs Royal Astronomical Society, XL., 11.

PHOTOGRAPHS OF THE SOLAR CORONA.

Professor Zenger, of Prague, announces a success which, if confirmed, will be of high importance. By using a new form of "achromatic lens in combination with mirrors of large aperture and very short focal length, so as to reduce the focal length to four times the aperture," Professor Zenger claims to have photographed the solar corona with the protuberances on it, and also to have examined the corona through the telescope. He describes its appearance to the eye as "a a bright circular ring . . . on an average one minute in height, and only in exceptional cases of nearly two minutes, and an eccentricity amounting to only two or three seconds of arc."

DISTRIBUTION OF PUBLIC TIME.

The importance of this subject, which is now attracting general attention, will render the subjoined list of the various time-balls, time-guns, etc., of the world, valuable. It has been compiled from the best sources available, but corrections and additions will no doubt be necessary in many cases, as the materials for the compilation are somewhat limited.

List of Time-balls, Time-signals, etc.

EUROPE.

Helsingfors.-There is a time-ball dropped at noon at the observatory, and a gun fired either on the guard-ship or

ashore.

Bergen.-A time-ball is dropped daily at noon.

St. Petersburg-Several public clocks in the city are controlled from the Pulkova Observatory, and a time-ball (or time-gun) makes a signal at noon daily.

Kiel.-Time-ball (or time-gun) for shipping.

Dantzig.-Time-ball at local noon and at Greenwich 14. Hamburg.-Time-ball (or time-gun) for shipping. Paris. The public clocks are controlled from the Paris Observatory.

Lisbon. A time-ball is dropped from the observatory at 14 Lisbon mean time, and one is dropped simultaneously on the south side of the river from Prayal flag-staff.