time dropping a time-ball erected by the Western Union Telegraph Company on their main building in New York City. The ball is dropped at New York noon, for the benefit of navigators and others. This ball has been in operation for four months without a single failure. It is always within 0.5 of the truth, and every day its error is published in the New York papers, so that it is practically a perfect signal.

A similar one is to be erected at Baltimore at the expense of the Baltimore Board of Trade.

The Observatory of Harvard College offers to supply standard time to railways and others in New England in extension of its present system, which is already widely useful.

The Navy Department has printed an important paper on the rates of chronometers as affected by temperature, by Lieutenant-Commander C. H. Davis, U.S.N., and the same subject is elaborately treated by MM. de Magnac and Villarceau in "La Nouvelle Navigation."

INSTRUMENTS AND OBSERVATORIES.

Astronomical Instruments.

Alvan Clark and Sons, of Cambridgeport, have just com. pleted an 11-inch photographic refractor for the Lisbon Observatory. It can also be used for visual purposes. The general design of its mounting is very stable and elegant. They have also finished the objective of a new 93-inch equatorial for Princeton College. It is constructed on Gauss's curves, and is said to be very fine and to have decidedly less outstanding color than the ordinary forms of this aperture. The crown-glass is capable of being rotated in the cell of the flint, and is thus separated from it. In this way it is intended to adapt this objective to photographic work. No crown-glass has yet been ordered for the 27-inch flintglass belonging to Yale College, and the M'Cormick 26-inch glass is still in the workshop, although it is fully completed.

Mr. Howard Grubb, of Dublin, publishes an important paper on the great telescopes of the future, in which he discusses, first, the advantages of each class of instrument, and, second, the effect upon these advantages of increasing

the size. He gives the considerations which indicate in what respect important advances in the art of instrumentmaking are to be looked for. First, Mr. Grubb says that beyond an aperture of 35.435 inches reflectors will have the advantage over refractors. Refractors have the advantage in their "greater permanence of collimation, and consequent suitability for ordinary observatory work, and for measur ing purposes," as also in permanence of the optical parts, in the fact that they have no central mirror to disturb the course of the rays, and the comparatively slight effect of air-currents upon them, since the tubes can be closed at both ends. The advantages of reflectors are absence of a secondary spectrum, better applicability for work in celestial photography, photometry, spectroscopy, etc.; the possibility of supporting them with perfect freedom from flexure irrespective of size (perhaps this conclusion of Mr. Grubb will not be generally accepted), and their general convenience for observing purposes. Mr. Grubb looks with most hope to large metal specula for the best results.

The question of the relative goodness of reflectors and. refractors has some light thrown upon it by recent observations of the satellites of Mars. These were both easily seen and measured in a refractor of 12 inches aperture. The outer satellite has been certainly seen with a refractor of 7 inches. With an 18-inch silvered-glass reflector, Key, of Hereford, was barely able to see the outer satellite when its exact position was known. At Marseilles the outer satellite has been observed with a refractor, but neither of the two has been seen with the reflector of 31 inches aperture, "on account of diffused light in the field." The silvering of this mirror has, however, deteriorated through age. With the 6-foot reflector of Lord Rosse the outer satellite alone was seen up to September 20, and "not well enough to measure it." At Melbourne neither satellite was seen with Mr. Grubb's 4-foot reflector, and we have no account of observations with Mr. Lassell's 2-foot reflector. As far as has been reported, not a single reflector has even seen the inner satellite.

At the same time Grubb also publishes his new illustrated catalogue of instruments, domes, etc., which is really an important addition to the literature of the subject. The cata

logues of two American makers, Buff and Berger, of Boston, and Fauth and Co., of Washington, are noteworthy in this connection. The latter firm has lately proposed to make a transit-circle for Princeton College, which, when completed, will be the first meridian instrument of large size made in the United States. A transit instrument for the same college has been completed by Kahler of Washington.

The 18-inch refractor for Strasburg has had several objectives made for it by Merz, among which a choice must yet be made, and until then work on the mounting is stopped.

Grubb, of Dublin, has just completed two 8-inch equatorials for Berlin and Dresden, in which he has incorporated his latest improvements with regard to illumination of field, wires, etc. One reading microscope suffices to read both declination and right-ascension circles.

The Naval Observatory of Washington has recently published a description of its principal instruments, with plates. It is Appendix I. to the Observations for 1874.

Titano-silicic glass prisms have been examined by Professor Stokes and Dr. Hopkinson, and the hopes that had been entertained of the utility of this glass in the correction of the secondary spectrum were not fulfilled. The phosphatic glass of Harcourt, while a success in this respect, is too soft for use in optical glass, and the new glass, in which a portion of the phosphorus was replaced by titanium, would have been suitable in this respect. The question is, then, as far from a practical solution as ever.

Experiments on the electro-static capacity of glass did not bear out Maxwell's conclusion as to the relation between the refractive index for long waves, the electro-static capacity, and the magnetic permeability.

The paper on the refractive indices of glass refers mostly to glasses which are articles of commerce, and hence is of immediate value. Specimens of hard crown, soft crown, titano-silicic crown, extra light flint, light flint and dense flint, extra dense flint, and double extra dense flint were examined, and an expression for the irrationality of dispersion of each of these glasses compared with a standard is obtained and tabulated. This table shows how little there is to choose between the glasses ordinarily used.

New Observatories.

Dr. Lohse contributes to the Astronomical Register for August an account of the Astrophysikalischen Institut, now building at Potsdam. It is on an elevated site, and the grounds contain 179,000 square meters. There are at present finished four dwelling-houses (three for observers and assistants) and the machinery-house. The observatory proper is in progress, and will be completed during 1877. One part of the scientific establishment is already completed, viz., a well of forty-six meters deep, with horizontal shafts connected with it. This is to serve for observations where a constant temperature is required, for observations on the temperatures of the soil, etc. The observatory will have three domes—a central (to contain a 12-inch equatorial by Schröder), a western (to contain an 8-inch by Grubb), and an eastern (to contain a 5-inch). A photoheliograph will be erected north of the central tower, and the physical, chemical, and photographic laboratories will be suitably placed in the main building. The work undertaken will be spectroscopic observations of the sun and stars, observations of the nebulæ and double stars, on the physical nature of the planets, etc., photographic researches of all kinds and photographic registration of sun-spots. The observatory is managed by a "Direction" of three members -Auwers, Förster, and Kirchhoff. At present there are three astronomers-Vogel, Spörer, and Lohse.

The Wilna Observatory was destroyed by fire on December 28, 1876. In spite of strenuous efforts, only some of the books and smaller instruments were saved. The refractor and the photoheliograph were totally destroyed. This is much to be regretted, as we owe to Wilna a large number of excellent photographs of the sun, a regular series of which was kept up. It is to be hoped that the negatives of these photographs have been preserved.

Ex-Governor C. C. Washburn intends, during the next year, to erect and equip an astronomical observatory for the University of Wisconsin. This gift will be made available by an annual appropriation for its support from the state. The firm of Clark and Sons are now making a 16-inch equatorial for this observatory.

The Troy Polytechnic School is fitting up an observatory in connection with its courses of study.

A new observatory has been founded at Lyons, France, of which André has been named director. Its meridiancircle was presented by M. R. Bischoffsheim, of Paris.

The observatory at Kiel is now in its new building, and has lately received a new refractor, by Steinheil, of eight inches aperture. Its meridian circle is engaged in observing a zone of stars of less than 10° N. P. D.

A sum of $12,000 has been devoted by Oxford University to the building of additions to the observatory.

Professor Young has accepted the chair of astronomy at Princeton. He will have a large telescope at his disposal.

The third volume of André and Rayet's Astronomie Pratique (History of Observatories) is concerned with the observatories of the United States, and will be found a useful book of reference. It is compiled from the notes of M. Angot, one of the editors. It supplements the older works of Loomis and Mailly.

Publications and Reports of Observatories.

The report of the Secretary of the Navy for 1877 contains that of the Superintendent of the Naval Observatory, which gives an account of the work of the past year. The 26-inch equatorial continues to be used in the observations of the faint satellites. The transit circle, besides its regular work of observations of the sun, moon, and major planets, has made a very large number of observations of asteroids, and is also engaged in the formation of a catalogue of the B. A. C. stars between 120° 0' and 131° 10' of N. P. D. The old meridian instruments are in use for completing Yarnall's catalogue, of which a second edition is in preparation. The investigation of the moon's motion is continued. The transit of Venus reductions are in progress. The photographs of the transit are now measured. The division errors of the ruled-glass scale micrometer have been carefully determined.

The bill introduced in the United States Senate, to provide for the removal of the Naval Observatory, directs the Secretary of the Navy to appoint a commission of three members to select a new site within the District of Columbia, which shall possess the advantages of healthfulness,