each zone. Whenever desirable or necessary, however, one or more may be observed during the progress of the zone. In the selection of zero stars the peculiarities of the instrument must govern. In general the mean of their right ascensions should fall nearly on the mean of the zone. Where this cannot be attained, the observer must decide whether he can rely more upon a uniform clock rate (for the determination of right ascensions), than upon an unchanged position of the instrument in reference to the meridian. With very northerly stars the selection should be made more with regard to the declinations of the zero stars, as an error in the clock rate has less influence here. The mean daily clock rate must not be used in the reductions, but the rate resulting from the mutual comparison of the zero stars for the zone.

The individual observer must judge whether a single microscope will give the requisite security in declination. Whenever the stars to be determined are observed on fewer microscopes than the zero stars, a thorough investigation of the reduction between the different systems of microscopes must be made throughout each zone. Where this reduction shows large changes, the use of all four microscopes is advised, and also the introduction of zero stars in the progress of the zone. Changes in the equatorial point may be determined also by horizontal

collimators.

Where two share in the work, so that one reads the microscopes, and especially where only one microscope is read, the influence of the warmth of the observer's body must be carefully investigated and allowed for. (Consult Bonn Beob., II, p. viii).

In the observation of double stars the position of the mean is to be obtained. It is therefore advisable to extract from the Positiones Media the double stars occurring in a zone.

Should a star designated there as "simplex" appear double, in case of nearly equal brightness, the mean should be observed as well as the brighter one.

An ephemeris of variable stars will be published annually in the Vierteljahrschrift, to furnish information in regard to the most favorable times of observing them on the meridian.

There are many stars of the 90m in the D. M. which are fainter, and it is much oftener so with those marked A. The observer is enjoined to spare no pains in observing these stars, and should the observation not succeed in the course of the zone, note should be made so that it may be attempted under more favorable circumstances. Where variability may be suspected because a star is invisible or excessively faint, it is advisable to immediately inform the committee or those astronomers engaged on variable stars, that these stars may be care

fully attended to. A notice to the Bonn observatory would render it possible to scrutinize the original papers for any error.

Unless special circumstances prevent, the magnitude is to be carefully estimated at the observation of every star. It is desirable that the participants conform their scale of magnitudes to that of the D. M., unless some other scale is already in use; in that case its relation to the D. M. scale must be determined. (Consult Bonn Beob., III, p. xviii, ff.) To guard against a difference between observations of bright and faint stars, observers are recommended to observe variable stars in different stages of their brightness.

The choice of wire intervals both vertical and horizontal requires mature deliberation. The former must be arranged according to the declination; rather wide for equatorial zones, and rather narrow for northern zones. For eye and ear observing, wire intervals of 10 to 12 are recommended. Equidistant wires and in zones near the equator intervals of a whole number of seconds are to be avoided. With the chronographic method intervals of 25 to 38 will suffice. The choice of narrower intervals cannot be recommended. The horizontal wires should not be too close, because the fainter stars cannot be seen distinctly through them. A definite precept cannot be given, as the magnifying power employed is closely connected with the question. The observer will do well to ascertain what distance suits him best. Bessel used at first a power of 66 times and afterwards one of 107 times; a third of 119 times, which was perhaps more proper, was not employed on account of its small field of view, a difficulty not to be encountered in the present work, as each star can be directly set upon. The distance of the horizontal wires was 8", which is probably rather narrow. The Bonn northern zones were observed with a power of 108 times, those published in vol. vi with one of 150 times. The horizontal wire interval at Bonn is 14", which is perhaps too wide; 10" to 12" may perhaps be more advantageous.

The final catalogue will be reduced to the equinox of 1875 0. It is earnestly recommended to make the reduction of the observations as soon as possible, so as not to be pressed at a later period by the work. Of course a definitive reduction is possible only when the accurate determinations of the zero stars are completed at Pulkova, which will take several years. Meanwhile the provisional places given at present will be so nearly correct that only small corrections will need to be introduced. This reduction will afford an opportunity of judging what security of position is arrived at and where perhaps a third observation is to be added. It will, it is hoped, appear from all the observations that the mean difference between two observations of the same star will not exceed 08.1 in R. A. (for north

erly stars proportionally more), and 1"-2 in declination. This indeed is completely sufficient. It would follow from this that the probable error of a mean of two observations is 08.04 and 005. But with the above mentioned mean differences among 22 chances the differences 0.25 and 3" would occur once, and among 142 chances those of 0.34 and 4" 1 once. Accordingly it would be advisable perhaps as often as differences occur which exceed 0-3 and 3"-5 to make a third observation for verification.

The observer will do well to inform himself completely on the subject of the introduction of the D. M., found in Bonn Beob., III. It is there noticed that sometimes two very close faint stars by their united light have given cause to the introduction of a 9 star which does not exist; such are to be observed if bright enough. If a 9m star should be found which is not in the D M., and these cases are very rare, it is to be observed and the fact carefully noted, to be published in the final catalogue.

To secure the internal connection and completion of the whole, the participants are urgently requested to transmit, before July of each year, annual reports on the work of the year to the President, or one of the Secretaries, of the Society.

ART. VI.-On Gahnite from Mine Hill, Franklin Furnace, New Jersey; by GEO. J. BRUSH.

THE rare species Gahnite has been again found at a new locality in a cross-cut made by the New Jersey Zinc Co. from the valley of the Wallkill river to an opening on the south end of Mine Hill. I collected specimens at this locality in the summer of 1869, and by blowpipe examination at that time determined the mineral to be a zinc spinel.

The mineral differs in its crystalline characters from the specimens of other localities in the frequent occurrence of the cubic plane; in fact the cubic planes are often the largest, so that the crystals are cubes with truncated dodecahedral edges and only small octahedral planes. There are also minute planes of the trapezohedron 2-2, truncating the edges of the dodecahedron; also others of the trigonal-trisoctahedron 3. Besides. these there are sometimes two planes between the cubic and the octahedral, which appear, from examination and approximate measurements by Prof. Dana, to belong to the forms 4-4 and 8-8. Their surfaces are rounded, and feeble in luster, and generally they are blended in a single curved plane, consequently the measurements are not entirely satisfactory. The inclinations on a cubic plane, obtained by Prof. Dana, are for 4-4, 160° 30′, for 8-8, 170° 30'.

The crystals vary in diameter from an eighth of an inch to over an inch and a half; generally, however, they are less than half an inch. The color of the crystals is blackish-green; in thin fragments, olive-green. Hardness = 7.5. Specific gravity 4.894.91.

Before the blowpipe the mineral is infusible. With the fluxes reacts for iron and manganese; and with soda on charcoal gives a zinc coating.

The analysis in the wet way was made by Mr. Joseph S. Adam of this laboratory. The mineral was decomposed by fusion with bisulphate of potash. The silica was separated in the usual manner, and the iron and alumina thrown down as basic acetates, and this precipitate was examined to insure purity. The iron was determined by titration with permanganate of potash. From the acetic solution the manganese was separated by bromine, and the zinc was thrown down from the filtrate by sulphid of ammonium. The small amount of magnesia was determined as pyrophosphate, care having been first taken to separate the minute traces of it which were found precipitated with the alumina.

Two analyses by J. S. Adam gave:—

This gives the relation of the oxygen of R and as 8:12:25 77, or 1:3.17, which would indicate that a small portion of the iron was present as protoxyd. We have but to assume 1:56 per cent of the Fe in the analysis to have existed as Fe in the mineral to reduce the ratio to exactly 1: 3.

This variety of gahnite shows a larger percentage of zinc than any heretofore analyzed, and is unique in its cubic habit. It is associated with black mica, apatite, calcite and a brownish variety of chrysolite. A partial analysis of this chrysolite by W. G. Mixter shows it to be a unisilicate of iron, manganese, magnesia and zinc, probably related to, and possible identical with, the zinciferous chrysolite described by Prof. W. T. Roepper, in this Journal, II, 1, 35. A tin-white metallic mineral imbedded in some of the gahnite crystals proved to have the pyrognostic characters of leucopyrite.

Sheffield Laboratory of Yale College, Nov. 1870.

ART. VII.-Meteors of November 13th-14th, 1870.

1. At New Haven.-On the morning of the 13th of November, 1870, Mr. Charles S. Hastings and Mr. W. B. Roberts, of the Sheffield Scientific School, and Mr. A. A. Murch, of the Academical department of Yale College, watched for meteors upon the tower of Graduates' Hall, in New Haven, from half-past twelve to half-past three A. M. The sky was clear, or nearly The following is the result of the count:

SO.

The moon was about one sign in advance of the radiant, and evidently diminished very much the number of meteors that could be seen. Probably two-thirds or three-fourths of the whole number were thus made invisible.

On the next morning, that of the 14th, the writer, with Messrs. O. Harger, E. S. Dana, C. B. Dudley, J. McNaughton and R. P. Maynard, watched from shortly after eleven P. M. of the 13th, onward, with the following result:

From 3h 45m onward the sky was so nearly overcast that regular counting was abandoned. Here and there open spaces in the clouds enabled us to be assured that up to six o'clock there was not a marked increase in the number of meteors. After half-past five, however, the clouds more nearly covered the sky.