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the attraction due to its mass must be superior to the sun's disturbing power. But at the moment when a rupture of the figure takes place in any direction, the sun's disturbing force must then and there be equal or erior to the attraction towards the comet's centre of gravity.

On these principles it seems easy to account for the forms of comets, their transformations, and even for their disruptions, as in the case of Biela's comet. Their divisions and subdivisions into swarms of small comets, or meteoric gaseous matter, scattered along the track of the original orbit, are also simple consequences of the same analysis.


Rotation PerioD OF THE PLANET. By H. S. PRITCHETT, of

Morrison Observatory, Glasgow, Mo. The great red spot on Jupiter which has attracted such widespread attention from astronomers for two years past was first observed at the Morrison Observatory in July, 1878. During 1879, the spot was observed on about seventy different nights and many observations were made of transits across the central meridian as well as careful observations of its physical appearance and surroundings. The observations were continued until Feb. 7, 1880. Observations were resumed again early in May and are being continued at the present time.

The observations have been all made by Prof. C. W. Pritchett with the 12 inch Equatorial of this Observatory and all with the same power, a positive eyepiece magnifying about 280 times. In what follows I shall attempt to give simply a résumé of what has been done, together with the approximate results arrived at in a preliminary discussion of the observations. The rough sketches which are given are intended to give a correct idea of certain phases of the spot and its environment without special regard to finish, or the special detail of the planet's disk. They represent in each case the inverted image as seen in the telescope.

In 1878 the spot was seen only once, July 9, at which time a drawing was made and the spot was described briefly as being a “rosetinted elliptical cloud.” Although sought for the next night and on succeeding nights, when it should have been seen near the center of the disk, nothing more was seen of it during this opposition. For the failure to see the spot during the latter part of 1878 I am at a loss to account.

The general appearance of the spot and the physical phenomena of the surrounding surface may be best indicated perhaps by the following extracts from the observing books during 1879 and 1880.

1879, July 2.-Spot observed last year again seen for the first time. The major axis is about one-fourth longer than last year and the minor shorter. Color a bright pink and tinge deeper than that of the belts.

1879, Aug. 8.-At 11.5h Gl.m.t., as for several days past, all the cloud masses called belts seem collected south of the equator except the broad and distinct northern equatorial belt. The surface sonth of the equator is dense with clouds except in the vicinity of the spot. Here the belt masses seem not to approach.

1879, Aug. 24.-Seeing very fine. For the first time fine red streaks were noticed extending out a short distance from the ends of

the spot.

1879, Sept. 6.- A white spot, which was first observed on Sept. 4, is again seen, but much farther to the preceding side, showing a motion either of this or of the red spot.

1879, Sept. 13.-Spot and belts are nearly of the same color (bright pink). The spot is always pink while the belts take all hues from white to ashy, gray, brown and red.

1879, Sept. 20.—The appearance of Jupiter to-night was magnificent. The northern equatorial belt is much redder than the southern which is almost blue. Between the two main belts is an irregular mottled space interspersed with numerous white patches. One white spot just north of the preceding end of the red spot at time of transit. Two small dark cloudlets, one a little preceding, the other following, the west end of the red spot. Far north of the equatorial belt are two very sharp dark lines curving as parallel circles. It is worthy of note that the red spot does not seem to change its position relative to the dark cloudlets above it, while in reference to the white spots situated between the equatorial belts it changes rapidly.

1879, Sept. 30 and Oct. 5.—The same dark spots are observed and keep the same relative position with respect to the red spot.

1879, Nov. 6.-A very fine needle-like streak extends from the following end of the spot. Definition fine. The environment of the spot is about the same as for many weeks.

1879, Nov. 25.—The dark gray masses before mentioned are still seen in the same positions.

1880, Jan. 14.–At 6h 42m Gl.m.t. the shadow of Sat. III was near the center of the spot as shown in the drawing. The shadow on the spot appears black as on the surface of the planet. Sometimes there appeared to be a slight penumbra around the black shadow, but there were thin clouds which gradually increased and at 7h 14m the observations were entirely interrupted by clouds.

At times, during breaks in the clouds, the spot and the black shadow in it and the belts were quite distinct; but it was impossible on account of clouds to observe the egress of the shadow from the spot.

1880, Feb. 1.—The spot was observed in full daylight. Both the belts and spot seem very distinct.

The spot was again observed for the last time on Feb. 7. Observations were commenced again on May 11, 1880.

On May 23, the seeing was good and a very good observation was obtained. In appearance the spot has changed but little from last year. It is decidedly farther north than when first seen in 1878. The preceding end slightly more acute than the following and the needle-like projection still exists.

1880, July 10.--There seems a remarkable absence of belts in both northern and southern hemispheres. The surface seems more luminous than usual. One large white spot exists in the equatorial belt directly north of the preceding end of the spot. There is a trough-like appearance between the belts reaching to the eastern limb.

1880, July 29.-Images very good. The belts are very brown, the northern much broader and darker, the interval between being much diversified by white patches almost continuous. A number of fine dark lines on the northern hemisphere.

These notes from the observing books seem to show that from July 1878 to July 1879 the spot became somewhat longer and narrower and moved farther north and nearer the equatorial belts. Since 1879, it has changed its form and size but little, preserving the same outline and color and showing the same minute projections from the ends. It has kept uniformly throughout the observations a deep pink color bordering on red.

The luminous border spoken of so frequently by the English observers was noted only once or twice by one of the observers in 1879, and nothing whatever is seen of it in 1880. Even on the occasions when it was suspected to exist, it was described as being but little brighter than the ordinary bright disk of the planet, and I am strongly inclined to the belief that the appearance of the luminous border is due chiefly to the effect of contrast between the bright surface of the planet and the spot and adjacent belts.

Transits of the shadows of satellites across the spot have been observed under favorable conditions on four different occasions. The shadow in every case appeared black just as it is seen on the planet's disk with a slight suspicion at times of a penumbra.

The great relative motion of the white spots with respect to the reil spot was often noted. In two of the sketches presented, those of July 10 and July 13, 1880, the motion of one of these spots in three days is very clearly shown. The fixity of the dark gray masses with respect to the red spot was also noted.

The persistence of this spot in size, color, position and general outline for so long a time, suggests very strongly that it is more than a displacement of the upper Jovian atmosphere and that its cause is analogous to the great solar upheavals which produce the larger and more permanent sunspots. Only a local cause operating from beneath the surface of Jupiter would seem to be able to produce a phenomenon so uniform and long continued. This phenomenon may give additional weight to the theory which regards Jupiter and the outer planets as secondary suns. The small change in the size of the spot is shown in the following measures (Table I, p. 178) made with a filar micrometer during 1879 and 1880. The measures are all reduced to the epoch of the first observations on July 10, 1879, and were made when the spot was at the center of the disk.

The small differences in the measures on different dates are probably accidental. The mean of the values of the two axes is 13.60“ and 3.98". Using Newcomb's value of the Solar Parallax, these apparent chords correspond respectively to distances


A. A. A. S., VOL. XXIX.

of 26400 and 7700 miles. So that if Mr. Proctor should be correct in his hypothesis that this phenomenon is caused by the breaking away of the upper cloud masses of the Jovian atmosphere, exposing the heated surface beneath, the rent thus made is large enough to receive three globes of the size of our earth placed side by side.

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About ninety observations of transits of this spot across the central meridian have been made at this Observatory. These observations were commenced July 10, 1879, and are now being continued.

The observations were made in the following manner. The movable thread of the micrometer was set perpendicular to and bisecting the apparent equatorial diameter using the Ephemeris of Mr. Marth. The transit of the preceding end was then noted. The transit of the center was obtained by placing one of the wires of the micrometer on each end of the spot and noting the time wlicn the segments cut off on each side became equal. The transit of the following end was noted in the same manner as that of the preceding end. The mean of the times noted for the transits of the two ends rarely differed from that noted for the transit of the center by more than one or two minutes.

From twenty of the observations of highest weight, commencing July 10, 1879, and continuing until Feb. 5, 1880, including 507 rotations of the planet, I have computed the rotation period. The observations after being corrected for the aberration time and the relative motion of the earth and Jupiter furnished twenty equa

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