The mass of Lexell's comet is therefore greater than that of the Earth's atmosphere, but appears to be far within the superior limit assigned by La Place. 25. I shall next apply the formula for the limit to Encke's comet. The comet of 1795, discovered by Miss Caroline Herschel, was subsequently recognized as Encke's comet. It was discovered on Nov. 7, 1795, and the discoverer observed that the diameter on that evening was "about 5'." "It had no nucleus, and had the appearance of an ill-defined haziness, which was rather strongest about the middle." I quote from the Notes of Cooper's "Cometic Orbits." On Nov. 21st, Olbers observed the comet, and found its diameter to be "about 3'." I shall therefore take the mean of these determinations, or 4', for the diameter on Nov. 14, 1795. Olbers states that the comet was "round, badly-defined, and without a distinct nucleus." The Elements of this comet, for 1795, were subsequently computed by Encke, and are given by Cooper, as follows: Perihelion Passage 1795, December 21.44098, Gr. M. T. From these elements and the Earth's place in her orbit, I have found for Nov. 14, 1795, Encke's comet has therefore more than three times the mass of the Earth's atmosphere. 26. I shall next apply the formula for the mass to Halley's comet. Arago says that on January 25, 1836, Sir J. Herschel made two measurements of the head of Halley's comet, as follows:Diameter of the head in the direction of right-ascension, 229.4" "declination, 66 66 66 66 66 237.3" Two hours after, the measurements were respectively 196.7" in R. A. and 252.0" "Dec. Whether these measurements were reduced to the unit of distance, or not, is not stated. I shall therefore compute the limit of the mass, first on the supposition that they have not been so reduced, and again by supposing that they have been reduced to the unit. I shall take the mean of the first two measurements, or 233.4" for the apparent diameter, which differs only about 44" from the mean of the four. From Cooper's "Cometic Orbits," the elements of Halley's comet are taken, at its perihelion passage in 1835, being the second set of elements computed by H. Westphalen. The inclination, however, has been changed to its supplement, because the motion is retrograde, in order to use Gauss's general formulæ. These elements are as follows: Halley's comet, perihelion passage Nov. 15.93927, 1835, Gr. M. T. = 304° 31′ 34′′ From these elements, and the Earth's position at the date, 1836, January 25a.5, I have found log. 2 (2)3 = 11.9331398 log. A 12.3974508 Hence > 34.331 A If, therefore, Herschel's observations were not reduced to the unit of distance, Halley's comet had a mass more than thirty-four times as great as that of the Earth's atmosphere. But if, as is probable, Herschel reduced his measurements to the unit of distance, then I find that Halley's comet had a mass somewhat more than forty-five times that of the Earth's atmosphere; or 45.098. A Regarding a comet as comparatively a small mass of gaseous matter, which largely changes its distance from the sun, the preceding analysis shows that the sun exercises a great and constantly varying influence over the comet's figure of equilibrium. As long as any completely closed figure of the comet is maintained, 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 superior 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. THE RED SPOT ON JUPITER: OBSERVATIONS UPON ITS PHYSICAL CHARACTER; ITS MAGNITUDE AND THE DETERMINATION OF THE ROTATION PERIOD OF THE PLANET. By H. S. PRITCHETT, of 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. In 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. 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 11.5" 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 south 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 nar |