Diagram of Brooks comet. TWO comets are now visible to the naked eye, one in the evening and one in the morning sky, and a third one may be seen with an opera glass or small telescope if one cares to look for it. The year 1911 has been rather remarkable in that no less than seven comets were discovered in the nine months before the end of September. The sixth comet for the year, known in astronomical circles as comet f 1911, was discovered abroad on the -evening of September 23rd, by Quenisset, while the seventh comet, or comet g 1911, was picked up on the morning of September 29th by Belgjawsky, making it the first comet ever discovered in Russia. For a few days after the discovery, it appeared is if Belgjawsky comet were going to fulfill the wish for a brilliant comet -expressed by the writer in Scientific American, September 30th, page 299. On September 30th, the morning after its discovery, on a rather bright sky due to the coming of dawn, Prof. Frost at the Yerkes Observatory saw the comets nucleus as bright as Regulus, the chief star in the Sickle with a tail 5 or 6 degrees in length. Under slightly more favorable circumstances, the comet would probably have been as bright as Halley , when this interesting visitor was present in the morning skies. Unfortunately, however, the comet did not live up to expectations, so that now we have not a comet in the sky so brilliant that all may see it without the aid of special directions. When a comet is discovered, it is necessary to calculate its orbit, and predict where it will be in future by the aid of an ephemeris As mentioned in the article on Brooks comet (Scientific American, September 30th) most of the orbits are computed on the assumption that the comet is moving in a parabola about the sun. The position of the comet in space at any time depends on five quantities which are called the elements of the orbit. The plane of the comet orbit in space is defined by itsrelation with respect to the ecliptic, and it takes two quantities 10 express this. One of these quantities is the line in which the comet plane and that of the earth, or ecliptic, intersect. This is called the line of nodes. The other is the angle between the two planes, or inclination. When the comet crosses the ecliptic, passing from the south to the north side, it is said to be nt the ascending node (symbol, y ). To determine the direction of this line in space, we need the longitude of the ascending node, which is the angle in the plane of the ecliptic from the vernal equinox to the ascending node. The third element needed is the direction of perihelion in the plane of the orbit. This is called the longitude of perihelion, and is the angle in the plane of the orbit from the ascending node to the perihelion point. The fourth element is the distance of the . comet from the sun when at perihelion, and the fifth, the time at which the comet is close to the sun. Each observation of a comet position gives two co-ordinates, right ascension and declination; and hence to determine the five -elements, three complete observations are necessary. Obviously, it would be exceedingly difficult to tell the exact curve in which the comet is moving from a very small section of its orbit, and when it is remembered that the center of a comet has nothing well-defined about it and is difficult to measure, it is not surprising that two preliminary orbits calculated by different persons from different observations perhaps, will disagree quite a little. After the elements are known, it is possible to calculate the righ: ascension and declination which the comet will have at dates in the future. A comparison of the calculated places and those observed will give a means of estimating the accuracy of the cometary orbit. Ordinarily the three observations used by the calculator for the comet's elements are those first published, which generally come on successive nights. A cablegram from Prof. Kobold of Kiel, Germany, to Prof. Pickering of Harvard College. and by the latter distributed to American astronomers, gave the information that Kobold had computed an ephemeris for Belgjawsky's comet from observations made September 30th, October 1st and 3rd. The elements were: Time of perihelion passage T = 1911 Oct. 10. d. 26 G. M. T. Perihelion minus node......... CJi) = 71 deg. 39 min. Longitude of node.............. q = 88 deg. 44 min. Inclination .................... z =96 deg. 38 min. Perihelion distance ............ q = 0.0304 right ascension. declination. 1911 Oct. 5 12 hrs. 1 min. 39 sec. +12 deg. 11 min. 9 12 hrs. 57 min. 53 sec. 11 deg. 33 min. 13 13 hrs. 50 min. 4 sec. 7 deg. 45 min. 17 14 hrs. 30 min. 30 sec. 2 deg. 16 min. a glance at the above elements at once showed to the expert astronomer that this was a most unusual comet. Here was a comet moving almost at right angles to the plane of the ecliptic, and remarkable for that reason. The unit for measuring distances to comets is the distance from the earth to sun, and here was a comet coming within 3,000,000 miles of the sun. This insured that the comet would be a brilliant one, but the perihelion passage occurring on October 10th told that the comet would soon dwindle in brilliancy. The right 'ascensions and declinations of the comet showed that it was moving south alid east, and since the comet was visible before sunrise, this meant that it would become invisible in the glare of the sun, to be seen later in the west after sunset. On October 17th, Belgjawsky's comet set at 7 o'clock, and may be now seen shortly after sunset at the position given on the diagram above. The comet is now but a shadow of its former self. The photograph shown is one by Prof. Barnard on September 30th at 4:34 a. M. with an exposure of only five minutes. The tail was 5 or 6 degrees long to the naked eye, but the exposure had to be cut short on account of the approaching dawn. On October 4th, another photograph by Prof. Barnard shows a tail of about 8 degrees, slightly curved. The convex side was toward the north with a streamer extending southward at an angle of 60 degrees with the main tail. Considerable structure showed in the main tail and it had a shredded appearance. On the next morning the sky was very hazy and Prof. Barnard could see only the nucleus. The original of the photograph of September 30th here reproduced shows much detail about the comet's head with a system of many streamers which go out from the head and give the tail a fan shape. On this same morning, comets Brooks and Belgjawsky could be seen at the same time with the naked eye—the former in the northeast near the handle of the Dipper, and the latter in the east near the horizon. Brooks's Comet. Brooks's has been a most interesting comet because it has remained visible for so long to the naked eye. On October 21st, this comet rise” at four o'clock about 20 degrees north of east, and rises about the same time for the week following, but each morning more nearly east. Brooks's comet may be found from the diagram in the constellation of the Virgin. It will be seen from this diagram that the paths of Belgjawsky's and Brooks's comets cross each other, but at the present time, the former is visible in the evening sky, east of the sun, the latter in the morning sky, west of the sun. The exquisite photograph of Brooks's comet by Prof. Barnard, reproduced here, was made at 9:10 P. M., September 22nd, with an exposure of three hours and a half, and shows a tail for about 9 degrees. The trails of light on the photograph are the stars which become elongated to lines of liglu as the comet moved with respect to tne stars while the photograph was being taken. The comet was followed closely for the 3 hours 30 minutes necessary to make the exposure by keeping the eye constantly at the eye end of the telescope attached to the camera for guiding purposes. The diagram of Brooks comet shows that it was picked up about as soon as it possibly could be. The diagram also shows that in November the comet will be moving directly away from the earth and will dwindle in brightness very quickly. Quenisset Comet. This comet is fainter than the other two, and has suffered in consequence. It was discovered 15 degrees from the North Pole. On October 19th it was in the middle of the Northern Crown. It passes almost due south about a degree a day from the Northern Crown into the Serpent, ft is very slowly increasing in brightness.