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permanent connection with the colony. In this condition they exhibit many diversities, and constitute a series of transition forms leading up to the highest, in which provision for a free and locomotive existence is complete. The embryo of the Hydroida is all but universally a ciliated body, the analogue of the winged seed of the plant, which diffuses the species."
The phosphorescence exhibited by the free medusan zooids, and by many of the tree-like hydroids, is not the least beautiful or wonderful part of their history. When any of the campanularians (Sertularia, etc.), common on our coast, are freshly dredged up and placed in pans of salt water in a dark room, a brilliant series of sparks is shown from cell to cell if the colony is irritated by a slight poke with a stick; and sometimes myriads of the free zooids illumine the sea with a tiny light in every bell. No esplanation has been offered of the precise nature of this phosphorescence, which is evidently under the creature's control.
It is curious to find the larva of a pycnogon playing a cuckoo part to one of the hydroids, Syncoryne eximia. “At a very early stage of their existence the young pycnogons gain access in some way or other to the interior of the zoophytes, and find their way through the cavity of the cænosarc into the budding polypites, which they occupy, using them as nests during the further stages of their development.”
In the preceding remarks we have endeavoured to present a few features of the type of the hydroid zoophytes in a popular form, in order to stimulate curiosity, and induce fresh students to observe them. We have been led to this course by finding Mr. Hincks's book unusually adapted to smooth the difficulties of beginners, while satisfying the demands of more advanced naturalists. He has adopted a simple, and we think, on the whole, natural' classification. He, of course, accepts Frey and Leuckart's well-established subkingdom, Cælenterata, and Huxley's class, Hydrozoa, with its three orders, Hydroida, Siphonophora, and Discophora. It is with the Hydroida that his labours lie. First comes the sub-order, Athecata, in which the polypites have no special cups to live in; the second sub-order is that of the Thecophora, or cup-bearers, and lastly come the Gymnochoa, which have no polypary, and which comprehend the Hydridæ.
We could easily have selected matter from Mr. Hincks's book to have shown its scientific character more fully, but as that will be taken for granted by naturalists who have been long familiar with his labours, we thought it preferable to do what is in our power to
secure for him a wider class of readers, who will be delighted with the aid he gives to the observation and study of some of the wonders of our shores. The plates which occupy the second volume are in the main good, but greater delicacy on the part of the engraver would have been more truthful and advantageous.
ASTRONOMICAL NOTES FOR MAY.
BY W. T. LYNN, B.A., F.R.A.S.
Of the Royal Observatory, Greenwich.
TAE PLANETS. — MERCURY will be very favourably placed for observation about the end of the month. He is at greatest elongation on the evening of the 29th, and does not set for a few days before and after that time until past ten o'clock. Even so early in the month as the 7th day, he sets a full hour after the Sun, or at half-past eight in the evening. His northern declination is very great, and he will be close to the Pleiades on the 8th, very near B Tauri on the 20th, and u Geminorum on the 31st.
Venus is in superior conjunction with the Sun on the morning of the 9th, and will begin to be conspicuous as an evening star about sunset towards the end of the month; setting, on the last day, at Sh. 36m., or half an hour after the Sun.
Mars continues to be observable during the whole of the first half of the night, being on the meridian at seven o'clock on the evening of the 1st day, and at six o'clock on that of the 26th. He is still in the constellation Leo, and will be very near Regulus on the 15th, and in conjunction with the Moon on the 18th.
Saturn now begins again to put in a claim for attention. He rises at the beginning of the month, soon after ten o'clock in the evening, and, at the end of it, at a few minutes past eight. The rings are this year well opened out for observation. The planet is in the constellation Ophiuchus (the limits of which here transgress within the zodiac), and of very great southern declination. He will be in conjunction with the Moon on the 26th day.
OCCULTATIONS.— The only phenomenon of this kind which will be of any particular interest, and take place in the early part of the night, is the occultation of Regulus, the bright star in Leo, which will pass behind the Moon (a little past her frst quarter) for a few minutes only on the night of the 18th. The disappearance will
take place at one minute, and the reappearance at ten minutes, after ten; the former at a part of the Moon a short distance to the right of the highest in altitude (as seen in an inverting telescope), and the latter a little more so; the occultation being, in fact, scarcely more than a graze.
The Moon.-Our satellite is new at 4h. 7m, on the afternoon of the 11th day, and observation of lunar objects may therefore commence on the evening of the 15th. On the 17th, the interesting regions about Hipparchus in the centre of the Moon may be studied; Tycho and its neighbourhood will be near the terminator on the 20th. On the 22nd, Schickard, near the Moon's south point, and the Mare Humorum to the north of it will be observable; on which night also the brilliant and well-known crater, Aristarchus, will show indications of its existence. The Moon is full at 3h. 23m. on the afternoon of the 25th day.
New PLANET.—A new planet (No. 108 of the minor planets), was discovered by Dr. Luther at Bilk, near Düsseldorf, on the 2nd of last month. This is the eighteenth which that astronomer has detected.
WINNECKE'S COMET.*— This comet was re-discovered by Dr. Winnecke himself at Carlsruhe, on the night of the 9th of April. It was very faint, but large and diffused, the diameter being as much as 6' or 8.
The comet was nearest the Earth (distant about sixty millions of miles), at the end of last month, but will not pass its perihelion until nearly the end of June. It will probably be observable during the first half of the present month until the increasing moonlight obscures it; but is not likely to be visible afterwards, unless with very powerful instruments. At the beginning of the month, it is still in the constellation Leo Minor, its place being on the 1st day, about R. A. 10h. 4m., N. P. D. 54° 10'.
We dwelt sufficiently on its previous history in our last number. In our next we shall give an account of the observations which may be made of it during this appearance, so far as they are accessible to us.
As we stated, in the January number of THE STUDENT, that our account of the observations of Encke's comet, made last summer, was probably nearly complete, it is proper now to notice that a series made by Drs. Kam and Hennekeler, at Leiden (an observatory which, under the direction of Professor Kaiser, hus for some years past been extremely active), has just been published : also two observations in the middle of August, by Dr. Brünnow, at Dublin. No remarks accompany the observations of any particular interest. The Leiden astronomers state that the comet was faint, and difficult to observe (" Astronomische Nachrichten,” No. 1747). No observations have yet been reported from the southern hemisphere.
CONJECTURED METEORS OF HALLEY'S COMET.—The Abbé Falb, of Graz, in Styria, has suggested that, if Halley's comet leaves meteors behind it, so as to form, as some other comets appear to have done, a more or less complete meteoric ring along its orbit, some of these may possibly be seen to pass over the Sun's disc about the 15th of May, when the Earth is in a line with the descending node of the comet's orbit. He adduces, also, some instances in one or two years of dark bodies having been actually seen, about that date, passing across the Sun. The description given, however, of these appearances by the observers does not seem at all consistent with the idea of their having really been meteors; and the Abbé is still more unfortunate in trying to connect a darkening of the Sun on the 12th of May, 1706, with the same cause, he having apparently overlooked the fact, that a solar eclipse took place on that day. Dr. Weiss denies the possibility of seeing meteors at all in such a position at the distance we must suppose them to be, if actually in the orbit of the comet. As this distance amounts to no less than three millions of miles, he appears to be justified in this assertion, unless some of the meteors are of a much larger size than we have as yet had reason to believe. Nevertheless it may perhaps be worth while to examine the Sun closely about the time in question. We need hardly remind our readers of the necessity, in observing the Sun for such a purpose, of keeping the eye-piece well in focus; since it has not unfrequently occurred that persons have mistaken merely terrestrial objects in a distant part of our own atmosphere for cosmical bodies. If it requires any considerable change of focus to see distinctly objects on the Sun from that necessary for scrutinizing any part of the Sun itself, this is a sufficient proof that they are really within the Earth's atmosphere.
VARIABLE Stars.—The subject of the periodical variations of light of many of the fixed stars being one of great and increasing interest, we propose to give each month the names of those stars which have been calculated to attain their maximum of brightness during some part of it. This will enable those possessed of moderate telescopes to assist in fixing the future mutations with greater accuracy. Ninety-five stars have already been determined to have a periodic change of considerable regularity: a few others have been thought to undergo changes of irregular period. In addition to these, there are two or three instances in which only one appearance of a star has been observed, apparently pointing to the conclusion, that on that one occasion only it increased so much in brilliancy as to become visible to us. One of the most remarkable events of this nature is
the well-known case of the star in Corona Borealis, known as T Coronæ, which is constantly visible as a telescopic star of the ninth magnitude, but in May, 1866, suddenly exhibited such an outburst of light as to be very conspicuous to the naked eye, perceptibly changing the ordinary appearance of the constellation in a manner which could not fail to arrest the attention of any observer who regarded it.
Of the ninety-five stars whose periods have, with more or less accuracy, been determined, thirteen have a period of less than a month, one of a little less than 50 days, three of about 70 days, one of a little less than 100 days, nine between 100 and 200 days, twenty-three between 200 and 300 days, thirty-two between 300 400 days, eleven between 400 and 500 days, whilst those of the remaining two amount to nearly two years in length. As the first fourteen (for R Lyræ, whose period is about 46 days, changes by so small an amount as to be always visible even to the naked eye) may be seen every month, at least during some part of it, we give a table of their places,* periods, and magnitudes when at maximum and minimum. It should be mentioned that astronomers have agreed to denote the variable stars by the capital Roman letters, beginning with R for each constellation, and taking the stars in the order of the discovery of their variability: thus, R Sagittarii indicates the star which was first discovered to be variable in the constellation Sagittarius. In those cases, however, in which the star had already a well-known name (as assigned by Bayer in the northern, or Lacaille in the southern hemisphere), this name was not allowed to be superseded by the nomenclature of the variable stars, as confusion might thus have ensued.