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In the following Table Professor Kirkwood compares together the aphelion distances of the several known comets of short periods with the mean distances of the several larger planets from the sun :

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It is also evident that the passage of the solar system through a region of space comparatively destitute of cometic clusters would be indicated by a corresponding paucity of comets. Such variations of frequency are, indeed, found not only in the records of comets, but also of meteoric showers which have been accidentally recorded, the greater number of the latter having been observed during the five centuries between 700 A.D. and 1200 A.D., and again in those following A.D. 1700, suggesting that during the former and, perhaps, again during the present period the solar system is passing through a cosmical or meteoric cloud of very great extent,-not less, indeed, on the received speed of the sun's proper motion, than fourteen times the width of Neptune's orbit. Professor Kirkwood adds, in particular reference to the August meteor-system, " The fact that the August meteors, which have been so often subsequently observed, were first noticed in 811 [see M. Quetelet's Catalogue of Star-showers) renders it probable that the cluster was introduced into the planetary system not long previously to the year 800. It may be also worthy of remark that the elements of the comet of 770 A.D. are not very different from those of the August meteors and of the third comet of 1682"*. With regard to the sun's passage through a meteoric cloud of the above-considered dimensions and constitution it is noticed that the num

38 * ber of cometary perihelia found in the two qua

2604
*62

P.C.
drants of longitude towards and from which the sun
is moving is 159, or 62 per cent., and that of peri-
helia in the two other quadrants is 98, or 38 per

P.c. cent., showing their tendency to crowd together about the direction of the sun's proper motion in space. The large excess of the number of the cometary perihelia closest to the sun in the forward quadrants, relatively to the direction of his proper motion in space, is also regarded as indicating the direction of the sun's motion through the meteorcloud in a manner which the facts of observation evidently corroborate.

* The interval between the perihelion passage of 770 and that of 1862 is equal to 9 periods of 121:36 years. Oppolzer's determination of the period of 1862, III., is 121-5 years. Hind remarks that the elements of the Comet of 770 are “rather uncertain,” but says “that the general character of the orbit is decided.” It may be worthy of remark that a great meteoric shower, the exact date of which has not been preserved, occurred in 770.

3. On the Periods of certain Meteoric rings. By Professor Kirkwood (read to the American Philosophical Society, March 4, 1870).-According to the computed elements of the Comet I. 1861 (by Oppolzer), first shown by Dr. Edmund Weiss (Astron. Nachr, no. 1632) to agree very closely with those of the April meteor-stream, its periodic time of revolution is 415.4 years. On the other hand, Professor Kirkwood points out that, without accepting a shorter periodic time of revolution, the former April displays recorded in ancient times do not agree with the time of revolution of the comet. Adopting a period of about 28} years for the cycle of returns of the April shower, the whole of the dates of its appearance selected by Professor Newton as agreeing well with those of its most recent appearance in the present century are represented with perfect accuracy by the following scheme :

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The periodical time of 28} years corresponds to an ellipse whose major axis is 18.59, and whose aphelion distance is very nearly equal to the mean distance of the planet Uranus. A remark of Mr. Du Chaillu is here believed to be rightly recalled, that he observed the April meteors in the equatorial parts of Africa almost as brilliant, and leaving streaks more enduring than those of the great November meteor-shower (of which he was also an observer in England, in the year 1866). If the date of Mr. Du Chaillu's obserFation was about the year 1860, á corroboration of Professor Kirkwood's cycle of 28 years repeated twice since the great display of those meteors in the year 1803 would be thence derived. The April meteor-shower was also sufficiently bright in the year 1863 to make its approach to an epoch of maximum brilliancy in about that year a somewhat probable conjecture.

Among the formerly recorded star-showers which appear to have certainly been connected with the December meteor-system, Professor Kirkwood points out a notice of such an occurrence in the year A.D. 901. Others are found to have taken place in the years 930, 1571, 1830, 1833, and 1836, with an apparent maximum in the year 1833, when as many as ten meteors were seen simultaneously. Finally, pretty abundant displays of the shower were observed in the years 1861, 1862, and 1863, with a probable maximum in the year 1862. These dates indicate a period of about 293 years, thus901 to 930....

i period of 29'060 years. 930 to 1571..

29-136 1571 to 1833..

29111 1833 to 1862..

29'000 A third meteoric shower, that of the 15th-21st of October, presents, again, a similar period of revolution. The recorded dates of apparitions which correspond in the times of their appearance with the present meteor-showers of the 15th-21st of October are the years A.D. 288, 1436 and 1439, 1743, and 1798, on each of which occasions a great number of shooting-stars were

22

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seen.

The periodic time of 27 years is well indicated by these dates,

thus:-

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A.D. 288 to 1439..

42 periods of 27-405 years each. 1439 to 1743..

27.636 1743 to 1798....

27500 “If these periods are correct, it is a remarkable coincidence that the aphelion distances of the meteoric rings of April 18th-20th, October 15th21st, November 14th, and December 11th-13th, as well as those of the comets 1866 I., and 1867 I. are all nearly equal to the mean distance of Uranus."

4. Beiträge zur Kenntniss der Sternschnuppen, von Dr. Edmund Weiss (Sitzungsberichte of the Imperial Academy of Vienna for January 16, 1868) presents a short summary of the mathematical problems required to be solved in the determination of the parabolic orbit, and the actual relative speed of the meteors' course in the atmosphere, from the known position of the radiant-point; and shows how approximate calculations of the velocities of shooting-stars have led to discoveries, in proving certain periodical meteorcurrents to be intimately connected with comets of which the orbits have recently been determined*.

5. The Fuel of the Sun, by W. Mattieu Williams, F.C.S. (8vo, 222 pp. Simpkin and Marshall).-An attempt to explain convulsions of the sun's surface by planetary disturbances of a universal atmosphere collected in greatest density about the larger bodies of the solar system, and agitated by tides arising from their several attractions, is the theory for the establishment of which a collection of the greatest interest of recent observations of solar physics has been brought into a small compass by the author of the work, and is well directed to explain the chief phenomena of solar physics. The corona is regarded (Chapter XIII.) as originating in solar projectiles driven from its surface with eruptive violence. In the following chapter the source of meteorites is conjectured to be the solar projectiles which thus pass beyond the boundaries of the zodiacal light; some of which being confined to revolve in two principal orbits outside of that luminary, and in several intermediate zones of irregularly and more thinly scattered projectiles, may be regarded as giving rise to the August and November, as well as to other minor and more or less regular meteoric displays. Somewhat more important speculations and descriptions of the meteorology of the moon and planets, as well as of the distribution of the nebulæ, suggesting the stellar origin of some of those bodies, occupy the greater portion of the remainder of the work.

* The velocity of the April meteors, or Lyraïds, of the 20th of April meteoric shower, relatively to the earth, is given in Dr. Weiss's list of radiant-points and relative velocities of cometary orbits, in the above paper, as 1-585, that of the earth in its orbit being unity. Adopting the value of 18.6 miles per second for the earth's mean orbital velocity, this gives the relative velocity of the Lyraïds, or April shower-meteors, 29:5 miles per second ; very nearly that observed (30 miles per second) in the case of the only shooting-star of the shower doubly observed, as described in this Report, on the night of the 20th of April last.

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Fifth Report of the Committee, consisting of HENRY WOODWARD, F.G.S., ,

F.Z.S., Dr. DUNCAN, F.R.S., and R. ETHERIDGE, F.R.S., on the Structure and Classification of the Fossil Crustacea, drawn up by

HENRY WOODWARD, F.G.S., F.Z.S. SINCE I had last the honour to present a Report on the Structure and Classification of the Fossil Crustacea, I have published figures and descriptions of the following species, namely :

DECAPODA BRACHYURA. 1. Rhachiosoma bispinosa, H. Woodw. Lower Eocene, Portsmouth. 2. — echinata, H. W. Lower Eocene, Portsmouth.

3. Palæocorystes glabra, H. W. Lower Eocene, Portsmouth. All figured and described in Quart. Journ. Geol. Soc. vol. xxvii. p. 90, pl. 4.

DECAPODA MACRURA. 4. Scyllaridia Belli, H. W. London Clay, Sheppey. Geol. Mag. 1870, vol. vii. p. 493, pl. 22. fig. 1.

AMPHIPODA. 5. Necrogammarus Salweyi, H. W. Lower Ludlow, Leintwardine. Figured and described Trans. Woolhope Club, 1870, p. 271, pl. 11.

ISOPODA, 6. Palæga Carteri, H. W. Lower Chalk, Dover, &c. Geol. Mag. 1870, vol. vii. p. 493, pl. 22. fig. 1.

7. Præarcturus gigas, H. W. Old Red Sandstone, Rowlestone, Herefordshire. Trans. Woolhope Club, 1870, p. 266.

MEROSTOMATA. 8. Eurypterus Brodiei, H. W. Quart. Journ. Geol. Soc. 1871, August. Trans. Woolhope Club, 1870, p. 276.

PHYLLOPODA. *9. Dithyrocaris tenuistriatus, M Coy. Carboniferous Limestone, Settle, Yorkshire.

10. Dithyrocaris Belli, H. W. Devonian, Gaspé, Canada. 11. Ceratiocaris Ludensis, H. W. Lower Ludlow, Leintwardine. 12. Ceratiocaris Oretonensis, H. W. Carboniferous Limestone, Oreton, Worcestershire.

13. Ceratiocaris truncatus, H.W. Carboniferous Limestone, Oreton, Worcestershire.

Figured and described in the Geol. Mag. 1871, vol. viii. p. 104, pl. 3. 14. Cyclus bilobatus, H. W. Carboniferous Limestone, Settle, Yorkshire. 15. torosus, H. W. Carboniferous Limestone, Little Island, Cork. 16. Wrightiö, H. W. Carboniferous Limestone, Little Island, Cork. 17. Harknessi, H. W. Carboniferous Limestone, Little Island, Cork. *18. radialis, Phillips. Carboniferous Limestone, Settle, Yorkshire, Visé, Belgium.

*19. Cyclus Rankini, H. W. Carboniferous Limestone, Carluke, Lanarkshire. [*20. Brongniartianus,” De Kon. Carboniferous Limestone, Yorkshire, Belgium.]

21. Cyclus Jonesianus, H. W. Carboniferous Limestone, Little Island, Cork. (These latter figured and described in the Geol. Mag. 1870, vol. vii. pl. 23. figs. 1-9.)

[Those marked with an asterisk have been already figured, but have been redrawn and redescribed in order to add to or correct previous descriptions.

Thus, for example, " Cyclus Brongniartianus" proves upon careful examination to be only the hypostome of a Trilobite belonging to the genus Phillipsia. Dithyrocaris tenuistratus is identical with Avicula paradoxides of De Koninck,

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&c.]

marus.

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Since noticing the occurrence of an Isopod, Palæga Carteri, from the Kentish, Cambridge, and Bedford Chalk, Dr. Ferd. Roemer, of Breslau, has forwarded me the cast of a specimen of the same crustacean from the Chalk of Upper Silesia. This, together with the example from the Miocene of Turin, gives a very wide geographical as well as chronological range to this genus.

A still more remarkable extension of the Isopoda in time is caused by the discovery of the form which I have named Præarcturus in the Devonian of Herefordshire, apparently the remains of a gigantic Isopod resembling the modern Arcturus Baffinsii.

I have also described from the Lower Ludlow a form which I have referred with some doubts to the Amphipoda, under the generic name of Necrogam

Representatives both of the Isopoda and Amphipoda will doubtless be found in numbers in our Palæozoic rocks, seeing that Macruran Decapods are found as far back as the Coal-measures *, and Brachyurous forms in the Oolitest.

Indeed the suggestion made by Mr. Billings as to the Trilobita being furnished with legs (see Quart. Journ. Geol. Soc. vol. xxvi. pl. 31. fig. 1), if established upon further evidence, so as to be applied to the whole class, would carry the Isopodous type back in time to our earliest Cambrian rocks.

I propose to carry out an investigation of this group for the purpose of confirming Mr. Billings's and my own observations, by the examination of a longer series of specimens than have hitherto been dealt with. In the mean time the authenticity of the conclusions arrived at by Mr. Billings having been called in question by Drs. Dana, Verrill, and Smith (see the American Journ. of Science for May last, p. 320; Annals & Mag. Nat. Hist. for May, p. 366), I have carefully considered their objections, and have replied to the same in the Geological Magazine for July last, p. 289, pl. 8; and I may be permitted here to briefly state the arguments pro and con, seeing they are of the greatest importance in settling the systematic position of the Trilobita among the Crustacea.

Until the discovery of the remains of ambulatory appendages by Mr. Billings in an Asaphus from the Trenton Limestone (in 1870), the only appendage heretofore detected associated with any Trilobite was the hypostome or lip-plate.

From its close agreement with the lip-plate in the recent Apus, and also from the fact of the number of body-rings exceeding that attained in any other group save in the Entomostraca, nearly all naturalists who have paid attention to the Trilobita in the past thirty years have concluded that they possessed only soft membranaceous gill-feet, similar to those of Branchipus, Apus, and other Phyllopods.

The large compound sessile eyes, and the hard, shelly, many-segmented body, with its compound caudal and head-shield, differ from any known Phyllopod, but offer many points of analogy with the modern Isopods I; and

* Anthrapalæmon Grossartii, Salter, Coal-measures, Glasgow. † Palæinachus longipes, H. Woodw., Forest Marble, Wilts.

It should always, however, be borne in mind that as the Trilobita offer, as a group, no fixed number of body-rings and frequently possess more than twenty-one segments, they

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