The work, when finished, will consist of twelve parts; and form three volumes in quarto of descriptive matter, and one folio, of two hundred and fifty Plates. It may be considered, with respect to Vertebrated Animals, as a complement to the celebrated Recherches sur les Os Fossiles, of Cuvier, and the Petrefacta Musei Bonnensis, of Goldfuss. We shall procure, as soon as possible, the early livraisons of this valuable publication, and present an analytical notice of them to our readers. Our zealous townsman, Mr. Weaver, of the Natural History Museum, possesses, by the liberality of J. Greaves, Esq., a most beautiful and interesting specimen of fossil fish, lately discovered in a stone-quarry, belonging to that gentleman, near Stratfordupon-Avon. Professor Agassiz, who obtained a knowledge of the existence of this specimen, and the loan of it for the purpose of his work, through the medium of Professor Buckland, of Oxford, has recently returned it to Mr. Weaver, with a most gratifying letter, of which we subjoin a translation; * and several impressions of the splendid drawing upon stone, which has been made from it under the eye of the Neuchâtel Professor. Of this drawing, a reduced but very correct copy, executed by Mr. Underwood, is given in our present number. The Ichthyolite, in question, appears to be an unique specimen of a new species of the genus Tetragonolepis, so named from the four-angled (tetragonal) figure of the scales. The circumstance, from which the specific designation angulifer,-a Latin adjective literally signifying angle-bearing,-applied to it by Professor Agassiz, is not equally obvious or comprehensible. Birmingham, Septr. 1835. *Sir, In returning to you the fossil fish which you had the kindness to confide to my care, I take the liberty of transmitting to you some copies of the drawing which I have published in my work. It is, unquestionably, one of the finest specimens of fossil fish with which I am acquainted. It has proved the more valuable, as it is a new species, of which I know no other specimen but that which you possess. Accept, Sir, with my thanks, the assurances of my distinguished consideration. Oxford, August 9th, 1835. L. AGASSIZ. 134 SIR, To the Editor of the Analyst. As the long-expected Comet of Halley is now actually visible, and many of your readers are, no doubt, anxiously anticipating the pleasure of watching its phenomena, I have thought that it might not be unseasonable to lay before them, through the medium of your Journal, a few notices as to the best method of making such observations upon it as may be of real value and interest. The general diffusion of scientific attainments would appear, at first sight, to render this a superfluous task; but I am convinced that the scarcity of accurate information respecting Comets, is much greater than is usually supposed: most of the popular statements, even down to the present time, being either very defective or shamefully erroneous; while such as are sufficiently explicit, and deserving of confidence, are not very accessible. I have therefore compiled, from unquestionably authentic sources, an enumeration of the phenomena whose existence it will be desirable to ascertain by observation in the present case. serviceable in drawing their attention to the points most deserving of notice, I hope that, with your sanction, they may be induced to communicate their observations to your periodical; which, if all made upon system, and with an uniform view to determine the presence or absence of certain phenomena, would not fail to possess great interest individually, and collectively, no small value and importance. If any of your astronomical readers should find it I remain, Sir, your humble servant, Tretire, near Ross, Sept. 16, 1835. THOMAS WILLIAM WEBB. HINTS TO OBSERVERS OF HALLEY'S COMET. It is well known to those who are in the habit of using telescopes, how little can be seen, in many cases, by a person whose eye is entirely unaccustomed to the object to be observed, and who has no previous notion of the particulars to which his attention should be more expressly directed. This, of course, will be peculiarly the case with an object so faint and undetermined as a Comet; and it is by no means improbable that, unless observers previously determine upon the points which require attentive examination, the Comet may pass repeatedly through the field of their telescope, without their being aware of the existence of phenomena which, taken in connection with older observations, may prove of considerable importance. We know so little as yet of the real nature and construction of these wonderful bodies, that we cannot presume to advance to anything like a generalization of the observations hitherto made, many of which would appear to be contradictory and inexplicable: we must therefore, at present, be content with accumulating as many well-authenticated facts as possible, deduced from a careful examination of every Comet that visits our system, from a combination of which we may, at some future period, be better able to develope the laws that govern their mysterious construction. The subjoined particulars appear to be very worthy of attention with this view; and they are humbly offered to the notice of the astronomical readers of the Analyst, in the hope that, by their united efforts, they may be able to ascertain some of them satisfactorily. In the first place, however, it may not be amiss to premise, to unpractised observers, a few hints on the management of their telescopes. No observation should be made with an erecting or terrestrial eye-piece, as the additional number of lenses occasions a great loss of sight: astronomical eye-pieces alone should be used; or a terrestrial eye-piece may be converted into an astronomical one, by unscrewing the tube nearest to the eye, and taking out the two lenses from its interior end; as the field of view may, however, in this case, be rather unserviceable, it will be a still farther improvement to remove the innermost of the two lenses at the eye-end, leaving only two lenses in the whole instrument (if an achromatic), viz: the object glass, and that next the eye. In this case, the edges of the field will be obscure, and its appearance not satisfactory to an eye unaccustomed to it, but an object in its centre will be seen very clearly and distinctly in an inverted position. The focal length of the telescope, it should be observed, will be greatly shortened by this adjustment, which, if previously untried, should be made, at first, upon terrestrial objects in the day-time. If the instrument has several astronomical eye-pieces, it will be proper to use them all, in any observation that is in the least doubtful; but the higher powers will be found deficient in light, in observing so faint a body; and, generally speaking, no higher power should be employed than is sufficient to develope satisfactorily the phenomena under examination: the advantage of a large field will be found to be very great in ascertaining the extreme boundaries of an object whose termination is frequently so difficult to be distinguished; and from the neglect of this precaution astronomers of eminence have occasionally been led into considerable errors. It may not be so generally known as it deserves to be, that the deposition of dew upon the object-glass may be entirely prevented by the employment of a tube of stiff paper or pasteboard, made to fit the end of the instrument; this not only saves much time, but obviates the necessity of wiping the object-glass, an operation seldom performed without some danger of scratching it. If, in addition to this precaution, the brass cap is placed upon the glass, before it is removed into the warm air of the house, damp will scarcely ever be formed upon it under any circumstances. The particulars which it is desirable to ascertain, may be enumerated as follow : 1. Whether the Comet has anything that can be called a moderately welldefined disc in the centre, or whether it is a mere luminous point, or only a general and gradual accumulation of light, without any apparent outline. The former, though not very usual, was certainly the case with this Comet at its return in 1682, when it is described, by Cassini, as exhibiting in the telescope a disc "aussi rond, aussi net, et aussi clair que celui de Jupiter.* 2. If it has a disc, whether that disc has an uniform illumination, or is brightest in the centre; and whether its appearance is invariable, in this respect, at different times. 3. Whether the disc at any time exhibits any spots, or anything of a mottled aspect, or is irregularly terminated, or seems to be composed of an assemblage of small fragments; and, in this case, whether their relative brightness and position are subject to any change. 4. Whether the disc is accurately circular or not. In 1682 the nucleus, according to Hevelius, had throughout an oval form,+ but this appears to be contradicted, by Cassini, in the passage already cited; and it is not recorded to have been noticed at its last return in 1759. (The previous returns, it will be remembered, were anterior to the employment of telescopes.) 5. Whether the disc, as it approaches the perihelion, manifests any symptom of phases. This most curious feature was actually exhibited by this Comet in 1682, according to Delambre. He states that Picard and La Hire *Memoires de l'Academie Royale, 1699. + Littrow, Beytrage zu einer Monographie des Halley's chen Cometen, Wien, 1834, p. 56 observed it, and "la figure en croissant est nettement dessinée dans les registres de l'Observatoire. Hevelius also observed, but only on one night, (Sept. 8, six days before the perihelion) a brilliant hook extending itself from the nucleus, in the direction of the tail, for a distance equal to the diameter of the head. This could hardly have been a true phasis; but the possible recurrence of such a remarkable phenomenon deserves to be carefully watched: perhaps this hook was actually what is described as a crescent by the Paris observers. 6. Whether it may be possible, if there is anything like a planetary disc, to observe the occultation of a fixed star by it; and, if so, whether it is a total obscuration of the star, or only such a diminution of its light as must necessarily result from its being viewed, as it were, upon a bright ground. 7. If there is nothing but a luminous point in the centre, whether this point is constantly visible every night, and during the whole of each obser vation. 8. If there should be merely a general condensation of light towards the centre of the head, whether this condensation is sudden or gradual; and whether it is equally gradual at different times, or, by its more sudden increase of brightness, and more defined outline at certain periods, affords a suspicion of an interior body obscured by a variable atmosphere. 9. Whether the central parts or nucleus of the Comet is subject to any remarkable variations in brightness from night to night, the air continuing equally clear; and whether anything like a periodical return can be detected in such variations. 10. Whether the nucleus, be it what it may, is situated in the centre of the coma, and whether there is any variation in this respect. In 1682 Flamsteed observed the nucleus of this Comet to be twice as near to the edge of the coma on Sept. 4, as it had been three or four days previously. 11. Whether the coma exhibits an uniform appearance on every side, or is more dense in any direction, or appears as if scattered and interrupted. 12. Whether the extension of the coma decreases, in proportion to the size of the nucleus, as it approaches to the perihelion; allowance being made for its increasing distance from the earth, and for the effect of twilight. 13. Whether there is any dark space included within the head, and surrounding the nucleus; or whether there may be more than one such interval, and whether it, or they, extend round the whole, or only a portion of the nucleus. 14. Whether the darkness of such included space approximates to the depth of colour of the exterior sky, and whether this darkness is variable in intensity upon different nights. 15. In 1456, three days before the perihelion, the head of the Comet, viewed (of course) with the naked eye, shone with the brilliancy and twinkling of a fixed star, so that many persons imagined it must have been in conjunction with several small stars.§ As the present position of the earth with respect to the Comet is far less favourable than during that most splendid of its recorded appearances, nothing of this kind can perhaps be expected, but it should at least be borne in remembrance, and attended to. 16. In 1607, when the relative positions of the earth and Comet were peculiarly similar to the present, and therefore a similarity of aspect may be expected, the head appeared, to the naked eye, not exactly circular, but, as Kepler expresses it, "quodam modo strumosa, deficiens a rotunditate."|| 17. The length and breadth of the tail should be carefully observed on every favourable opportunity. Much will, in this case, depend upon the state of the atmosphere and the eye of the observer. In 1456 the tail was not less than 60o in length. Picard estimated it at 30°, in 1682; while He velius made it but 12° on the same night, and never more than 16o. In 1756, La Nux, in the clear atmosphere of the Isle of Bourbon, traced it for 47°.* 18. Whether the tail progressively increases in breadth to the extremity, or, on the contrary, assumes a tapering form. 19. Whether one side of the tail is better defined than the other, as was distinctly observed by Flamsteed in 1682†, and whether this appearance is constant. 20. Whether the tail is curved, and whether it deviates, at its origin, from exact opposition to the sun; and whether there are any symptoms of a reverse or reflex curvature at its extremity. 21. Whether the tail, when near the horizon, vanishes entirely, as if obscured by a cloud, as is asserted by Apian to have been the case in 1539.‡ 22. Whether the tail is divided lengthwise into two or more streams by a dark space; whether such space is visible in the neighbourhood of the nucleus, or appears uninterruptedly to surround it; whether its depth of shade is uniform; and whether it extends to the extremity of the tail. 23. Whether any smaller lateral streams accompany the principal tail in the vicinity of the head. 24. If there should be several branches in the tail, whether they are all of equal brightness, and whether the difference in this respect is constant; whether their relative length and breadth remain unaltered; and whether there is anything like an interruption anywhere in their length. 25. Whether the light of the tail, or of its several sub-divisions, is uniformly distributed in the direction of its breadth; or whether it is most vivid at the edges, or in any other part. 26. If there should be anything remarkable in the form of the tail, whether the periodical recurrence of that peculiarity may justly lead to the idea of a rotation. 27. Whether there are any traces of an anomalous tail, i. e., a stream of light in the opposite direction, tending towards the sun. 28. In 1456, the tail was said to have resembled at times a sparkling flame, which gave rise to the conjecture that it might be in conjunction with numerous small stars. Such a phenomenon may possibly, at least, recur. 29. It is peculiarly deserving of attention that in 1607, to which return, as has been already stated, the present bears the nearest resemblance, the tail exhibited that very peculiar, but unquestionably authenticated, appearance of coruscations or fluctuations in length and brightness. Kepler's words are very clear and decisive: "testabantur enim de Cometa anni 1607 omnes qui diligenter illum fuerunt contemplati, caudam jam brevem, mox in ictu oculi longam apparuisse, non tamen, quod Cardani verba et exempla innuere videntur, (concerning the Comet of 1556) flamma, ut sic dicam, ad latera expatiante, sed solum prorsum ruente. Adroque mihi, qui debili visu sum, vix aliter apparebat hæc cauda, quam ubi inclaruit seu promicuit: tunc enim non mediocriter longa apparebat, et oculos evidentes]] movebat."§ If such fluctuations can be observed, it will be very desirable to note, if possible, their frequency, and whether they may not affect the breadth as well as length, of the tail. If there are several distinct branches, it should be carefully noticed whether the coruscations pervade them all. 30. The colour of the Comet should be remarked, and whether the different parts exhibit any difference or contrast in this respect. In 1759, the nucleus was said to be whitish, and similar in colour to the planet Venus. Such rapid variations have frequently been perceived in some Comets, that even hourly observations have ascertained changes of a very obvious and |