tained. Hydro-carbon gas is generated and pumped into the retort, and as it is consumed the supply is maintained. By this process the alumina is reduced, and the metal aluminium remains as a spongy mass, mixed with carbon. This mixture is then remelted with metallic zinc, and when the aluminum has collected in the metallic state, the zinc is driven off by heat. The reduction is due to the action of the hydro-carbon under pressure. The time for reducing one hundred pounds of alumina earth, cryolite, or other compound of alumina should not be more than four hours; and when hydro-carbon gas can be obtained in a heated and compressed state, the reduction takes place in a still shorter period.

CORRESPONDENCE.

VARIATION IN LUNAR PHOTOGRAPHY.

To the Editor of the Student.

MY DEAR SIR,-I have just met with a somewhat remarkable instance of variation between the photograms of De la Rue, 1858, Feb. 22, and of Rutherford, 1865, March 6, which shows the great importance, not only of obtaining moon pictures through a series of years, but also of carefully and systematically examining them. Lohrmann, in 1823, figured

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a plain on the S.W. of Hipparchus, surrounded by mountains as in the margin, with a crater on the W. border, which he considered sufficiently important to designate as No. 51, Sec. I. This crater he described as a little pit. On De La Rue's photogram the W. border is traceable with a very shallow crater on it in precisely the position indicated by Lohrmann. It is, however, so faint on the photogram as to be easily overlooked, and very unlike the conspicuous craters on the W. of Hipparchus. This faint shallow crater appears on the photogram to be larger than drawn by Lohrmann. No trace of it exists on Rutherford. The plain is there, and the interior mountain border running, as in De La Rue, N.N.E. and S.S.W., which is not the direction given by Lohrmann. In the position of Lohrmann's crater is a kind of platform, the middle portion of which is very slightly depressed, and not at all of the character shown on De La Rue's photogram, which clearly testifies to the accuracy of Lohrmann's drawing. The form of the border is decidedly different in the two photograms. De la Rue's differs very slightly from Lohrmann's. Rutherford's has a projection from the W. border, inclosing the site of Lohrmann's crater, as shown above.

Lohrmann gives 6° of brightness for the surface of the plain seen in its meridian. On both photograms it is not more than 36. 1868, Dec. 23, I found it only 43, at about the same epoch of phase as that of the two photograms. Lohrmann gives the surface of the plain as per

fectly smooth. On Rutherford's photogram the W. parts of the plain is furrowed by a valley which can be traced opening on the floor of Hipparchus.

At the present moment I have only access to the photograms I mentioned. Later moon pictures for further comparison are very desirable. I am, my dear Sir, yours very truly,

Cynthia Villa, Walthamstow,

January 29th, 1869.

W. R. BIRT.

THE LATE TRANSIT OF MERCURY.

To the Editor of the Student.

SIR,-In Mr. Lynn's notes of the late Transit of Mercury, which appeared in the January number of THE STUDENT, he mentions the apparent distortion of the planet shortly before passing off the solar limb. No particulars of this occurrence having been given, I venture to send a few notes of my observations, both of this phenomenon and of others which appear to have escaped notice by your able correspondent.

Just previous to the contact of the planet with the solar edge, I thought I saw on the side of the planet farthest from the sun's limb a circular slip appear detached from the body of the planet, except at the S. (real) pole, where it was connected. It lasted but a few seconds, and then the planet resumed its original figure.

Immediately after the above appearance (?) Mercury presented on the same side a flattened aspect; and soon after the same flattened appearance was noticed on the side next the solar limb, so that the planet seemed to have for a short time a square protuberance on each side. The so-called protuberance nearest the solar limb then suddenly narrowed and lengthened, and assumed for an instant a ligature-like appearance, connecting the planet with the solar edge, much the same as the diagram shown in illustration of Mr. Lynn's paper. Immediately after, the planet moved on to the solar edge, and was seen for some seconds as a triangular notch on the sun's edge.

This phenomenon soon became unsteady, then jagged, and finally passed off the solar disc.

It would be out of place for me to make any remarks upon these appearances, but simply state the phenomena as they appeared to me, and to say that I used a small, but powerful refractor, and that the atmosphere was tolerably clear during the observation.-Yours, etc.,

H. W. EMONS,

Secretary, Hackney Scientific Class.

2, Percy-road, Wells-street, South Hackney, London, N.E.,

February 6th, 1869.

LITERARY NOTICES.

HANDBOOK OF NATURAL PHILOSOPHY. By Dionysius Lardner, D.C.L., formerly Professor of Natural Philosophy and Astronomy in the University College, London. "Optics." Sixth Thousand. Edited by T. Oliver Harding, B.A., Lond. With Two Hundred and Ninety-eight Illustrations. (James Walton.)-This well-known work, now re-edited by Mr. Oliver Harding, contains a large body of optical information, illustrated by numerous diagrams. In adapting it to the present date the editor has not gone quite far enough in his alterations and additions to give complete satisfaction. Thus the paragraph on spectrum analysis is very meagre, and the chapter on optical instruments contains no description of a spectroscope. Mention is made of Nachet's binocular microscope, but Wenham's far more important pattern is omitted. Under the head of telescopes, those with silvered mirrors are left out, although they are wellknown to practical astronomers as instruments of very great importance, placing large apertures and great accuracy within the reach of moderate We think it is a great fault that ordinary works on optics do not give distinct ideas of the nature of light waves. Light propagated in all directions from a luminous point proceeds, if not interrupted, in a series of consecutive spherical shells, and unless this is understood the term "ray" becomes associated with erroneous ideas belonging to the now abandoned theory of corpuscular emission-that is of particles shot out in straight lines.

means.

Dr. Lardner was not a very accurate thinker, and the present editor has not sufficiently corrected his errors. Thus we are told that "the intensity of light which issues from a luminous point diminishes in the same proportion as the square of the distance from such point increases." We have italicised the word in which the blunder lies. It is obvious that if we walk away from a luminous point we do not thereby diminish the quantity of light issuing from it, although our eyes may receive less and less. If the Lardner dictum were true, all the gas-lights of London would be absolutely put out as any particular traveller receded from them in a railway trip.

THE ELEMENTS OF THE LATIN SYNTAX: with Short Exercises for the Use of Schools. By W. H. Harris, B.A., B.Sc., Lond. Univ., F.G.S. (Hodder and Stoughton.)—This is a very praiseworthy effort to make Latin Syntax more intelligible to pupils. We commend it to the attention of the scholastic profession, and of private students, in the conviction that they will find it a useful book.

THE THEORY OF OCULAR DEFECTS AND OF SPECTACLES. Translated from the German of Dr. Hermann Scheffler. By Robert Brudenell Carter, F.R.C.S. (Exam.), Fellow of the Royal Medical and Chirurgical

It

Society, Consulting Surgeon to the Gloucestershire Eye Institution, with Prefatory Notes and a Chapter of Practical Instructions. (Longmans.)— This is an elaborate treatise adapted exclusively to professional use. deals mathematically with the varieties of optical defects to which eyes are subject, and indicates the kind of artificial help that will be most effective. It is a learned painstaking work, which scientific oculists and spectacle makers will find worth studying; but we could extract nothing from it adapted to the general reader.

FIRST LESSONS IN ANCIENT HISTORY FOR YOUNG PEOPLE. By the Rev. T. Woolmer, author of "Child Training." (Hodder and Stoughton.)— Some parts of this little book are nicely done, but the author sticks to the orthodox chronology of the creation of the world, and places his Egyptian dates far below Bunsen and Lepsius. About the present state of Egypt he talks great stuff. "Her lands are untilled," "her fields unsown," "her cities deserted," etc., etc., expressions not according with fact.

NOTES AND MEMORANDA.

SWEDISH ARCTIC EXPEDITION OF 1868.-Professor A. E. Nordenskiöld observes, in a letter to the President of the Royal Society, "that the polar sea is far more open in the autumn than at any other season of the year, but that even then the passage is soon stopped by dense and impenetrable masses of broken ice. That during the winter the polar basin is covered by unbroken ice, and that the freezing of the surface begins as early as the end of September. That an autumn cruise north of 80° lat. is attended with unusual dangers, owing to the darkness and storms then prevailing, no ship being able a long time to sustain a night storm among large rolling pieces of ice and a cold of -15° Cent. If the ship has the good luck not to be more or less damaged by the unavoidable encounters with ice mounts, it will soon, by the immediate freezing of the washing waves, be itself quite covered and pressed down with ice. The idea of an open and comparatively milder polar basin is quite chimerical; on the contrary, 20′ to 30′ north of Spitzbergen, a region of cold seems to begin, which no doubt stretches round the pole. The only plan to attain the pole, from which success can be expected, is that adopted by most English arctic men, namely, of going northward by sledges in the winter, either from Smith's Sound or Seven Islands. The expedition is said to have collected valuable information on Arctic natural history, temperature of currents, etc.

TERTIARY PLANT BEDS OF NORTH GREENLAND.-In his annual address the President of the Royal Society stated that the collection of fossil plants brought from Greenland by Mr. Whymper, and sent to Professor Oswald Heer for examination, would shortly be returned to London, and a complete series of specimens deposited in the British Museum. The deposits belong to the lower miocene, and it is remarkable that so far north as 70° lat., at Disco, two cones of magnolia were found.

NEW PHOTOMETRIC STANDARD.-Mr. Crookes recommends as a substitute for the "sperm candle," which is apt to vary, a glass lamp holding about two ounces. The aperture for the wick is 0-25 of an inch in diameter, and the wick is composed of fiftytwo pieces of platina wire held in a platina tube. The fuel is alcohol sp. g. 805, and pure

benzol boiling at 81° C., mixed in the proportion of 5 to 1. The photometer he uses operates with polarized light, and is upon the same principle as one by M. Jansen.

MICROSCOPIC PLANTS AND ANIMALS IN ERUPTIVE ROCKS.-In a pamphlet pub. lished at Leipzig, "Über eine Mikroskopische Flora and Fauna Krystallinischer Massengesteine (eruptivgesteine) von Gustav. Jenzsch," the author speaks of numberless forms of plants and animals met with by him in polished sections of certain rocks called eruptive, and supposed to be of igneous origin. He instances the Melopyr of Zwickau, and a quartz porphyry of Halle. He has met with diatoms, polythalama, and polycystina, and rarely with arcella amongst the "polygastric" infusoria. He describes zoospores and the conjugation of algae. He likewise speaks of a new infusorian "Rynchopristis," and a rotifer "Trikolos."

DESCENT OF GLACIERS.-The Rev. Canon Moseley comes to the conclusion, from mathematical calculation, that the weight of a glacier, together with the weight of any snow mass behind it, would not account for its peculiar descending motion at the slopes which are observed. The glacier moves not as a whole, but with different velocities in different parts. "It moves faster at its surface than deeper down, and at the centre of its surface than at the edges." Thus it suffers constant disruption, and the parts are reunited by regelation, as Faraday explained. The displacement of particles one over the other in this motion is known in mechanics as shearing, and Mr. Moseley shows that the resistance to this movement is so great that the weight of the mass could not account for its descent; and that some other force much greater, and producing internal molecular displacements, must come into play. (See "Proc. Roy. Soc.")

TYNDALL ON BLUE SKY AND POLARIZATION OF SKY LIGHT.-For the details of Professor Tyndall's experiments we must refer our readers to the "Proceedings of the Royal Society." He uses a glass tube about a yard long, and 2 to 3 inches wide, into which he introduces the vapour to be examined, and then illuminates it in a dark room with a beam of electric light. Vapours are selected which suffer chemical decomposition when thus acted upon, and form precipitates. He says, “by graduating the quantity of vapour, this precipitation may be rendered of any degree of fineness, forming particles distinguished by the naked eye, or particles which are probably far beyond the reach of our highest microscopic powers. There is no reason to doubt that particles may be thus obtained whose diameters constitute but a very small fraction of the length of a wave of violet light." When the vapours are sufficiently attenuated and illuminated by an electric beam, a blue cloud appears visible in a dark room, but not in ordinary daylight. These particles polarize light, but not according to "the law of Brewster, which makes the index of refraction the tangent of the polarizing angle." Employing substances of widely different refractive indices, he has found the finelydivided matter of the "blue cloud" give a maximum of polarization in a direction at right angles to the illuminating beam. He considers that water particles in the more minute state of division exist in the higher regions of the atmosphere, and have the same effect.

PLANT REMAINS INTERSTRATIFIED WITH BASALT.-At a recent meeting of the Geological Society, Mr. W. H. Bailey described a layer of red clay, 4 to 8 inches thick, separated by a conglomerate bed from the underlying basalt, and from the upper layer of basalt by earthy beds, aud containing fir cones, leaves of dicotyledonous plants, and a few elytra of beetles. Mr. David Forbes remarked that the existence of this leaf-bed might be regarded as affording ground for belief in the non-igneous origin of basalt, but from experiments he had made he found half an inch of clay sufficed to protect vegetable forms from destruction, when a mass of melted slag was allowed to flow over them.