conflagration has not reached stand in the clearest relief as they are seen for probably the last time; but in a dozen spots, at both sides of the bridges, sheets of flame and awful volumes of smoke rise to the sky and positively obscure the light of the sun. I am making these notes on the Trocadéro. Close and immediately opposite to me is the Invalides, with its gilded dome shining brightly as ever."

Another as follows:-"As I drive along the green margin of the placid Seine to St. Denis, the spectacle which the capital presents is one never to be forgotten. On its white houses the sun still smiles; he will not refuse his beams spite of the deeds which they illumine. But up through the sunbeams struggle and surge ghastly swart waves and folds and pillars of dense smoke; not one or two, but I reckon them on my fingers till I lose the count."

Twenty-four hours later, the change has come. "The rain is now falling heavily, has been falling heavily all day, and may do something for burning Paris. The sound of artillery has died away;" and from another writer :-"A heavy smoke hangs over Paris and rain is constantly falling."

I believe it has often been remarked that rain generally follows a heavy cannonading, but in this case there is an almost unexampled artillery fire and tremendous conflagration at the same time, accompanied by a sudden and violent change in the atmospheric conditions. From where I am writing we noticed a remarkable change on Thursday morning, and about 2 P. M., after intense closeness and oppression, a rain of a tropical character set in for twelve hours or more. On many occasions in Queensland, I noticed that in seasons of drought, after extensive grass fires, causing intense heat, heavy thunderstorms generally followed. GEORGE PEAKCE SEROCOLD

Rodborough Lodge, Stroud, May 27

Alleged Daylight Auroras

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SEVERAL letters having appeared in recent numbers of NATURE, giving what the writers consider to have been undoubted instances of aurora visible in the daytime, you will, I hope, allow me to state the reasons why I still adhere to the views expressed in my former communication on this subject. And, first of all, I must beg your correspondent Mr. Jeremiah not to think me uncourteous if I dismiss at once, as unworthy of serious criticism, the cases which he has dug out of monkish chronicles. It is likely enough that some of these old records may be imaginative descriptions of nocturnal auroras, and as such they are not without intercst, but I cannot admit them as competent witnesses on a point of nicety.

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A more modern instance adduced by the same correspondent will be found at p. 7 of NATURE for May 4, under the title "Aurora Borealis, seen in the daytime at Canonmills." In this case it is difficult to know what relation is intended between the title and the account which follows. The account describes the clearing off of the clouds in a mass from the north-west, with the production of an azure arch," the centre of which "reached an elevation of 20°." If I reply to this that the clearing off of clouds is not an aurora, even though they clear off in a compact body from the north-west, leaving an azure arch," I may be met by the rejoinder that nobody said it was; and yet I strongly suspect that the writer had some confused idea that he was describing an auroral arch, and I am certain that nine out of ten readers, misled by the heading, would take the same view. Stripped of the cloud-phenomena, all that remains of the Canonmills aurora is the appearance of some 'very faint perpendicular streaks of a sort of milky light," which could be traced across the segment of blue sky, but were "extremely slight and evanescent. Considering the probability that the observer regarded the cloud-arch as auroral, which it certainly was not, and considering how his judgment would be likely to be biassed by that idea in the interpretation of "extremely slight and evanescent appearances, I think we may fairly regard this testimony as particularly weak.

In NATURE for Dec. 8, 1870, Mr. Cubitt describes and figures a double auroral arc which he saw in broad daylight on the 25th October. It was "some 25° above the horizon, and almost due east. In my first letter I expressed a doubt of the correctness of this observation on the ground that auroral arcs are not seen in the east. My criticism has since been challenged on two distinct issues. Mr. Jeremiah insists that an auroral arc may * NATURE, vol. iii. p. 126.

extend towards the east, and that what Mr. Cubitt saw may have been the eastern extremity of a northern arc. A reference to Mr. Cubitt's letter and illustration will show at once that if what he saw was any part of an arc, it was the apex and not an extremity. But another correspondent, Mr. Reeks, in NATURE of Dec. 29, 1870, in criticising my remark, makes a statement which is more difficult to answer. He affirms positively that in Newfoundland he has many times seen the arch nearly due east, that is, as he explains, with "the extremities pointing N.N.W. and S.S.E." I would suggest, however, in reply to this statement, that in an extensive auroral display there may be fictitious arches, produced by the accidental correspondence of streamerson either side of the " cupola. An arch of this kind may easily extend from N.N. W. to S.S.E., spanning the entire heavens It is essentially different from the true auroral arc, which, untii much stronger evidence to the contrary is adduced, I shall still believe to be invariably transverse to the magnetic meridian, Obviously, Mr. Cubitt's are was not of the kind that Mr. Recks describes.

I pass on to a record of daylight aurora, which, more than any other that I have seen, demands a careful investigation. I refer to An Account of an Aurora Borealis seen in full Sunshine, by the Rev. Henry Ussher, D.D.," said to be taken from the Transactions of the Royal Irish Academy for 1788, and quoted by the Rev. T. W. Webb in NATURE for May 11. Dr. Ussher's account, it must be admitted, is most particular and complete. He describes "whitish rays ascending from every part of the horizon, all tending to the pole of the dipping needle, where, at their union, they formed a small thin and white сапору similar to the luminous one exhibited by an aurora at night." Nothing can be more precise. But is it not also a trifle too wonderful Surely, if any part of an aurora is to be seen by daylight, it must be just one here and there of the most vivid beams. That the whole phenomenon should be visible at noon-day in all its com pleteness, just as at night, even to the faint extremities of the streamers in the magnetic zenith, is to my mind so entirely in conceivable that not even the authority of a doctor of divinity can command my faith in it. I can much more easily believe that the sky presented a remarkably symmetrical arrangement of radiating cirri, and that the observer, impressed by the recollec tion of the aurora of the previous evening, persuaded himself that the "rays coruscated from the horizon to their point of union." The confirmation by "three different people is of little value unless their observations were independent.

To those who have no clear conception of the difference be tween cirrus and aurora, the foregoing arguments will be meaning less. Some persons write very loosely of "luminous cirri," and I have even seen described the transformation of cirrus cloud 220 aurora as it grew dark. I believe that there is no connection between the two phenomena beyond an occasional and parely accidental similarity of form, and that when the two co-exist, the cirrus, instead of being the seat of the aurora or deriving lumi nosity from it, only serves to obscure its brightness, and, if dense enough, may appear in the form of dark bands across the auroral light, the latter being, as I conceive, at a very much greater elevation.

I adverted in my former letter to the argument that may be drawn from the non-visibility in the day-time of other lights com parable with the aurora, and I will only now add the following suggestion. If the auroras that occur in this country are occa sionally visible in daylight, it might be supposed that the much grander displays of the Arctic regions would be habitually visible in daylight. But is the fact so? Clifton, May 23

Aurora Australis

GEORGE F. BURDER

TRAVERSING the Indian Ocean 44° S. 65° E., I observed, September 24th, 1870, 4h. till 13h. Greenwich time, a south polar light of great intensity and splendour. After my arrival at Manado (Celebes) I was just writing a few lines about it for the readers of NATURE, with the purpose of knowing whether at the same time an aurora, or at least disturbance of the magnetic needle, had been observed on the northern hemisphere, when I saw in NATURE (Nos. 49, 50, and 51, 1870), several interesting descriptions of aurora borealis observed September 24 in England, &c. I am not aware whether many observations of southern polar lights have been recorded, but I remember that those which Cook described in the year 1773 were coincident with aurora borealis observed in Friesland, and others observed in 1783

at Rio Janeiro were coincident with polar lights in the northern hemisphere. At all events I believe that the attention of men of science is not sufficiently directed to this coincidence of northern and southern polar lights, at least not as much as it deserves in respect to the theory of polar lights at all; and I should be very glad if, in consequence of this notice, authorities would discuss this highly interesting phenomenon in NATURE.

I shall later, according to my diary, accurately describe the display of this splendid aurora australis, and mention the influeace which it perhaps or probably had on the abnormal meteorological phenomena, which I observed during the succeeding days. ADOLF BERNHARD MEYER

As an ardent and not inexperienced votary of photography, I am fully alive to the value of photographic evidence, and regard with enthusiasm each fresh victory which photography achieves, yet I cannot myself look with any very great degree of satisfac. tion upon the photographs of the late solar eclipse either as examples of photography or as evidence contributing to our knowledge of solar physics. In saying this I make no reflection whatever upon the ability or efforts of those by whom the pictures were produced. On the contrary, I am aware that when these pictures were taken the first grand requisite of photographic success-a clear view of the object to be represented-was scarcely to be obtained. Briefly; from a technical point of view, the pictures are of but indifferent definition, and the identity of the coronal rifts in the Cadiz and Syracuse photographs not satisfactorily conclusive, in addition to which in the picture by the American observers, the so-called coronal light extends a long way over the lunar disc, which seems to me to preclude the possibility of its being other than a phenomenon of terrestrial meteorology. A few weeks ago, when the sky appeared almost cloudless, I observed a beautiful lunar halo, very much resembling the so-called corona, which I apprehend no one would attribute to anything but atmospheric moisture. Why, then, in the instance of a sky burdened with innumerable clouds, should we attribute the halo of light surrounding the solar disc to other than atmospheric causes, even though there should be something which might be mistaken for a coincidence in two distinct photographs of one or other of the rifts which were characteristic of that halo?

PERMIT me to state that the presumed "important bearing on the so-called "Eozoon Canadense," of the principal fact noticed in the communication entitled Paleozoic Crinoids, which appears in NATURE of May 25th, is discussed in a paper by Dr. Rowney and myself, contained in the forthcoming number of the Proceedings of the Royal Irish Academy, now on the eve of publication. The paper referred to is a reply to the articles by Drs. Dawson and Sterry Hunt, which appeared in the last (second) number of the Proceedings. Glenoir, near Galway, May 29 WILLIAM KING

WITHOUT going into the vexed question as to whether Eozoon Canadense is or is not of organic origin, I may be permitted to express some surprise at the new, and, to say the least of it, startling theory broached by Mr. Perry in last week's NATURE, of the vaporous formation of a certain limestone. The only facts brought forward in support of this view are, its occupying pockets, its foliations, and its conformation with irregularities of surface in the pre-existing rock. All these could be as well accounted for on the supposition of deposition from aqueous solution, without doing violence to the fact that carbonate of lime is not volatile at any temperature. E. T. H.

THE INEQUALITIES OF THE MOON'S

MOTION

THE following is an abstract of the method of computing the inequalities in the motion of the moon which are due to the action of the planets, proposed by Prof. Newcomb in the paper presented to the Academy of Sciences of Paris on April 3.

When we consider the movements of the sun, moon, and earth, under the sole influence of their mutual attraction, the position of each of these three bodies in space will be given in terms of eighteen arbitrary constants, and of the time. The problems of the relative movement of the moon around the earth, and of the movement of the centre of gravity of the earth and moon around the sun, have been solved with a degree of approximation sufficient at least for the purposes of astronomy. Thus, we have the co-ordinates of any two bodies relatively to a third, or relatively to the centre of gravity of the system, in terms of twelve elements and of the time. It only remains to add the expressions for the uniform movement of the centre of gravity in a straight line, to have the general expressions for the co-ordinates of each body.

We have then only to consider the action of the planet to vary the eighteen elements according to the method of Lagrange, to have the movements of each of the three bodies under the influence of the attraction of the planet. Unfortunately, the expressions thus obtained are at first extremely complicated. We have to compute a coefficient corresponding to each combination of the elements taken two and two. The entire number of the coefficients is, 17 x 18 therefore, 153. And each coefficient contains eighteen products of the partial differential coefficients of the co-ordinates of the three bodies relatively to the elements. These latter differential coefficients are so complex that the formation of any one product would be a considerable labour. The direct formation of the coefficients required is therefore impossible. The paper in question is principally devoted to an explanation of the simplifications which may be introduced into the problem.

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It is first shown that all the coefficients formed by combining any one of the six elements which fix the position of the centre of gravity with any of the twelve elements of the relative motion, vanish identically, while the combinations of those six elements with each other give only the principle of the conservation of the centre of gravity. This leaves only sixty-six combinations. It is then shown that, if the elements are divided into two classes, the first class being the mean longitudes, the longitudes of the perigees, and the longitudes of the nodes of the sun and moon, and the second the mean distances, eccentricities, and inclinations, the coefficients vanish whenever the two elements combined belong to the same class. The number of coefficients is thus reduced to thirty-six, and they are simply the differential coefficients of six functions of the elements of the second class. These functions are formed an extremely simple process when we have the rectangular co-ordinates expressed as functions of the elements

and the time.

The remainder of the process is simply one of the development of a very complex perturbative function, and is of no especial interest.

THE HELIOTYPE PROCESS

T one of the recent soirées of the Royal Society given Α1 Edwards and Kidd exhibited at work the new heliotype by General Sabine at Burlington House, Messrs. process, whereby photographic pictures can be very rapidly copied in by the aid of the printing-press. The process is very inexpensive, and so rapid that if one of the pages of NATURE were sent to the works, it could be

copied by photography, and within two or three hours after receipt, pictures could be turned out as fast as the printing-press could work them off. A few days ago I went over the works to examine the process, and a gentleman, who brought an engraving to the proprietors just as I arrived, saw the press printing off very good copies before I left, the interval being about two hours. The works are at some distance out of London, free from the smoke and dust.

The following is an outline of the history of the process:-Mr. Mungo-Ponton, of Clifton, discovered some years ago that if a dried film of gelatine and bichromate of potash be exposed to light, the film is afterwards insoluble in warm water. M. Poitevin afterwards noticed that where light had acted upon such a film, it took greasy ink just like a lithographic stone, whereas those parts on which light had not acted, absorbed water. In the attempt to produce pictures on this principle, he poured a mixture of warm gelatine and bichromate of potash over a lithographic stone, or plate of metal, and when the film was dry he exposed it to light under a negative. Where the light had acted the film became waterproof, and where it had not acted the gelatine swelled up like a sponge. This surface of hills and valleys prevented him from getting good pictures when he attempted to print from it on the lithographic principle.

Messrs. Tesse du Motay and Marechal tried the process just mentioned, and by carefully selecting their subjects, choosing those only in which there was little contrast of light and shade, they reduced the elevations and depressions on the surface of the film to a minimum, and thus obtained some very fair pictures, but after a very few had been printed off, the gelatine printing surface broke up. The next man who took up the process was Albert of Munich. Before his time, whenever a sufficiently thick film of gelatine to stand wear and tear had been used, the elevations and depressions were so great that the film could not be inked. Albert took a plate of glass about half an inch thick, covered it with a thick coating of bichromated gelatine, and after it was dry exposed it all over to light to make it insoluble. Afterwards he covered the surface thus prepared with a very thin coating of sensitive gelatine, on which the picture was printed from the negative. By this process he obtained some exceedingly beautiful and perfect pictures, and he is producing them by this plan at the present time.

Mr. Ernest Edwards took up the process at this point about a year ago. He made a thick leathery film at the outset by adding alum to the warm gelatine solution. He found that films so prepared still retained the lithographicstone-like property; they will scarcely swell up in water at all. They are insoluble, and they resist the wear and tear of the printing-press very satisfactorily.

The working details of the heliotype process are as follows. The films are prepared upon large sheets of accurately levelled finely ground glass, technically known as "greyed glass"; about 22 inches by 18 inches is a convenient size. The surface of the glass is first polished by means of a clean piece of rag, with a little solution of wax in ether; the exceedingly thin film of wax thus left upon the glass permits the dried gelatine film to come off easily. The glass plates after being waxed are levelled, and then a measured quantity of a warm mixture of gelatine, bichromate of potash, chrome alum, and water, is poured upon each plate from a jug with a piece of muslin tied over its mouth. The temperature of the solution in the jug is about 150° Fahrenheit, and after it is poured over the plate it sets in a very few minutes, but it requires a much longer time to dry. Curiously enough, until it is dry it is not sensitive to light; this fact was found out accidentally, for at first this part of the operations was carefully carried on in yellow light.

After the film has set, the plates are taken into a dark room to dry. If any of the fumes given off by burning gas

escape into this room, they act upon the film just as light would do, therefore although a gas stove is used to dry the plates, the products of combustion are very carefully car ried off. The gas stove used in the works was invented by Mr. George, a dancing master at Kilburn. It is a closed iron cylinder, into which air is admitted by one pipe coming from outside the house, and the products of combustion are carried off by another. A third iron air pipe enters the bottom of the stove, curves round its sides in a spiral, and then emerges through the iron plate forming the top. Air from outside the house is warmed in this spiral, after which it escapes into the drying-room, which is kept at a temperature of from 90° to 120°. At a temperature of 90° the films take about twenty-four hours to dry. As they dry they contract slightly, and thus separate themselves from the glass. These dried films are technically termed "skins"; they are of an orange colour, and about one-tenth of an inch thick. The picture is printed on them from a negative, and a faintly visible image is formed; when this image is fully out the films are

removed to a dark room.

Here each skin is floated in water, and caught upon the surface of a thick plate of zinc; a flat piece of wood, edged with india-rubber is then scraped with considerable pres sure over the film, so as to squeeze out all the water between the skin and the zinc. As the film still continues to absorb moisture, it is thus fixed to the zinc with the whole pressure of the atmosphere. After this the zine with its attached film is left for half-an-hour at least in a large vessel of water, for the superfluous bichromate of potash to soak out, and then the film is no longer sensi tive to light. If the film be thus soaked for several hours, or even days, it does not suffer.

The film, upon its zinc plate, is now ready for the printing press. It is damped between each impression, just like a lithographic stone. Then it is inked, and the best roller for the purpose is found to be one made of india-rubber, backed inside with "india-rubber sponge "to give additional softness. Ordinary lithographic ink is used. If stiff lithographic ink be employed, the surface will only "bite" where light has acted most; if thin ink be used, the leathery surface will only bite in the half tones of the picture; hence each picture is produced by at least two inkings, and advantage is taken of this circumstance to use two colours, and get warm shades in the half tones. It is very interesting to see the picture gradually growing under the inking process. By this method doubleprinting is executed with a single pull at the press. Ordinary Albion hand printing presses are used.

The negatives worked from in this process have to be "reversed," and they may either be reversed at the time they are taken, or afterwards. In the former case, instead of the lens of the camera being pointed direct at the object or picture to be photographed, a mirror, silvered on its front surface, is interposed at an angle of 45°. Another method of reversal is to take an ordinary unvarnished negative, and coat it either with a solution of indiarubber, or a solution of gelatine and alum. When the film is dry the plate is accurately levelled; it is then coated with a pool of collodion as thick as it will hold, and this collodion is then allowed to dry. Next the film is cut through with a penknife near the edges of the picture, and the plate is placed in water, where the negative soon floats off the glass, after which it is dried between blotting paper. The flexible negatives thus obtained are very durable, except when bad india-rubber is used in reversing them.

When a batch of pictures has been prin'ed from any particular skin, the film is taken off the zinc plate, and put away until wanted again. Mr. Edwards says the skins will stand a vast amount of wear and tear, and he showed me one from which he said 1,500 pictures had been printed, the last impression being as good as the first, and the skin ready for further work if necessary.

By this process many of Mr. Nasmyth's lunar pictures have been copied, and while on the premises I saw some work then being executed for Mr. Ruskin, and others known in the world of art and science. Bones, and some descriptions of anatomical specimens, are very easily photographed and printed by this process, which is also well adapted for landscapes and architectural subjects. If it be desired, a glaze is given to the finished prints in a very simple way. A little powdered magnesia is sprinkled over the surface of the print, and it is then placed on a smooth board and rubbed with a pad of flannel. Magnesia belongs to the soapstone family, and when used in this way it very readily gives a surface polish to paper. WILLIAM H. HARRISON

PARIS NEWS

FOR OR some time past the Académie des Inscriptions et Belles Lettres has held no regular sitting, almost all the members being refugees in Versailles or elsewhere. A special commission has been given to M. Rénan, one of its most distinguished members, to inspect the ruins of the Parisian monuments which have been destroyed by the Communists. M. Rénan, before publishing his last books on religious matters, had been sent to Mesopotamia to do the same work as Mr. Layard. Private letters received from the distinguished commissioner have been read before a group of members of different academical bodies at Versailles, sitting almost in an official capacity as the Academy for Inscriptions and Belles Lettres. It was stated that the Louvre buildings had escaped, and the bulk of artistic works will be saved from the conflagration. But the private Imperial Library in the old building of the Ministry of State has been destroyed. The value of this collection was chiefly historical, a number of the volumes being of special value from the fact of their having been presented to the several Kings and Emperors of France during the last three centuries. There were also some manuscripts of value, and collections of drawings for the study of art in the Museum. It was intended to open it shortly as a special art library for the use of students at the Louvre. The National Library, formed by Richelieu, was not burned down as has been rumoured; the building has entirely escaped. But it appears that steps had been taken by the insurgents to destroy it like the Serapion was at Alexandria when Omar took possession of the city. The Luxembourg buildings and Museum were saved only by the prompt exertions of the troops, when the insurgents were actually setting fire to them. The Luxembourg holds within its precincts a valuable library, where have been collected parliamentary documents from every nation and of every period. It was said to be the most valuable in the world in this respect. The collection of pictures is the richest in the world for works of living painters belonging to the French school. Courbet, the member of the Commune, had not been admitted to this, esteemed the highest honour by French artists. The Sorbonne is almost entirely saved, the walls only having been pierced by gun-shot or shells. The collections are most valuable, and very serviceable for students. We have no special news from Sainte Geneviève, a library largely used for law purposes on the Place du Panthéon; but it is supposed that the library is safe, as the insurgents were prevented from exploding it, though an immense quantity of powder had been deposited in the cellars, and it was used as an arsenal during the whole of both sieges. The Institute is safe, although it appears steps had been taken for its destruction. The Mazarine Library close to it is most valuable for works of the 17th and 18th century, as well as the library of the Arsenal. But according to every probability this last establishment will be entirely lost, owing to the vicinity of the Grenier d'abondance, a place

where an immense number of goods were collected, and which was ignited. A commission of the Academy of Sciences will be issued to study the different processes used by the insurgents for burning the Tuileries, Palais Royal, &c. Hay, soaked with petroleum, appears to have been very often resorted to, as well as canisters full of the same substance. In some instances petroleum had been poured from outside into the cellars, and an ignited match thrown into the impregnated air. The stories of firemen throwing petroleum from fire-engines are, we are happy to say, unfounded.

DREDGING OF THE GULF STREAM

WE are much gratified to learn from Harper's Weekly that preparations are now being made, under the direction of the Superintendent of the Coast Survey, for a very complete and thorough investigation of the deep-sea bottom, and especially of the channel of the Gulf Stream off the eastern coast of America, with an examination also of the Straits of Magellan and of a part of the Pacific Ocean. A steamer is now being built, which will shortly be launched, with the special object of continuing the deep-sea dredgings which, under the direction of Count Pourtalès, have given the Survey so much reputation.

It is expected that the arrangements will be completed by the end of August, and that the whole matter will be specially in charge of Prof. Agassiz, assisted by Count Pourtalès, whose experience eminently qualifies him for the post.

The plan of operations is, first, to run a line of dredging across the Gulf Stream between New York and Bermuda, and, if necessary, far enough eastward to completely cross the Gulf Stream current. The course will be thence to Trinidad, where a careful examination will be entered into to ascertain whether there is any difference in the deep-sea fauna of the adjacent waters and that of the coast of Florida. The expedition will then probably proceed to San Paulo for the purpose of examining the deepest known portion of the Atlantic, reaching to, at least, five thousand fathoms. From San Paulo it will again cut across the Brazilian current, and after possibly spending some time on the coast between Buenos Ayres and the Straits of Magellan will proceed by a zigzag course to the Falkland Islands, in the neighbourhood of which the expedition will remain for some time, for the purpose of solving certain important problems relating to both the deep-sea fauna and to that of the coast. It is next proposed to spend, at least, a month in the Straits of Magellan during the summer season of that portion of the globe. The work at the Straits being completed, the party expect to pass up along the western coast of Chili, next to the island of Juan Fernandez, and thence across to Callao. From this point the course will be to the Gallapagos, and thence across the Chilian current to some point on the west coast of Mexico-possibly to Mazatlan. The Revillagigedo Islands will next be visited, whence the party will proceed to San Francisco.

The entire exploration will probably occupy ten months, and bids fair to be the most important attempt ever made at determining the character of the fauna of the deep seas. The experience gained in all the former American and foreign expeditions of this kind will be freely used on this occasion; and no pains will be spared in the way of outfit to render the whole undertaking an entire success.

The fact that this expedition is under the direction of the Coast Survey is sufficient guarantee that nothing will be neglected to secure satisfactory results in the way of investigations upon the physics of the ocean, as well as its natural history, as it is intended to make use of the most approved apparatus for the determination of depths, temperatures, specific gravity, and chemical composition of the waters, &c.