almost exclusively. The article is divided into the two sections of plane and solid geometry. At the commencement the student is recommended by the weight of Prof. Cayley's advice to trace a number of curves, and he draws a few simple ones, so drawing attention to a point upon which Mr. Frost, in his "Curve-Tracing," strongly insists. Prof. Clifford, too, we believe, had it in his mind to publish an account of some methods which "are exceedingly simple and easy of application; they partake more of the nature of a manual craft than of a purely intellectual occupation, and may so be used as a rest from severer studies; and, as we can only imagine things of which we have seen the like by appealing directly to the senses, they extend those powers of concrete realisation which the growing complication of modern analysis renders daily more desirable." The methods he alluded to are "Projection, a process by which no alteration is made in the order, the class, nor in any other purely descriptive property of a curve ; " then "those modifications of form which leave the order of a curve unaltered;" then "those changes which exercise no effect upon the class." In the last two cases he proposed to use a process which he used to call "the composition of curves, by which a curve of any order or class may be built up out of the simplest elements." We fear that we have lost this proposed sketch, with the many other sketches he had outlined and lived not long enough to endue with a vitality he could so well have given them. After the illustrations referred to Prof. Cayley discusses shortly the metrical theory, and obtains the several familiar equations both in plane and solid geometry. In short paragraphs polar, trilinear, point, and line co-ordinates are described, but not applied. We have noted scarcely any misprints in the first article, but in the second there are several, all of which are easily detected. The figures are very well done.

We would draw attention to the article on Geodesy by Col. Clarke, which we have read with much pleasure. It is well illustrated, and the eighteen columns treat of the following matters :-Horizontal angles, astronomical observations, calculation of triangulation, irregularities of the earth's surface, altitudes, longitude. These are as fully discussed as need be in a sketch of the subject, and we shall expect that Col. Clarke's more extended work on Geodesy, referred to in NATURE, vol. xxi. p. 423, will take its place as a standard work for some time to come.

Geology occupies at the present day so important a position in the circle of the sciences that it deserves to be treated, in any modern cyclopædia, with no niggard hand. A slender essay, confined to a survey of the broad features of geology, would have been sadly disappointing in such a work as the " Encyclopædia Britannica." It is therefore satisfactory to observe that Prof. Geikie, to whom the editor entrusted this article, has put a liberal interpretation upon his trust. He has treated his subject with a fulness worthy of a great and growing science, and worthy too of the noble plan upon which the Encyclopædia has been projected. The masterly article which he has contributed to the new edition stretches over more than 320 columns, and is thus longer than most of the kindred articles, such as those on 66 Astronomy" and "Chemistry." Possibly it

might have borne, here and there, a little condensation, but on the whole it is admirably fitted for its place. It stands forth as a solid and comprehensive monograph which, if reprinted from the Cyclopædia, would form one of the most substantial treatises in our geological literature. But the article is not only substantial, it is, like all Prof. Geikie's writings, eminently readable. The cardina virtues of an encyclopædist are accuracy and conciseness of expression, and he usually finds but little scope for the play of literary graces. Prof. Geikie, however, is far too polished an author to write upon any subject in an unattractive style; and the present article is sufficient to prove were proof needed-that his graceful pen does not fail him, even when discoursing on the knottiest point in geology. The comprehensive nature of this article, and the originality with which the subject is treated, may be best shown by explaining the seven-fold division adopted by the author. First he deals with the Cosmical Aspects of Geology, and not only discusses the shape and the motions of the earth, but stretches his survey to the probable history of the solar system. Then he inquires into the nature of the materials of the earth's substance-an inquiry which falls under the head of Geognosy. In the early part of the article the author may seem to trench a little upon subjects which are treated in other articles, but this is almost inevitable in any cyclopædia. It is not to be expected that the several essays shall just touch each other without overlap, like the pieces of a neatly-jointed mosaic. The geognostic division of the article is followed by a section on Dynamical Geology, and this in turn by one on Structural Geology, or the architecture of the earth. Under the head of Paleontological Geology Prof. Geikie sketches the history of life as revealed by the fossiliferous deposits, while in the following section on Stratigraphical Geology he traces the chronological succession of events in the history of the stratified rocks. Finally a chapter is devoted to Physiographical Geology, or a discussion of the origin of the physical features of the earth's surface.

To see for the first time a great actor play the part of a familiar character is a treat; but the pleasure is seldom quite free from a mixture of disappointment. His reading of the part is usually not our pet and peculiar one, and we are, as it were, bullied into contentment by the great power of the performer. We felt something akin to it when we read the article "Heat" by Sir William Thomson, though the feeling was of course unreasonable. It often happens when for the second time we see a great actor play a great part we yield ourselves to his charm without a trace of intellectual reserve; so it will most likely be when next we read the article "Heat." At all events, the readers of NATURE may be assured that there is little in this article that they can justly find fault with, whatever they may miss to find that they expected. Could it be otherwise, when the author is the pupil of Regnault, the colleague of Joule, one of the patriarchs of the modern science of thermodynamics, the greatest living authority on the theory of heat in Britain? We shall therefore most modestly discharge our function by pointing out to our readers what they will find in Sir William Thomson's article; and by slightly indicating some points on which, to our regret, he has withheld his opinion.

The article opens with a discussion of the sense of heat, and of the distinction between heat and temperature. We are thus introduced to the conception of latent heat, which is explained at some length. The two leading methods of calorimetry, viz. calorimetry by latent heat, and thermometric calorimetry, are then discussed in general terms; and the results of the comparison of the different calorimetric units by Regnault and others are given. Then follows a full account of the origin of the modern theory of heat, which regards it as energy, and measures it by the equivalent amount of work. We thus have a third method of calorimetry, which is called dynamical calorimetry. Of the thirty-five pages occupied by the whole article eighteen are devoted to thermometry. This is the most important, and certainly the most interesting part of the article. After discussing a theoretical (and to some extent practical) system of thermometry by mixtures of hot and cold water, the thermoscope being the sense of heat in the hand, the author gives an elaborate classification of the different possible kinds of thermoscopes. Then comes an extremely interesting discussion of the merits of the different kinds of thermometers with arbitrary scales. The defects of the mercury-in-glass thermometer, and the advantages which led Regnault to prefer the (constant volume) air-thermometer are fully explained. We do not remember to have anywhere seen so full, and, it is needless to say, so philosophical an account of Regnault's results of the comparison of the different thermometric scales. The rest of the part devoted to thermometry is more or less speculative. The absolute thermodynamic scale of temperature, invented by the author himself, is defined; and its great advantage pointed out, viz., that it gives us a definition of temperasuch that, if a thermometer were graduated according to it from observation of one class of thermal effects in any one particular substance, it would agree with a thermometer graduated according to the same thermodynamic law from the same class of effects in any other substance." Thermodynamic formulæ are investigated in a variety of cases for graduating thermometers, according to the absolute scale, from experimental data concerning the thermometric substance. A number of instruments are described in detail which are intended to realise these cases in practice. We are thus introduced to the water steam, mercury steam, and sulphurous acid steam thermometers, and the constant pressure hydrogen thermometer. These instruments are mostly new as to their details, and all of them are new in the sense that they have not been practically used hitherto. Nevertheless a great future is predicted for them. It would appear that Sir William Thomson has himself constructed models of them all; but whether he has used any of them in practical work he does not say. It has doubtless occurred to many of our readers, as it has to us, to have doubts and difficulties about thermometric measurements. Nowhere could we find better reasons for our scepticism than in the earlier part of Sir William Thomson's discussion of the systems of thermometry at present in use; we shall look, therefore, with all the greater interest for some farther account of the practical working of these new instruments. Their success, were it even but partial, would be an immense gain to thermal science.

ture

Thermal capacity and specific heat are next defined; and a brief account of the leading features of the results. of different experimenters is given, without detail as to the methods employed in obtaining them. For further information we are referred to the articles on "Thermodynamics," "Matter," "Liquid," "Steam." The remaining five pages of the article deal with the transference of heat. Radiation is explained and distinguished from other modes of transference; but to our great regret is dismissed very briefly. A criticism of the work of the various experimenters in this department from an authority like Sir William Thomson would have been most interesting. There is still much doubt and difficulty hanging over the subject of the diathermancy of gases, for instance; we need scarcely mention as an illustration the famous controversy which has raged over water vapour. It may be however that these and kindred matters are to be treated under " Radiation" or "Light"; although we are not referred to these articles. The general principles of the theory of the conduction or diffusion of heat, as laid down by Fourier, are explained; and a most interesting critical account is given of the earlier attempts to measure conductivity. The explanation of the causes of the failure of Clément and Péclet in measuring high conductivities, such as that of copper, is very instructive, and should be closely studied by those engaged in like researches. Of the methods in use for measuring the thermal conductivity (or diffusivity as the case may be) of solids, Sir William Thomson prefers that of Angström, and recommends along with it the use of thermoelectric methods for determining the temperatures along the experimental bar. The mathematical theory of this method is given, and its connection with the researches of Forbes and Thomson on underground temperatures pointed out. The method of Forbes for measuring the conductivities of metals in absolute measure is described in general terms; and the results obtained with it by Tait are given, and compared with those of Ångström and Thalèn. We regret that no mention is made of the recent attempts to measure the conductivities of liquids and gases. The only result given is that of Bottomley for water, and no description of the method accompanies it. It is quite true that the success of many of these attempts has been somewhat doubtful; but, for that very reason, a criticism of the methods by a competent and impartial authority would have been most opportune, and useful as a guide to future experimenters. Appended to the article are a series of ten tables of thermal constants, and a reasoned synopsis of the principal mathematical formulæ that occur in the theory of diffusion. This last is a most valuable part of the article; for it could be given only by a master of the subject, and it is likely to be extremely useful to many physicists, who have sufficient knowledge to enable them to use such formulæ, but not sufficient mathematical power to find them for themselves, or sufficient time to hunt them up from ponderous treatises and half-forgotten memoirs when they want them.

We sincerely congratulate the editor and publishers of the Encyclopædia on the high degree of success which continues to attend their great undertaking.

OUR BOOK SHELF

Life and Her Children: Glimpses of Animal Life from the Amaba to the Insects. By Arabella B. Buckley. (London: Edward Stanford, 1880.)

AFTER light came life, and with that life there came its two great functions-growth and development. With the simplest as with the most complex forms there is the same eager race to be run, to increase in size, to multiply, and thus replenishing this earth, to die. "Life and Her Children" is a praiseworthy and admirable attempt to tell us something of the Children that Life sends forth, and of their history. Its main object is to acquaint young people with the structure and habits of the lower forms of life; but in our deliberate judgment it will do a great deal more. None will read its introductory chapter without advantage, and few will read the volume through without enjoyment. Within its narrow limits of 300 small pages no candid reader would expect to find all the details that might be wished for, or all the illustrations that might be desired. What constitutes the book's chief charm is the marvellously simple yet quite scientific style which runs through it, the food for thought and future study which it affords, and the truly philosophic glow which lights up its every page. The volume gives a general account of Life's Simplest Children, the Protozoa. The word "slime" does not seem to us quite a happy term by which to designate the living protoplasm of these creatures; this word conveys the idea of a something adhesive or glutinous, or of a something thrown off a living organisma something without a structure (sordies, eluvies)—and there seems somewhat of a "contempt for nature," a thought certainly never present in the author's mind, in the use of such a word. Jelly would seem a more appropriate word, as conveying the idea of the consistency requisite for life, and would have the sanction of use. Thus the Noctilucæ, called in this volume "tiny bags of slime," were described, if we mistake not, by their discoverer as "tiny spherical gelatinous bodies," and Prof. Huxley says, Noctiluca may be described as 'a gelatinous transparent body about the one-sixtieth of an inch in diameter.'"

The chapter on "How Star-fish Walk and Sea-Urchins Grow" is excellent. The story of how the five curious little oval jelly bodies swimming about by their jelly lashes in the depths of the smooth water in some English bayended in becoming respectively a lily star, a brittle star, a starfish, a sea-urchin, and a sea-cucumber, is well told, and woodcuts, though they make one see as in a glass darkly, help in their own way to make the meaning plain. In the "Outcasts of Animal Life" a difficult problem is treated of. It need not surprise one that it is not solved. The last four chapters tell of" the Snare-Weavers and their Hunting Relations (spiders)"; the Insects which change their coats but not their bodies, and those which remodel their bodies within cover of their coats; "the Intelligent Insects with Helpless Children, as illustrated by the Ants." This volume thus tells of the greater part of the living invertebrate animals as they are spread over the earth to fight the battle of life. Though in many places the battle is fierce and each one must fight remorselessly for himself and his little ones, yet the struggle consists chiefly in all the members of the various brigades doing their work in life to the best of their power, so that all while they live may lead a healthy, active existence. The little bird is fighting his battle when he builds his nest and seeks food for his mate and his little ones; and though in doing this he must kill the worm, and may perhaps by and by fall a victim himself to the hungry hawk, yet the worm heeds nothing of its danger till its life comes to an end; and the bird trills his merry song after his breakfast, and enjoys his life without thinking of perils to come. So Life sends her Children forth; and it remains for us to learn something of their history.

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LETTERS TO THE EDITOR

[The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts. No notice is taken of anonymous communications.] The Editor urgently requests correspondents to keep their letters as short as possible. The pressure on his space is so great that it is impossible otherwise to ensure the appearance even of communications containing interesting and novel facts.]

Prof. Tait and Mr. H. Spencer

As Mr. Spencer has already got the length of calling some of my statements "fictions, pure and absolute," it is time that this discussion should cease. But it is necessary that I should at least show my reasons for having made the statements in question. They will be found ample.

Mr. Spencer's pamphlet, which originated this discussion, and in which I am the first subject brought up for vivisection, bears on the title-page that it deals with Criticisms.

The only passages of mine which Mr. Spencer quotes, which can possibly have the slightest reference to himself, and which can in any way be construed into criticisms, are but two in number. In these, or in one of them, the cause of his attack on me must be sought.

The first is mainly a verbal transcription from Mr. Kirkman, and as such it is none of mine; but in introducing it I inadvertently (though correctly) spoke of the "Formula of Evolution" as a definition.

The second is a passage from a different part of my article on Sir E. Beckett's book, and its application is to materialists and agnostics in general.

This latter passage did not appear to me capable of having roused the vivisection-instincts of so calm a philosopher as Mr. Spencer, especially as it was not applied to any one in particular. Of course, then, I at once assumed that the former passage contained the offence which was to be expiated; and I was confirmed in this idea by the way in which Mr. Spencer put his formula alongside of the Law of Gravitation. I could not have ventured to suppose that Mr. Spencer "did not even know that he was in the habit of saying formula rather than definition." This naïve confession cannot but be correct. Had it been made in Mr. Spencer's pamphlet, I should not have thought it necessary to say a word. It explains at once his frequent entire misapprehensions of my meaning. So I give up my plausible theory of the origin of Mr. Spencer's attack on me; and shall, henceforth, ascribe that attack to my having made a singularly apt and telling quotation from Shakespeare!

With regard to the other parts of the discussion, I feel that I need not add anything to what I have already said; except on one point, an important one.

Mr. Spencer has employed an old remark of Prof. Huxley as to what mathematics can, and cannot, do; but he has not employed it happily, for the question at issue is really this :-Is it correct to speak, at one time, of force as an agent which changes a body's state of rest or of motion, and again to speak of it as the time-rate at which momentum changes or as the space-rate at which energy is transformed?

I answer that there is not the slightest inconvenience here; except, perhaps, in the eyes of those metaphysicians (if there be any) who fancy they know what force is. Such phrases as "the wind blows," or 'the sun rises," though used by the most accurate even of scientific writers, would otherwise (on account of their anthropomorphism) have to be regarded as absolute P. G. TAIT

nonsense.

Geological Climates

It was with great surprise I read Prof. Haughton's unqualified statement in last week's NATURE, that-"It is impossible to suggest any rearrangement of land and water which shall sensibly raise the temperature of the West of Europe,"-since I had, as I thought, in my recently-published volume-"Island Life"-not only "suggested" such a rearrangement, but also adduced much evidence to show that it had actually occurred throughout the periods when both the West of Europe and the Arctic regions enjoyed a much higher temperature than they do now. I will now briefly re-state my "suggestion," and will also make a few remarks on the general causes of difference of temperature, which may serve to render the subject more intelligible.

It is now well known that places in the temperate zones owe their temperature at different seasons only partially to the amount of direct sun-heat they receive, but very largely to the amounts of heat brought to them by currents of air. Thus we explain, not only the mild winter climate of our islands as due to the prevalence of westerly and south-westerly winds which have become warmed by passing over the Atlantic, but also the wonderful inequality of temperature at different seasons of the year. When we have warm spring-like days in mid-winter, it is because these warm currents of air are passing steadily over our islands; while continued hard frosts are as clearly due to masses of cold air from the north or north-east which drift down to us, often with no perceptible wind. Again, when in April and May we have days as cold as those of December and January, they can always be traced to northerly or easterly currents of air, and are probably often connected with the southern drift of the icebergs at that season. It is clear then, that if south-westerly winds were to continue throughout the winter, the severity of that season would be entirely abolished; and the same effect would be produced if by any means the winds from the north and east lost their severity.

Now the source of the constant warmth of our westerly winds is admitted to be the influx of warm water into the North Atlantic-chiefly by the Gulf Stream; and this warm northward flow of tropical water, being primarily due to the trade-winds, is not confined to the Atlantic, but is equally present in the other great oceans, and similar effects are produced in them, though nowhere to so great a degree as in our islands, owing to our insular position and the great extent to which Europe to the east of us is permeated by water as compared with North America or Asia. The North Pacific, with its great Japan current, is probably quite as warm as the North Atlantic; but Vancouver's Island, though further south than London, has not so mild a climate; and this can be clearly traced to the great mass of land to the east and north of it, the lofty snow-clad mountains, and the absence of those deep gulfs and inland seas which do so much to ameliorate the climate of Europe.

Prof. Haughton states, in his "Lectures on Physical Geography," that the Kuro Siwo, or great Pacific current, is two and a half times as large as the Gulf Stream, while the Mozambique current, which forms the outflow of the warm waters of the Indian Ocean, is one and a half times as much, so that these two currents have together four times the bulk and heating power of the Gulf Stream. If therefore these two currents at any time obtained an entrance into the Arctic Ocean, it is difficult to over-estimate their effect on its climate. The Gulf Stream, of which probably not half passes northwards of our islands, gives to Iceland the same winter temperature as Philadelphia, and keeps the North Cape (far within the Arctic circle) permanently free from ice, and this, notwithstanding the powerful counteracting influences of the lofty Scandinavian mountains on the one side, and the huge ice-clad plateau of Greenland on the other. Suppose that only an equal proportion of the Kuro Siwo entered the Arctic Ocean, is it not probable that no seaice at all would form there? While, if Greenland were less elevated and thus ceased to be an accumulator of ice, the combined effect might be to render the whole Polar area free of icebergs. This would at once do away with the chief source of winter cold to all north temperate lands, and ameliorate the climate of America as much, proportionately, as that of Europe. But we have yet to consider a still more powerful agent in ameliorating the climate of Western Europe in Secondary and early Tertiary times. The heated waters of the Indian Ocean have now no northern outlet, and only penetrate the continent in the sub-tropical Red Sea and Persian Gulf. Now if we suppose the waters of the Bay of Bengal and the Arabian Sea to have had northward outlets through the heart of the Euro-Asiatic

continent, penetrating in two or more directions into the then much more extensive Arctic Ocean, we should have an agency at work which would render the presence of any permanent ice in The cooling agency of ice being once abolished, the comparathe North Polar area as impossible as it is now in Scotland. tively small area of the Polar as compared with the Tropical seas (about one-tenth) would facilitate the raising of the temperature of the former to perhaps 15° or 20° F. above the freezing point, and this would not only give the Arctic lowlands a climate quite sufficient for the vegetation which we know they supported, but, by doing away with the only source of our winter cold, would give our islands a perfect immunity from frosts and render them capable of supporting the vegetation now characteristic of sub-tropical lands.

That the modifications of land and sea here indicated did exist throughout a considerable portion of past geological ages, and that the existing consolidation of the great northern continents, to which the possibility of our present Arctic climates is mainly due, is a comparatively recent and abnormal phenomenon, I have At endeavoured to prove in the work already referred to. present I have only undertaken to show, that a "suggested" rearrangement of land and water adequate to raise the temperature of Western Europe to a very sensible, or even to a very large extent, is "possible.” ALFRED R. WALLACE

Photophonic Music

I HAVE not yet met with any reference to the capabilities of the photophone for giving musical harmonies. Might not some curious effects be got in some such way as this:-Suppose a disk perforated with holes in four concentric circles corresponding to the notes of a chord; a beam of light to be sent through each circle to a lens and disk of rubber with tube (as Prof. Bell has described), the four tubes debouching in a cup-shaped cavity to be applied to the ear; lastly, the disk to be rotated variably by means of a small windmill or otherwise. Another arrangement might be to make the beams cf light pass through the holes to selenium cells in four telephone circuits, the four telephones being placed in one frame, against which the listener's ear would be put, or coupled in pairs, one pair put to either ear. Again, might not harmonised tunes be obtained thus:-Suppose a broad open drum of wood or cardboard rotated uniformly on a screw forming a vertical axis. The drum is perforated in a spiral band of four lines of holes (for the light), corresponding to the notes of the harmonised air to be produced. This spiral band passes before four rubber disks or selenium cells (as in the former system), but arranged vertically and placed within the drum, at the lower part. The drum, it will be understood, works gradually down the axis, presenting a continuous four-line series of holes before the receiving apparatus. Again, a long continuous strip of cardboard, with four rows of holes, might be passed before the receiver in any convenient way. M.

The "Philosophy of Language

THOUGH it is my principle never to answer any criticism of my writings, I find myself obliged to deviate for once from this rule by the character of your highly esteemed review, and by the desire to find a discerning appreciation from your readers, whose judgment has for me the greater value, as it is the main aim of all my works to restore the relations between the science of mind and natural philosophy. Therefore you would oblige me very much by publishing the following short remarks:The critic of my brochure ("Max Müller and the Philosophy of Language,”) says, Nor is speech the deliberate product of a conscious will." Now it is the real aim of all my works on the philosophy of language to show how the human will— before dark and unconscious-grows to consciousness by language and human activity intimately connected with it. Can there be the least doubt of this, even if I refer only to the motto of my "" Origin of Language,"-"Language has created reason, before language man was without reason"? Your critic has made me say just the contrary of what I really have said. Besides, it would have been only fair if the critic had pointed to the following little passage of my brochure :— "Max Müller has since expressed his full assent to this view," (viz., my theory of the origin of language). Mayence, November 11

LUDWIG NOIRÉ

[I gladly accept the author's assurance that he adheres to the view that "language has created reason." At the same time his

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express words as well as the general bent of his argument seemed to point in the opposite direction. Thus at p. 81 he writes :— Language is a product of association. . . . Language is a product of an active, not of a passive, process; it is the child of will, not of sensation." The statement that language is "the child of will" seems to me practically identical with the assertion that "speech is the deliberate product of a conscious will," because the will here spoken of, being "an active process," is necessarily conscious.-A. H. KEANE.]

Notes on the Mode of Flight of the Albatross WHEN watching the albatross one is struck with the fact that the bird gets up to windward without appearing to use his wings to a degree sufficient to account for the same. The sailors are satisfied with the explanation that he beats to windward. The conditions are of course not analogous to those of a ship sailing to windward. If the wind be very light, or if there be a calm, occasional powerful and obvious flapping of the wings occurs. If there is no wind, the birds often settle on the water round the ship. In very heavy weather the birds disappear altogether, probably settling on the water. Except that for breeding they resort to the islands, I believe they frequent the open ocean, where the surface is seldom without more or less swell.

On watching the flight of the albatross, one observes that in order to rise from the water violent and obvious flapping of the wings is necessary, which is continued some time after the wings cease to strike the water. After a start has thus been effected, if there is a fresh breeze, the wings are kept almost motionless. Sometimes the bird goes some distance with the impetus derived from the flapping of the wings at the start, but sooner or later he turns so as to expose the plane surface of his wings full to the force of the wind, rising at the same time some height above the water, and drifts off to leeward, thus soon acquiring the velocity of the wind; then swooping down into the hollow between two swells, he turns his head to windward, and keeping close to the surface of the water, sails along more or less against the wind for a surprising distance; finally, rising over the crest of a wave comparatively high into the air, and turning with his wings as before, so as to catch the wind to the fullest extent, he again lets himself drift off to leeward.

Thus the manoeuvre he performs seems to consist in drifting with the wind in such a way as to attain its velocity very soon, and then turning round so as to make use of this velocity to carry him in the contrary direction.

Of course if he still remained exposed to the wind which had imparted to him its velocity he would not travel far against it before he came to a standstill, and he would certainly make no progress to windward; but by keeping close to the surface of the water, and as much as possible in the hollows between the waves, he is almost out of the wind; and in this comparatively calm region the impetus derived from the wind will carry him a long distance in exactly the opposite direction to that of the wind itself.

Νο

This manoeuvre appears to be an important factor. doubt the almost imperceptible movement of the wings may assist, though that this alone is insufficient to account for the progress to windward appears evident from the powerful efforts made with the wings in rising from the water and in calm weather. I have never had an opportunity to observe the albatross flying over land or over level water. If the manoeuvre above described be an important factor, the birds then would have to use their wings much as they do in very light winds on the ocean. If very strong winds were blowing, they would have to settle on the land or in the water in order to remain at the locality. ARTHUR W. BATEMAN

A General Theorem in Kinematics

PROF. EVERETT (ante, p. 99) has overlooked in the introductory paragraphs of Prof. Schell's paper, to which he refers for the original statement of the theorem re-discovered by Prof. Minchin, the acknowledgment: "Der Mittelpunkt der Beschleunigungen und jene beiden Kreise wurden bereits 1853 von BRESSE gefunden." The reference is to the Journal de l'Ecole Polytechnique, tom. xx., "Mémoire sur un Théorème nouveau concernant les Mouvements Plans, etc." By means of the "two circles" Bresse determines the point c (7) “qui aura une accélération totale nulle" (p. 82), and then by very ingenious applica

YOUR correspondent "P. C." (NATURE, vol. xxii. p. 607) asks information concerning a work, in English or French, on geometrical optics, thoroughly explaining the optical construction of telescopes and microscopes. I am not aware of any such publication these last forty years, but deem it possible that it may interest your correspondent to know of the existence of such a work in German by von Littrow, entitled "Dioptrik, oder Anleitung zur Verfertigung der Fernröhre." It was published, I believe, in Vienna about 1838. W. G. LOGEMAN

High Burghal School, Haarlem, Holland, November 17 [Littrow's "Dioptrik " was published at Vienna in 1830 in 8vo.-ED.]

Ozone

IF a slip of the prepared paper, used for testing for atmospheric ozone, be carefully moistened on one side with alcohol, using a clean camel-hair brush, on burning off the spirit and immersing the slip of paper in water the paper changes to a deep purple colour, as deep as No. 8 in Negretti and Zambra's scale of colours for ozone.

Is this due to the development of ozone? as, according to Schönbein, heat destroys ozone. Leicester, December 5

DU

J. P.

PLANTS OF MADAGASCAR URING the present year no less than four separate collections of plants have been received at Kew from Madagascar, including in the aggregate about a thousand species, represented by specimens complete enough to be botanically determinable. As the hills of the interior of the island attain an elevation of 10,000 feet, its range of climate is considerable. We now know not less than two thousand Madagascar flowering-plants, and probably have almost exhausted its ferns, to which the collectors have paid special attention, and which are about 250 in number, so that we may consider ourselves in a position to draw broad general conclusions as to the botany of the island.

Amongst the tropical types there are a considerable number of endemic genera. The lemurs find their parallel in the vegetable kingdom in the Chlænaceæ, a natural order whose nearest affinities are with Tiliacea, Dipterocarpeæ, and Ternstromiacea, which is strictly confined to Madagascar, and comprises four genera and about twice as many species, to which the Rev. R. Baron, in these new collections, has added a well-marked novelty in a second species of Leptolana. Altogether there are certainly not less than fifty genera confined to the island, some of them very curious types, as Dicoryphia in Hamamelideæ, Ouvirandra in Naiadacea, Asteropeia (placed in the "Genera Plantarum" in Samydacea, but which Mr. Baron's excellent new specimens will most likely have to be removed to Linacea), Macarisia in Rhizophorea, Deidamia and Physena in Passiflorea, Hydrotriche in Scrophulariacea, Canctia, Tánnodia and Sphærostylis in Euphorbiacea, Pachnotrophe in Morea, Calantica in Samydacea, and several each in the orders Rubiacea, Melastomacea, and Composite. To these endemic types the new collections add at last three, Kitchingia, a fine new genus of Crassulacea allied to Bryophyllum, with five or six species named after the collector of the first of the four parcels, Rhodocodon, a monotypic genus of gamophyllous Liliacea allied to Hyacinthus, and Micronychia, in Anacardiacea, also monotypic, figured lately in Hooker's Icones. Besides these the tropical flora of the island contains a large proportion: first, of endemic species of genera known elsewhere; second, of species