mere "feverish colds"; yet all such cases would have to be isolated in view of a threatened epidemic.

The extracts from the various Sanitary Acts appended to these papers form a very convenient work of reference for those interested in the subject, while the counsel's opinion on the powers of sanitary authorities as to influenza leave us very much where we were before. Dr. Sisley has, however, done valuable service in calling public attention to the inadequacy of our existing sanitary laws as a means of checking the spread of such a disease as influenza, and many will cordially indorse his opinion that "much improvement in this respect is not to be hoped for (until the sanitary service is consolidated, and becomes one fold under one shepherd--a Minister of Public Health."

OUR BOOK SHELF.

Anthropogeographie. Zweiter Theil. "Die Geographische Verbreitung des Menschen." Von Friedrich Ratzel. (Stuttgart: J. Engelhorn, 1891.) THE first part of this work was published about nine years ago, and is still highly valued by all who care to study geography and anthropology from strictly scientific points of view. The present volume will also be found worthy of the author's reputation as one of the foremost authorities on all questions relating to the connection between man and the physical conditions by which he is surrounded. In the first part Dr. Ratzel deals with the habitable part of the globe, tracing the process by which man has taken possession of it, indicating the development of his ideas regarding it, and noting the characteristics of its northern and southern borderlands and of its vacant spaces. The second part he devotes to various aspects of statistics, discussing, among other things, the relations between density of population and degrees of civilization. In the third part are considered the traces and works of man on the surface of the globe-a subject which leads the author to treat of cities and their importance as historical centres, of ruins, roads and other means of communication between communities, and geographical names. The fourth and last part relates mainly to ethnographical questions, including questions as to the diffusion of ethnographical characteristics, and the origin of ethnographical affinities. The work is not only full of thought and learning, but has the advantage of being written in a fresh, clear, and vigorous style.

Within an Hour of London Town: Among Wild Birds and their Haunts. By "A Son of the Marshes." Edited by J. A. Owen. (Edinburgh and London: W. Blackwood and Sons, 1892.)

"A SON OF THE MARSHES" is now so well known that any new book by him is sure to find readers and admirers. He does not, of course, make important contributions to science. His writings merely record the impressions produced upon him by various aspects of nature in which he happens to be especially interested. But his impressions are so thoroughly true, and are presented in so vivid a style, that they may always be studied with pleasure. Even his talk about very common things has a certain charm, for he observes them accurately, and brings out by skilful touches their relations to other things that are not quite so intimately known. The present volume has all the characteristics of his previous books, and should do a good deal to foster in the mind of "the general reader" a liking for some of the more attractive facts and ideas of natural history.

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 intended for this or any other part of NATURE. No notice is taken of anonymous communications.]

Exchange of Professorial Duties.

THE proposal of my friend Prof. Anderson Stuart, explained in the subjoined letter, seems to me one which may very probably commend itself to the professors and governing bodies of some of our Universities and University Colleges; and I therefore venture to ask for its publication in NATURE. By correspondence twelve months in advance, such an exchange as is here suggested could be arranged (with the assent of Senate, Council, or other authority), and would undoubtedly, where practicable, be of very great interest and advantage, not only to the teachers concerned, but also, in no less degree, to their classes. E. RAY LANKESTER.

Oxford.

Shepheard's Hotel, Cairo, February 13, 1892.

DEAR PROFESSOR RAY LANKESTER,-In conversations with teachers in Europe during my two visits (1890-91, 1891-92) they have again and again said how much they would like to visit the colonies for pleasure, health, or the opportunity of study, as the case might be; but of course they could not, being bound by their duties. On the other hand, the benefit to the colonial teacher of a periodical visit to the older centres of learning has all along been recognized.

Soon after my return to Sydney in March 1891, it occurred to me that it would be easy to secure at once a visit of a European teacher to the colonies and of a colonial teacher to Europe by a temporary exchange of duties. Every now and again it happens that a teacher must provide for the duties of his office by a substitute, as is done by the colonial teacher when absent on leave, and by the European teacher most frequently, perhaps, when ill. Why, then, should not two teachers in a subject, who could trust each other, agree to apply for leave of absence, each proposing the other as his substitute for the time specified? I cannot see that any governing body could reasonably object to the proposal, and so the arrangement would be concluded.

Immediately on my return to Europe in October last I spoke of the matter, and amongst others to yourself; and since then I have discussed it with many friends, one of whom referred to it approvingly at a recent meeting of the Royal Colonial Institute. The project becomes the more feasible the more one studies the details of carrying it out. Practically one's attention is confined to America and Australasia. A study of the periods of the University terms, and of the steamship time-tables, shows that it is feasible for the latter, which is by far the more distant-about five weeks.

Of the pleasures of the voyage, and of the advantages to be derived by the residence in the other land, I need not speak, for each may form his own estimate of these; but that "there is something in" the thing I am persuaded, and I should be glad of your help in ascertaining what that something may be. I am, dear Professor Ray Lankester, Yours faithfully,

T. P. ANDERSON STUART,
Professor of Physiology, University of Sydney.

Magnetic Storms.

EXACTLY twenty-seven days from the magnetic storm and splendid aurora of February 13-14, which has already been mentioned in NATURE, there was on March 12 another very fine aurora in the United States and Canada, and it also was accompanied by a powerful magnetic storm. This correspondence to the time of a synodic revolution of the sun, to which attention has been called by the writer many times within a few years past, is interesting, showing as it does that the motion of rotation is concerned to an important extent in the recurrence] of magnetic accumulating constantly showing that solar disturbances have storms and their accompanying auroras. The evidence is their maximum effect upon terrestrial magnetism when at the eastern limb and at or near the latitude of the plane of the earth's orbit. If the great sun-spot to which the aurora of

I WAS very sorry to find from Mr. Dunn's letter (p. 511) that I had not reported his evidence on the occurrence of young pilchards with perfect accuracy. He admits that the misunderstanding was probably not altogether my fault. It seems that in the days before the railway existed in Cornwall, and when seines were largely used at Mevagissey for the capture of pilchards, small pilchards under 8 inches in length, of the same size as French sardines, were often taken in vast numbers, but were either allowed to escape, or used only as manure. The sentence in my article, therefore, which states that Mr. Dunn had never seen such pilchards must be corrected, and I make the correction most willingly, regretting that I should have unconsciously misrepresented Mr. Dunn's statement.

But I must warn your readers against the idea that my article on the growth of the pilchard contained nothing which Mr. Dunn had not discovered and made known years ago. The letter to which he refers in Buckland's "Familiar History of British Fishes" deals with the subject of curing pilchards in oil, and contains nothing whatever about the rate of growth of the fish. It merely mentions that if small fish were wanted they could be had in quantities: "Some seasons their smallness is a pest to the fishermen, and millions have been returned to the sea after being inclosed in the seines, because of being no money value." But Mr. Dunn acknowledges that I correctly reported him as saying that no such small sardines have been taken since the factory at Mevagissey was started, and that no pilchards of the same size as French sardines have ever been tinned in Cornwall. As for his exhibit of a series of pilchards from those an inch in length up to the two years old full-grown fish, I find that it is only catalogued in the Polytechnic Society's Report, and that no dimensions are mentioned, nor any description given. My published evidence on the rate of growth in this species was therefore by no means superfluous, and I am glad to find that my conclusions confirm those which he had already formed, but for which he had sought no satisfactory means of publication. I have often received and acknowledged with the greatest pleasure valuable information from Mr. Dunn in this instance I was unaware that he had collected any evidence on the subject beyond that which I acknowledged in my article.

:

But while correcting misunderstandings on my part, Mr. Dunn misunderstands part of my article far more seriously. I stated that the adult sardine of the Atlantic coast of France was of the same size as the full-grown Cornish pilchard, while the sardine of the Mediterranean, taken at Marseilles, was considerably smaller. I did not say that the English pilchard was "larger than those of other countries," and I did not say that the Spanish pilchard was smaller than the Cornish. My "informants" were Prof. Pouchet for France, and Prof. Marion for Marseilles; and the accuracy of their published observations on the mere question of size is not in the least affected by any grave doubts, however much italicized, on Mr. Dunn's part. Plymouth, April 5. J. T. CUNNINGHAM.

Ornithology of the Sandwich Islands. YOUR correspondents, Prof. Newton and J. E. Harting (p. 532), are a little hasty in their conclusions referring to the Banksian collection.

In order to make things clearer, I will go a little further back in the history of this matter.

When the Linnean Society removed from 32 Soho Square, Dr. Brown was left in possession of that portion which had been built upon and used by Sir Joseph Banks as his museum.

Mr. John Calvert, partly out of veneration for the old house where so many men of science had from time to time met together, and partly from want of additional space for his very extensive museum and library, secured a long lease of these premises, including the old museum; so by that arrangement Dr. Brown

became his tenant. Now, it is a well-known fact that a portion of the Banksian collection was never removed from these premises, and remained the property of Dr. Brown, at the death of whom, Mr. J. Calvert arranged with Dr. Bennet (one of his executors), who had been for months removing van-loads of books, herbariums, and other articles of scientific interest, to purchase and take over, with the premises, various cases of birds, sundry articles, and the remainder and refuse of this large collection.

In two cupboards on the south side of the gallery were the ethnological relics collected during the voyage of the Endeavour, as well as many manuscripts in the autograph of Sir Joseph Banks, together with some of the catalogues of his collection.

On November 10, 1863, there was a sale at the rooms of Mr. J. C. Stevens by order of the Council of the Linnean Society. We soon detected the case of birds, which matched in every particular the cases that we purchased of Dr. Brown's executor; it had the same handwriting at the back, undoubtedly in the autograph of Sir Joseph. We also detected, in a cabinet of fos-ils and minerals which had belonged to Dr. Pulteney, one of the volumes of Sir Joseph Banks's catalogue, which matches the other volumes we had previously obtained: that volume still contains the stamp of the Linnean Society.

Lot 174 of this sale was a very large lot in boxes and a cabinet; added to which was a good proportion of the dirt and dust of bygone times. This collection had been formed by Dr. Maton with great care and industry, and contained various figured and type specimens, being all named in the quaint nomenclature of that period. At the bottom of one of the drawers of the cabinet was a letter in the autograph of the great Linnæus.

We purchased all these, together with others in that sale. The Duchess of Portland, Sir Ashton Lever, and Sir Joseph Banks, were the great collectors of that period; and the Owhyhee case of birds might have been obtained by Sir Joseph either by purchase or otherwise at any date during his life. We have this case marked Owhyhee in the undoubted autograph of Sir Joseph Banks. The birds are all badly set up, and one has fallen from its perch, but underneath each one is a number which is referred to in one of Sir Joseph's catalogues.

In our Museum there are several thousand specimens which formed portions of the three collections just named, with regard to which we have absolute proof of identification, and in some cases the old lot tickets still remain.

As our museum is densely packed in several houses, and in some instances there are large cabinets four and five rows deep, it is not possible at a few hours' notice to dig out all and everything connected with this matter; but I will at the earliest opportunity bring the Banksian collection to the front, which I shall give a full description of in print, for the satisfaction of all those who are interested in the matter.

As to the collection of eggs of Mr. J. D. Salmon, we knew this collection well, but have never seen it since his death. There is not one single specimen in our museum that belonged to that collection, nor did we ever make a catalogue of the same, as the very exhaustive and elaborate catalogue made by the owner would be amply sufficient for all purposes.

63 Patshull Road, N.W.

ALBERT F. CALVERT.

First Visible Colour of Incandescent Iron. HAVING read in your number for March 24 (p. 484) a letter on the above subject, I thought it might prove interesting to try a similar experiment with the carbon filament of an ordinary incandescent lamp. That used was an Edison Swan 16 candlepower 80 volt, and the method employed to heat it was to pass a gradually-increasing current (supplied from accumulators), using a water resistance gradually diminished by the addition of very dilute sulphuric acid in sensibly equal portions. The room in which the experiment was performed was very carefully darkened, and the observers were kept in darkness some minutes before the current was switched on, the dilute acid being added, so that, after visibility had been reached, five additions should bring the lamp to dull redness (by diffused daylight. The number of the experiment being called out, each observer wrote this down, together with his impression of the colour, in the dark, so that the retina was not affected by any extraneous light throughout. Each observer closely inspected the filament till he felt satisfied as to the colour, and then rested

his eyes in the dark till the next observation. There were twenty-five observers. The result seems worthy of notice.

(I) Of the twenty-five all agree that the colour of the filament is at first very pale. Thirteen call it very pale yellow, three call it white, seven a faint pink, two a bluish white.

(2) All agree that, as the temperature rises, the tint grows deeper and redder, passing through orange before reaching crimson. The words used to designate the final tint reached in the experiment vary from deep reddish orange to copper colour, dark red, blood red, crimson.

I may add that some of the observers had had considerable practice in observation, and their eyes were known to be normal so far as the perception of the tints of the visible spectrum is concerned. There is no reason to suppose that more than one, or two at most, possessed any abnormal sense of colour.

Assuming that in the cases of iron and carbon light of greater frequency of vibration is emitted as the temperature rises, in addition to the light emitted at lower temperatures (the vibrations causing which are merely increased in amplitude), is it not possible (1) that the selective power of the pigments of the retina at first scarcely comes into play, the slower vibrations acting on all to a certain extent, on the red more than the green, and the green more than the violet, in the normal eye? or (2) Does not the fact that all colours are more difficult to distinguish in a faint light, .g. moonlight, make it likely that very weak irritation of any part of the retina (I mean a part which causes the sensation of light, and that coloured, when the irritation is stronger) is perceived as "light," the indication of specific absorption not being strong enough in comparison with the total amount of irritation to produce the sensation of any special colour in the light perceived? or lastly, if we do not make the above assumption, it would seem that iron and carbon at all events emit, when first visible, light of far wider limits of frequency of vibration than, so far as I know, is generally admitted.

Some photographic experiments which I hope may throw fresh light on the subject have been begun. Eton College Laboratory, April 4.

T. C. PORTER.

Self-Registering Weather-cock.

I SHOULD be grateful if any of your readers would kindly recommend me a simple, inexpensive instrument, to automatically register the movements of a weather-cock above the roof.

Such an appliance must roughly indicate the direction of the wind at the time being.

Some years ago, a London builder put me up a very expensive instrument, which, beyond making considerable noise, was utterly useless. J. LAWRENCE- HAMILTON.

30 Sussex Square, Brighton, April 11.

ONE

THE ROLLING OF SHIPS.

NE fact that often strikes the thoughtful traveller by sea is that, notwithstanding the great and numerous improvements of recent years which have made life on shipboard pleasant and luxurious, little or nothing has been done to steady a vessel when she meets with waves that set her rolling heavily from side to side. The tendency seems to be rather in the direction of increased than of diminished rolling; for the steadying influence of sails, which makes the motion so easy and agreeable in a sailing ship, is fast disappearing in large steamers. Masts and sails add appreciably to the resist ance of large fast steamers; so they have been cut down in size year by year till such fragments of sail as still remain are so small compared with the size of the ship as to retain little power to reduce rolling.

Shipowners and seamen do not show much sympathy with the discomfort and misery that rolling causes to most passengers. They perhaps get anxious about an Occasional vessel that acquires the evil reputation of being a bad roller, because passengers may be frightened away and the receipts fall off in consequence; but beyond wishing, or attempting, to deal with abnormal cases, nothing seems to be thought of. Rolling is considered incurable, or as not of sufficient importance to trouble

about. Yet there is nothing which would contribute so directly to the comfort of landsmen at sea, or do so much to change what is for many misery and torture into comfort, as to check and reduce as far as possible the rolling proclivities of ships.

The laws which govern rolling are now well understood, and it is strange that this knowledge has not enabled an effective means of control to be devised. What is stranger still is that well-known means of mitigating rolling-such as the use of bilge keels-are employed in but very few cases. A ship rolls about a longitudinal axis which is approximately at her centre of gravity, and the rolling is practically isochronous at moderate angles in ordinary ships. The heaviest rolling occurs when the wave-period synchronizes with the natural period of oscillation of the ship. Many vessels are comparatively free from rolling till they meet waves of this period, and if such meeting could be avoided, excessive rolling could be prevented. Some vessels have periods as long as fifteen to eighteen seconds for the double oscillation, and as these would require to meet with waves 1300 to 1500 feet in length, in order to furnish the conditions of synchronism, it is seldom that they suffer from heavy or cumulative rolling. Such waves are, however, not rare in the Atlantic.

The limits of heavy rolling are fixed, of course, by the resistance offered by the water and air to the transverse rotation of the ship, which is very great because of the large areas that directly oppose motion in a transverse direction. But for this resistance, and the condition that rolling is only isochronous within moderate angles of inclination, a few waves of the same period as that of a ship would capsize her.

The two most obvious modes of preventing heavy rolling are, therefore, (1) to make the period of rolling of a ship as long as possible, so as to reduce the chances of meeting waves whose period will synchronize with it, and (2) to increase the resistance to rolling. The period of a ship varies directly as her radius of gyration, and inversely as the square root of her metacentric height. Hence the period may be increased by increasing the moment of inertia of the ship, or by decreasing the metacentric height. In armoured war-vessels the moment of inertia is large, on account of the heavy weights of armour on the sides, and the heavy guns that are either placed at the side or high up above the centre of gravity. Ordinary steamers have no such weights concentrated at great distances from the centre of gravity, and their moments of inertia are determined by the distribution of material in the hull that is fixed by structural conditions and by the stowage required for their voyages. Metacentric height cannot be reduced below a certain amount, which is necessary to prevent too easy inclination of the ship, or crankness, in still water. On the whole, we may regard the longest periods that the largest ships are likely to have with advantage to be about those named above, i.e. fifteen to eighteen seconds.

Length of period cannot give immunity against occasional heavy rolling; but increase of resistance reduces the angles of roll at all times, and especially when the angular velocity is greatest and the rolling is worst. Such resistance is furnished by the frictional resistance of the bottom of a ship and by the direct resistance of projecting parts of the bottom, such as the keel and the large flat surfaces below at the stem and stern. This resistance can be largely increased by means of bilge keels. The value of bilge keels is recognized in the Royal Navy, and the ships of the Navy have been fitted with them for many years with highly beneficial results. The advantage of bilge keels was proved beyond all doubt many years ago by careful experiments made in this country and in France; and the late Mr. Wm. Froude showed, by the trials he made of H.M.S. Greyhound

twenty years ago, that bilge keels of excessive size-3 feet 6 inches deep, and 100 feet in length, on a vessel 172 feet long-had only an insignificant effect upon speed throughout great differences of trim.

It is strange that the mercantile marine should not yet have adopted bilge keels, and obtained the undoubted advantage they give in steadiness. The number of ships that have them is comparatively few. There is an almost universal opinion and prejudice against their use, and the largest and finest passenger steamers have no bilge keels. This is in spite of the fact that, in cases where bilge keels have been fitted to try to check heavy rolling and they have been of suitable size and properly placed it has been found that the angles of rolling have been reduced by nearly one-half. There is a prevalent belief-which has no foundation in fact that bilge keels are very detrimental to speed. We have said that Mr. Froude's experiments showed the contrary, even trials made in still water; but it appears certain that at sea any trifling loss of speed which still-water trials might show would be more than compensated for by gain in speed when the vessel is prevented from rolling through large angles from side to side, and undergoing great changes of underwater form at every roll. Experience with ships that have had bilge keels added after running for some time without them shows that there has been no appreciable difference of speed or increase of coal consumption on their voyages.

on

Another, and a more heroic, method of stopping or reducing rolling would be to counteract the inclining moment of the ship caused by the ever-changing inclination of the waves by an equal and opposite moment, which would vary as the inclining moment varies. This has been attempted at different times and in various ways. It is essential to any degree of success, however, that the opposing moment brought into operation should be completely under control, so as always to act in the manner and to the extent required. The attempts to obtain a steady platform by freely suspending it, and making it independent of the rolling of the ship, have failed-apart from the practical difficulties of carrying out such an arrangement on a large scale--because the point of suspension oscillates when the ship rolls, and the platform acquires a rolling motion of its own. Weights, made of heavy solid material, which move from one side to the other of a ship subject to the action of gravity and rotation, fail because they cannot be made to act continuously in the manner required.

A degree of success has been achieved by admitting water into a suitably prepared chamber and leaving it free to move from side to side as the ship rolls. This has been done in several ships of the Navy, the case of the Inflexible being that which was the most carefully experimented upon. The movement of this internal water follows the inclination of the ship, but it lags behind, and thus tends to reduce the inclination. Its effect can be regulated by the quantity of water admitted into the chamber and by its depth. The Inflexible Committee state in their report that comparatively small changes in depth increase or diminish largely the extinctive power of the water. For various reasons-one of which is that while such a chamber is very effective in a moderate sea it fails in a rough sea when the rolling of the ship is greatest-and perhaps partly on account of the destructive and disturbing effect of 100 tons or more of water rushing from side to side of a ship over 60 feet wide-these water-chambers appear to have gone out of use in the Navy, and they have been given up in the City of New York and City of Paris, which vessels were said to be fitted with them when first built and placed upon the Atlantic.

Mr. Thornycroft has devised a means of checking rolling by moving a weight, under strict control, from side to

side of a vessel so as to continuously balance, or subtract from, the heeling moment of the wave slope. It consists of a large mass of iron in the form of a quadrant of a circle, which is placed horizontal, with the centre on the middle line of the vessel, and there connected with a vertical shaft. The shaft is turned by an hydraulic engine, which is very ingeniously controlled by an automatic arrangement. The heavy iron quadrant is swept round from side to side, revolving about its centre, to the extent that is required to counteract the heeling moment. In a paper read on the 6th instant before the Institution of Naval Architects, Mr. Thornycroft said:

"The manner in which the controlling gear works will be better understood if we imagine a vessel remaining upright among waves, while near the centre of gravity of the ship we place a short-period pendulum suspended so as to move with little friction; this will follow the change in the apparent direction of gravity without appreciable loss of time, so that any change in the wave angle and apparent direction of gravity cannot take place without due warning, which will indicate the time and amount of the disturbance. It is therefore only necessary to make the motion of the ballast bear some particular and constant ratio to the motion of this short-period pendulum to keep the balance true. The inertia of a heavy mass will cause some loss of time, as we can only use a limited force for its control; but it is possible to accelerate the phase of motion and overcome this difficulty so far as to get good results.

"If, now, we imagine the ship to roll in still water, the effect of the combination just described will be to balance the ship's stability for a limited angle; but this defect is removed by the introduction of a second pendulum of long period, which tends to move the ballast in the opposite direction to the first one, and enables the apparatus to discriminate between the angular motion of the water and that of the vessel.

"I find, however, that the long-period pendulum is rather a delicate instrument, and that its function can be served by a cataract arranged so as to always slowly return the ballast to the centre, and this device has the effect of accelerating the phase of motion, which, in some cases, we also require.

"We are therefore able, by very simple parts, to construct an apparatus which will indicate the direction and amount of motion necessary to be given to the ballast at a particular time so as to resist the wave effort; this power of indicating may be converted into one of controlling by suitable mechanism. The loss of time due to inertia of the necessary ballast is not always unfavourable when the apparatus has to extinguish rolling motion, the greatest effect being obtained when the ballast crosses the centre line of the ship at a time when it is most inclined to the water surface, and this corresponds to a quarter of the phase behind the motion of the short pendulum."

The apparatus has been working for some time in the steam yacht Cecile with very good results. What the objections may be to applying it to the largest passenger steamers remains to be seen. A moving weight of something like 100 or 150 tons would probably be required in such vessels. The power necessary to control the movement of the weight appears to be small, and Mr. Thornycroft's invention seems at any rate to show the way towards obtaining the long-desired boon of substantially reducing, if not checking altogether, the rolling of ships. If it succeed in doing upon a large scale only a portion of what is claimed for it in the way of anticipating and counteracting the heeling effect of waves, without the possibility of acting in an erratic or undesirable way, we may hope to see it adopted some day in passenger

steamers.