in Egypt has been extraordinary. There has been much rain and very little khamseen, and now there is a brisk north wind blowing, which generally follows the dust wind. Hence many weather-wise people say that the khamseen is over; if so, of course, all the better. On the other hand, to-night from our sandbank we have witnessed a sunset rendered transcendentally beautiful by clouds over fully one half of the sky. It is possible therefore that if the present weather continues, the sky will not be quite so free from vapour as it is generally in Upper Egypt. To avoid the khamseen, General Stone, who has had the region reconnoitred, has suggested to the English party to occupy an eminence to the northwest of Akmin, a village a little higher up the river than Sohag.

Near Sohag, Sunday

I had got so far at 11 last night, when the time came for closing the mail bag, although we were fast aground, and apparently with less chance than ever of getting off. There were two mail bags, however, made up after all, for the service is so interfered with higher up the Nile that I am still in time on Sunday evening to send a letter which ought to catch the next Brindisi mail, though whether it does or nor is very doubtful, for we have been aground again twice to-day.

So, as we have at last arrived at our station, I will endeavour to give an idea of the proposed arrangements. In the first place, we have found the steamer on which the various parties are to live as the guests of the Khedive moored close to the shore, at a point where it trends north and south, or very nearly so, about half a mile below Sohag. This position, which has been selected by the French party-the first to arrive-is a very admirable one for two reasons. First, the constant wind during the last week has been from the north, and by keeping a staff of people watering the foreshore of the Nile, all dust is obviated. To the north of the place of observation trees, and what looks like grass from a distance, grow close to the margin of the river; so that the dust can only be of nearly local origin; while a long stretch of sandbank to the north, running east and west, is far enough away to deposit its sand in the Nile before it can reach us.

Secondly, the khamseen, if khamseen there is to be, will have to travel a mile and a half along the Nile before it can enter the observatories; and it is thought this amount of water surface will have an important effect in reducing the amount of sand in the air, even in its case also. Nous verrons. These considerations have induced the English party to take up ground close to the boat and their French confrères. The hills which look so tempting in the mass are simply impossible as places of observation. With the means at command here it would take a week to get the instruments up, much more in position; while at Akmim, which is only two or three miles away, there does not seem to be any spot more favourable, taking everything into consideration, than the one here. At five o'clock to-morrow, then, the work will begin, and the next week will be a busy one, for in spite of the fresh breeze and the clouds-for there are very distinct clouds to-day--work on the sand becomes very oppressive in the middle of the day, and there are heavy weights to move, which the observers must move themselves. The scene from the ship is already interesting. To the north two tents and various shelters, to the south one tent. These will increase to six to-morrow. Here and there groups, looking down the bank, stealthily from between the trees. There is a pretty thick grove of acacia trees, which shelter us somewhat from the rays of the setting sun, still fierce in this latitude. Here and there, skirting the grove, a sentinel with fixed bayonet keeping guard. At the extreme south, tents for the military, and a long line of piled arms.

Across the water the scene is novel and beautiful in the extreme. The main Nile, in which the bat is anchored,

is here about half a mile wide, but there is an island about two miles long, and a wide stretch of water beyond that. This island forms, with the river, the foreground of the landscape. With an opera-glass we can see the Fellaheen cultivating the ground almost to the water's edge in places, and looking after their crops of maize or their flocks of goats. Here comes a veiled Rachel to the sacred river to fetch water for a house in an indistinct flat-topped village, sheltered in a large group of beautiful palm trees. The arm of the river beyond the island we cannot see, but a background is not lacking. A long line of mountains, we may almost call them, full of geological tracery, are now, as I write, almost blood-red in the light of the setting sun, and are surmounted by that grey purple one always sees to such advantage in Eastern lands-both grey and purple haze in a few hours to give way to the silver dawning of the moon, now terribly dwindling in her visible surface, and reminding the astronomers of the coming seventy seconds in a most forcible manner.

The proceedings at the end of the first day on which the English and French parties found themselves together as guests of the Egyptian Government naturally included some toasts-that of his Highness the Khedive, proposed by M. Trépied, and that of the English and French nations, proposed by Moktah Bey, and responded to by Mr. Norman Lockyer. The arrangements on board are as perfect as those made at the various stations on shore, and one's national pride is a little touched at the idea of what the Government reception would be of a party of Egyptian astronomers coming to England to observe an eclipse of the sun.

ANEMOMETRICAL OBSERVATIONS ON

BOARD SHIP'

IT is known that the determination of the velocity of the wind in the ocean has always been one of the desiderata of meteorological observations. Maury devoted much attention to this subject, and to determine, at least approximately, the velocity of trade-winds, he was compelled to work on a very unsafe basis-the velocity of ships during different parts of the year-and to put aside all observations made in accordance with the scale of Beaufort as unreliable. But it is obvious that the velocity of a ship depends on so many circumstances quite independent of the wind itself (such as the shape of the ship, the surface of its sails, the disposal of the cargo, and so on), that its velocity is but a very imperfect means of measuring the velocity of wind. Besides, the relation which exists between the force of the wind and the velocity of a ship, under different angles between the direction of both, is a new source of error, as this relation has not yet been established with accuracy, and can be established only by means of anemometric measurements. The necessity of trustworthy measurements of the velocity of wind at sea was so well understood in England that the Royal Society and the British Association established in 1859 two anemometers-one on the Bermuda Islands, and the other at Halifax. But it is known that the force of the wind is usually lessened on continents and islands.

Therefore it was absolutely necessary to make anemometrical observations on board ships, and a few attempts had already been made in this direction. Prof. Piazzi Smith invented an anemometer which might be established on board a ship, and which merited the highest eulogy from Maury, but Lieut. Domojirov does not know if any observations were made with it. Emil Bessel, during the Arctic expedition of the Polaris, made a series of observations with an anemometer on board his ship, but he does not explain, neither the methods of observation, nor the corrections he applied to his measurements.

In

A Domojirov, in the Izvestia of the Russian Geographical Society, vol. xviii. 1882, fasc. 1.

1879 Col. Rykatchoff, of the St. Petersburg Central Physical Observatory, established, on board the Nayezdnik. an anemometer of his own construction, which was observed during the ocean cruise of the clipper; and the schooner Nordenskjöld, which unhappily was lost in 1879 at Yesso Island, had also an anemometer, which was taken afterwards on board the Russian clipper Djighit by M. Domojirov. The observations on board of the Djighit were carried on with this anemometer (of Casella) put in such an apparatus (like that of the lamps on board of ships), as always maintained it in a vertical position, even during the heaviest seas, when the ship oscillated for 30° on one side, and 35° on the other. The apparatus was put on a 16-foot-long pole, which was pushed out for each observation on the wind-side of the ship, from the boat, and thus exposed to the full force of the wind for ten minutes. The height of the instrument above the sea was 26 feet. The direction of wind was determinated by means of a vane, and its true direction computed from the apparent one, on the principle of the parallelogram of forces, by taking into account the velocity of the ship. When the angle between the direction of the wind and the direction in which the ship goes is known, as well as the seeming velocity of wind (measured by the anemometer), and the velocity of the ship, the true velocity of wind is easily determined by means of simple computations, or of the tables published for that purpose by M. Rykatchoff (Russian Marine Review, February, 1880). A series of experiments having been made for ascertaining in how far the calculated figures agree with the true ones, M. Domojirov arrives at the conclusion that these figures are quite reliable; determination having been made during the progress of the ship, she was immediately stopped, and the determination made anew, both results always being quite identical. But the measurements from the side-boat are very tedious and even dangerous during heavy seas, and each observation occupies no less than three men for about twenty minutes. Therefore M. Domojirov proposes to apply electricity to register the rate of the anemometer.

The observations on board the Djighit were made five and six times per day from March 23 to May 30, and the complete results, with all elements for calculations, are published in the papers of M. Domojirov. The northeastern trade-wind, observed on the passage from Japan to the Sandwich Islands and back, had a very regular force of from 5 to 9 metres per second; the south-eastern trade-wind experienced on the passage from the Sunda Islands to the Seyschels, had a velocity of 4 to 9 metres per second, and the south-western winds on the passage from Port Victoria to Aden, had velocities from 12 to 15 metres per second.

It would hardly be necessary to insist on the importance of such observations for meteorology, as well as for practical purposes, and we hope that soon the still prevailing prejudices as to the possibility of anemometrical observations on board ships having disappeared, and more convenient methods of observation having been devised, the anemometer will become on board ships as necessary an instrument as the log and barometer.

P. K.

animal mechanics; but having seen some of the results obtained by Mr. Muybridge, at San Francisco, with photographic pictures taken during an exposure of the 1-500th of a second, he was very desirous to have the same process adapted, so as to admit of its being applied to the taking of birds flying. In September, 1881, on a visit of Mr. Muybridge to Paris, he brought with him some photographs of birds taken on the wing, but these unlike the invaluable series taken by the same gentleman of horses and men, were not the representation of a series of continuous attitudes, but rather represented the bird in the posi tion it happened to be in at a moment of time; whereas, to explain the fail and rise of the wings and the positions of

INSTANTANEOUS PHOTOGRAPHY OF
BIRDS IN FLIGHT

PROF. E. J. MAREY has lately published in the pages of our contemporary, La Nature, an article on a "photographic gun," the illustrations to which, with a somewhat shortened account of the process, we are enabled, through the courtesy of the editor of La Nature, to present to our readers. M. Marey's researches on animal locomotion are well known; his experiments carried on by the graphic method were productive of most valuable results, and they corrected and explained many debated points in

FIG. 1.-The Photographic Gun.

the body, it was, above all things, important to have a series of rapid photographs taken of the same bird over a period during which the whole mechanism was in action, so as to allow of the movements to be afterwards studied at leisure. After deliberating over this subject during the last winter, at last the idea of a photographic gun occurred to him; but the immense quickness with which the movements should succeed one another, in order to bring a series of sensitive surfaces across the lens, at first presented great difficulties in the constructing of the machine. It was necessary to have images taken successively ten or twelve times in one second, in order to

movement. A cover over all keeps the light out from the rear of the instrument. It will be noted that during the instant of exposure, the sensitised surface is steady, and when the exposure is over, it is at once passed away. Pressure on the button (b, Fig. 2, 1) sets the machine in motion. Before applying this instrument to the study of the flight of birds, an experiment was made with it on a black arrow, made to rotate against a white back ground well lit up. The speed of the rotation of the arrow was about 5 metres a second. The marksman, at a distance of 10 metres sighted on the centre of the target, on which the eye could perceive nothing save a confused grey shadow, so quick was the rotation of the arrow but on the development being completed, twelve images were to be seen, disposed in a circular manner, and each showed not only the arrow, but its shadow, as sharp as if the original had been immovable. Another experiment, equally successful, was made on a pendulum beating seconds. For to be more certain as to the duration of the exposure, M. Marey next adapted to the gun a chronographic apparatus, so that the time intervening between the taking of each picture could be with precision ascertained. After all these preliminary essays, the photography of animals in movement was attempted; and in Fig. 3 there will be seen the photographic representations of a sea-gull, in which the twelve successive attitudes assumed during the space of a single second by this bird during flight are ascertained. On other occasions other success

FIG. 5.-Sea-gull; end of depression of wing.

ful photographic series of a sea-gull in flight were taken when the bird was seen less in profile. The sea-gull gave exactly three strokes of its wing each second, so in the twelve photographs of each stroke four succeeding stages were reproduced. The wings at first elevated to their greatest, then commence to be lowered; then in the following image they are seen at the lowest point of their course; and in the fourth image are again on the rise. In enlarging these images, figures seen from a good distance were obtainable, but the sharpness of the enlargements left a good deal to be desired; for the negatives were somewhat granular, no doubt owing to some slight fault in the photographic process. The reproduction of these images by the heliographic process gives excellent silhouettes (as seen in Figs. 4 and 5); the originals, when examined under the microscope, showed even the wing-feathers distinctly.

On comparing the indications thus given by the photographic process with those already attained by the graphic process, a confirmation of most of the principal points obtained by the latter were obtained; but otherwise so far, the latter did not seem to add much to our knowledge of the mechanism of flying. However, ere deciding that this is so, numerous observations on different birds flying and in different conditions of flight, during calms and storms, and with and against the wind, must be taken. Attempts were also made to photograph the bat, but its small size, its flight during the dusk, and its capricious method of flying made it a difficult subject; but some of the experiments revealed interesting results. The angle

of oscillation of its wings is very extended, especially from below, when the two wings form two vertical planes sensibly parallel.

These extremely interesting researches of M. Marey are only, as it were, in their infancy; he intends pursuing them much further, and his results will be looked for with great interest by all those who study the subject of animal motion.

DR. FRITZ MÜLLER ON SOME DIFFICULT CASES OF MIMICRY

IN N his original explanation of the cause of mimicry, Mr. Bates referred to the occurrence of many cases in which species of different genera of Heliconidæ resemble each other quite as closely as the mimicking Leptalides and Papilios resemble species of Ithomia and other Heliconoid butterflies. In these cases both the imitating and the imitated species are protected by distastefulness, and it was not therefore clear how the one could derive any benefit by resembling the other. Accordingly, Mr. Bates did not consider these to be true cases of mimicry, but to be due, either to identical parallel variations of externally similar form, or "to the similar adaptation of all to the same local, probably inorganic,

conditions."

Examples of this close resemblance of species of different genera of protected groups have now become very numerous, and they often extend to three or more distinct genera, some species of which imitate each other in most parts of tropical America, each changing in a corresponding manner as we pass from one district to another.

In my Address to the Biological Section of the British Association at Glasgow, in 1876 (reprinted in "Tropical Nature"), I connected these cases with a number of others in which peculiarities of colour or of form appear together in several groups not closely allied, but always among those inhabiting the same locality and as frequently among unprotected (that is, eatable) as among protected groups of butterflies; and I concluded, generally, as Mr. Bates had done, that these curious phenomena were due to "unknown local causes."

66

Thus the matter rested, till, in 1879, Dr. Fritz Müller published in Kosmos a paper on Ituna and Thyridia; a remarkable case of Mimicry in Butterflies"; and in 1881 a second paper, entitled "Remarkable cases of acquired resemblance among Butterflies," in which he gives a solution of the problem as really a case of mimicry. The first of these papers was translated by Mr. R. Meldola, and communicated to the Entomological Society of London in May, 1879, and the same gentleman has kindly furnished me with a translation of the second paper (the title of which is given below), which discusses the whole question in great detail, and devotes much space to a criticism of my suggested "unknown local causes" as a sufficient explanation of the phenomena. I may at once say that I admit this criticism to be sound; and that Dr. F. Müller's theory appears to me to afford a clue (with some slight modifications) to most of the cases of close individual resemblance of not-nearly-related species of butterflies yet observed. I therefore wish to state, as briefly as possible, the exact nature of the explanation now afforded us, and this is the more necessary because Dr. Müller's theory did not receive much support when brought before the Entomological Society, nor did it then satisfy Mr. Bates, the discoverer of the true meaning and importance of the phenomena of mimicry as interpreted by the doctrine of Natural Selection.

The explanation depends on the assumption, that some at least, if not all, young insectivorous birds learn by experience that the Heliconoid butterflies are distasteful, and in so doing sacrifice a certain number of individuals 1 "Pemerkenswerthe Fälle erworbener Aehnlichkeit bei Schmetterlingen." Von Fritz Müller. (Separat-Abdruck aus Kosmos," V. Jahrgang, 1881.)

"

of each distinct species. But if two species, both equally distasteful, closely resemble each other, then the number of individuals sacrificed is divided between them in the proportion of the square of their respective numbers; so that if one species (a) is twice as numerous as the other (6), then will only lose one-fourth as many individuals as it would do if it were quite unlike a; and if it is only one tenth as numerous then it will benefit in the proportion of 100 to 1. It is an undoubted fact that the species of protected butterflies, like those of other groups, differ greatly in abundance of individuals, some being very rare while others are among the commonest of all butterflies. The proportion of ico to 1, therefore, is far below the amount of benefit an uncommon species might derive by resembling a common one. The benefit to be derived is thus clear, if the protected species are subject to the danger of attacks by young birds before they learn that such species are uneatable. I agree with Dr. Müller that they are exposed to this danger; and when we consider the great number and variety of insectivorous birds in South America the danger must be considerable, and quite sufficient to render it important for a numerically weak species to reduce it to a minimum, although to a species abounding in individuals it may be of little importance. It has been suggested that young birds have an hereditary instinct enabling them to distinguish uneatable butterflies antecedent to experience; but this seems in the highest degree improbable. It has no doubt been shown by Mr. Darwin that monkeys in captivity have a dread of snakes, and Mr. Jenner Weir believes that birds have an instinctive knowledge of uneatable caterpillars. But even admitting that in these two cases there is an instinctive hereditary aversion, it does not follow that the same will occur with regard to protected butterflies. Snakes form one well-marked group, and it is not alleged that monkeys distinguish between poisonous and harmless snakes; and caterpillars can also be readily divided into the two classes of edible and inedible by their green or brown (protective) colours on the one hand, and their gaudy or conspicuous colouration or hairy bodies on the other. But the protected butterflies have no such general mark of inedibility. Their colours and forms vary greatly, and cannot as a group be readily differentiated from those of other butterflies; and it is not to be accepted without actual proof that a young bird knows instinctively every Heliconoid or Danacoid butterfly in its district, as well as the protected Papilios and moths, almost infinitely varied as they are in colour and marking, among the equally numerous and equally varied butterflies of other groups It therefore seems clear to me that we have here a vera causa for the acquisition of true protective mimicry by the less abundant species of inedible butterflies.

There is however yet another cause which may have led to mimicry in these cases, and one which does not appear to have been discussed by Dr. Müller. The fact that the majority of butterflies are edible and are actually eaten by birds and other insectivorous creatures, while a considerable minority are distasteful and are thus protected, renders it pretty certain, a priori, that there exist many degrees of distastefulness. Certain species appear to be rejected by all insectivorous creatures, while some, though not eaten by birds, may be devoured by lizards, dragon-flies, or spiders. Some, too, may be eaten by some birds and rejected by others, and no ornithologist will think it strange or improbable that a trogon should have somewhat different tastes from a tyrant-shrike or a swallow. Again, in some species the distastefulness may extend to all the stages of egg, larva, pupa, and perfect insect, while in others it may be confined to one or more of these stages; or special dangers may exist for one species which are absent in the case of another. But it is evident, that, if these differences exist, it will be advantageous for the less protected to mimic the more com

pletely protected species, and the fact of the affinity between the different genera, with perhaps some tendency to revert to a common style of colouration or marking, will afford facilities for the development of this class of mimicry even greater than occur in the case of the distinct and often remote families of completely unprotected butterflies. We need not, therefore, be surprised to find whole series of species of distinct genera of Heliconoid butterflies apparently mimicking each other; for such mimicry is antecedently probable on account of the greater need of protection of some of these species than others, arising either from some species being less distasteful to certain enemies, or less numerous, and therefore likely to suffer to a serious extent by the attacks of inexperienced birds. When these two conditions are combined, as they often would be, we have everything necessary for the production of mimicry.

The explanation now given, so far as it refers to the various degrees of protection, may be extended to explain those cases in which various groups of Nymphalidæ or other families appear to mimic each other; such as Catagramma, Callithea, and Agrias in one series, and Apatura with Heterochroa in another. In my "Tropical Nature (p. 257) I have remarked--" Here, again, neither genus is protected, and the similarity must be due to unknown local causes"; but this is more than we know, and I now think that some of these groups-perhaps Catagramma and Heterochroa-are partially protected, and the advantage of sharing in this partial protection has led species of altogether unprotected and much persecuted groups to gain some protection by mimicking them, whenever their general form, habits, and style of coloration offered a suitable groundwork for variation to act upon.

If these views are correct we shall have the satisfaction of knowing that all cases of mimicry are explicable by one general principle; and it seems strange to me now that I should not have seen how readily the principle is applicable to these abnormal cases. The merit of the discovery is however wholly due to Dr. Fritz Müller; and it is to be hoped that he will complete his work by obtaining, if possible, evidence of its correctness. The chief thing required is an experimental proof of various degrees of inedibility in butterflies, during the different stages of their life-history; and also some observations as to the comparative abundance of the species of protected butterflies which mimic each other. If to this can be added the proof that such groups as Catagramma, which seem to be the objects of mimicry, are partially protected by inedibility, the chief remaining difficulty in the application of the theory of natural selection to all known cases of protective imitation will have been cleared up. ALFRED R. WALLACE

NOTES

IN reference to the Darwin Memorial, to which we referred last week, the honorary secretaries have issued a circular asking for contributions to the fund. In this memorial it is stated that though the works of Charles Darwin are his best and most enduring memorial, it is felt by his many friends and admirers that these should not be the only one. They are desirous of handing down to posterity the likeness of a man who has done so much for the advancement of natural knowledge. They wish also to establish a fund associated with his name, the proceeds of which will be devoted to the furtherance of biological science A committee has accordingly been formed, of which Mr. T. G. Bonney and Mr. P. Edward Dove are the hon. secretaries. The committee is one of the highest influence, comprising the leading foreign ministers, the two Archbishops, and the best-known names in all ranks and professions.

JOHANN CARL FRIEDRICH ZOLLNER, whose death we recently announced, was horn at Berlin on November 8, 1834.