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considerable rocking and using boats of more advantageous forms than mine, if it will be possible to have a much higher speed than 2000 metres per hour. It appears also that the available force will be hardly sufficient to struggle successfully against strong winds and currents.

I do not therefore prognosticate too confidently any practical value to the motor, but should be very glad if some of your readers would inform me as to any similar experiments which may already have been made. H. LINDEN.

Zoological Station, Naples, February 19.

Blind Animals in Caves.

As a reader of Mr. Herbert Spencer's writings and a disciple of his, I shall be very glad to lift Prof. Lankester's glove. In the first place I would point out that the process he describes is not natural selection in the ordinary sense; natural selection is the death of the unfit and the survival of the fittest. In the suggested process neither the animals with perfect eyes, nor those with imperfect, are destroyed in the struggle for existence; they are simply segregated. But this is of minor importance. The question is whether there is any foundation for the hypothesis suggested.

Prof. Lankester supposes that the individuals born with defective eyes have remained in the dark places, while those with perfect eyes have followed the glimmer of light and escaped. But he has overlooked the fact that blind cave-animals are born or hatched at the present day with well-developed eyes. It is clear, therefore, as in every other case to which the law of recapitulation applies, that the variations to which the evolution is due occurred at a comparatively late period in the life of the individual. Why did not all the individuals escape when they were young, and could still see without spectacles? When the imperfection of the eyes did occur, what ground is there for assuming that it was a congenital variation? It seems to me perfectly certain that it was a deterioration of the eyes caused by the fact that the individual had lived in the dark all its life. In short, I hold that the law of recapitulation in development, the law of metamorphosis, or biogenetic law, as Haeckel called it, is itself a sufficient proof of the inheritance of acquired characters. This argument has never been met or even considered by any of those who talk of congenital fortuitous variations without defining them.

The evidence for the statement I have made is, I confess, not quite complete, but it is sufficient for my present purpose. In Semper's "Animal Life," p. 80, there is an account of Pinnotheres Holothuria, based on the author's direct observations. This species lives in the respiratory trees of Holothurians, and in the adult the eyes are degenerate and invisible on the exterior of the animal. The young is hatched as a zoæa with perfect typical eyes; even when it enters the host it retains its eyes, but afterwards the eyes degenerate and become covered over by the carapace. In the common mole, to take an instance among mammals, the optic nerves are degenerate in the adult, so that there is no connection between eye and brain; but in the embryo both eyes are connected with the brain by well-developed optic nerves. I am not at present acquainted with any observations on the young of Proteus, or the blind fish Amblyopsis, or the blind Crayfish of the mammoth cave, but I am quite confident that the young in all these cases have relatively welldeveloped eyes. At any rate Prof. Lankester to support his theory must prove that they are blind from the beginning; for if they are not then it is clear that the variations which we have to consider took place during the life of the individual living in the dark, and consequently the support of Prof. Lankester's suggestion vanishes. Prof. Lankester again writes of the deep sea as though it were as destitute of light as the mammoth cave, or the subterranean home of the Proteus, but this is notoriously not the case. With regard to fishes, Dr. Gunther says that below the depth of 200 fathoms small-eyed fishes as well as large-eyed occur, the former having their want of vision compensated for by tentacular organs of touch, whilst the latter have no such accessory organs, and evidently see only by the aid of phosphorescence; in the greatest depths blind fishes occur with rudimentary eyes, and without special organs of touch. Dr. Günther mentions fiftyone species of fishes living at depths beyond 1000 fathoms, and among these only three Aphyonus gelatinosus, Typhlonus nasus, and Ipnops Murrayi are blind. It is, I think, sufficiently evident that the biology of the deep sea is quite different from that of subterranean caves or habitats. J. T. CUNNINGHAM, Plymouth, February 27.

BESIDES panmixia and emigration of the more perfect-eyed individuals, as explained by Prof. E. Ray Lankester, allow me to suggest another cause for the dwindling of the eyes in cavedwelling animals.

Prof. Weismann says that the degeneration "can hardly be of direct advantage to the animals, for they could live quite as well in the dark with well-developed eyes." I submit, however, that in a place permanently dark the eye is not merely useless, but, as a delicate and vulnerable part, it becomes a positive source of danger to the animal. No longer helping the creature to avoid obstacles or danger, it is, in proportion to its size, exposed to injury, destructive inflammation, and the attacks of parasites in a manner which must not seldom lead to the death of the individual. As other senses become more acute, and the eye recedes, this danger diminishes, and when the eye has become a mere rudiment, "hidden under the skin," its presence ceases to be a disadvantage, and so degeneration does not proceed to complete suppression.

It is a wonder that Mr. H. Spencer should have overlooked Prof. Lankester's explanation, for the English editor of Prof. Weismann's fifth essay has not failed to call attention to it. Mirfield, February 27. A. ANDERSON. [Darwin has himself drawn attention, in regard to burrowing animals, to the conditions pointed out in the above ("Origin of Species," 6th edition, p. 110).—ED.]

Foraminifer or Sponge?

I AM glad to find that Mr. Pearcey agrees with me in regarding Neusina Agassizi, Goës, as identical with Stannophyllum zonarium, Hæckel. But with respect to its systematic position I do not as yet see sufficient reason to differ from Prof. Hæckel in regarding it as a sponge, although I have never observed flagellated chambers and cells any more than he. The large masses of foreign bodies always present in this organism offer very serious difficulties in sectionising it, and as long as we are not absolutely certain about its cellular structure we are justified in thinking with Hæckel that general appearance and the presence of oscula, pores, subdermal cavities, horny skeleton, &c., are sufficient to characterise the form as a sponge.

Mr. Pearcey mentions six genera of Foraminifera which he thinks approach closely to Stannophyllum. I am sorry I cannot see much similarity. The chitinous lining in the tube-like body of some Foraminifera certainly bears not the slightest resemblance to the distinct fibrous stroma of Stannophyllum, which reminds me much more of the filaments of the true horny sponge Hircinia. If anything tells in favour of Mr. Pearcey's view, it is the concentric lines of Stannophyllum, which recall the foraminiferal rather than the sponge type of growth.

The final decision of this question can of course only be expected from an examination of the cell structure. University College, Liverpool, R. HANITSCH.

February 25.

A Magnetic Screen.

DURING the last vacation St. John's College, Oxford, has been lit with the electric light, and a transformer of the dynamomotor type, weighing over seven tons, has been placed within about sixty feet of the electrical testing room of the Millard Laboratory, which is furnished with several reflecting galvanometers. I greatly feared that the instruments would suffer much from the magnetic field of the large transformer. When it was found that no other space could be given up for the machine, I devised a method of construction which the Oxford Electric Lighting Company very kindly carried out for me when building their dynamo house. My method is to construct a wall of scrap iron round the three sides of the dynamo nearest to our laboratory. The iron wall is about eight inches thick, and is made by building two brick walls parallel to one another, and filling the interspace with scrap-iron; a delicate magnetometer used for testing the field at unprotected and protected points equidistant from the magnets, when the machine is in action and not so, shows that the iron wall is an effective barrier to the magnetic influence. I venture to make known this method of shielding off a magnetic field, because in these days of electrical invasion it may be of use in protecting physical instruments from being seriously disturbed, and rendered useless for any but the roughest determinations. FREDERICK J. SMITH.

Trinity College, February 28.




COING back now to the photographs, the next one was taken with the view of illustrating the effect on the inclination of the waves of the velocity of the bullet. In this case the bullet was aluminium; it was only one-seventh the weight of the regulation bullet. In consequence of its lightness it travelled about half as fast again as the ordinary bullet (not ; times as fast as it would have done if the pressure of the powder-gases had been the same in the two cases), and in consequence of the higher speed the inclination of the waves is still greater than in the previous case. Further, in this case the bullet was made to pierce a piece of card shortly before it was photographed. The little pieces that were cut out were driven forward at a high speed, but, being lighter than the bullet, they soon lost a large

only about half as fast as it does in air, and which w not explode or even catch fire when an electric spark made within it, or directly act injuriously upon the photographic plate. The increased inclination of the waves is very evident in Fig. 10.

These waves, revealed by photography, have a ver important effect on the flight of projectiles. Just as it the case of waves produced by the motion of a ship which, as is well known, become enormously more ener getic as the velocity increases, and which at high veloc ties produce as a matter of fact an effect of resistance: the motion of the ship of far greater importance than the skin friction, so in the case of the air waves produced by bullets; in its flight the resistance which the bullet meets with increases very rapidly when the velocity is raised beyond the point at which these waves begin to be formed. This being the case, I have thought it might be teresting to see whether the analogy between the behav iour of the two classes of waves might be even neare than has already appeared, and on turning to the beautif

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part of their velocity; they had in consequence lagged behind when they were photographed, but though travelling more slowly (they were still going at more than 1100 feet a second) they yet made each its own air wave, which became less and less inclined as the bits lagged more and more behind; each, moreover, produced its own trail of vortices like that following the bullet. The well-known fact that moving things tend to take the position of greatest resistance, to avoid the effect of which the bullet has to be made to spin, is also illustrated in the photograph. The little pieces that are large enough to be clearly seen are moving broadside on, and not edgeways, as might be expected.

In order to illustrate the other fact that the angle of the waves also depends on the velocity of sound in the gas, I filled the box with a mixture of carbonic acid gas, and the vapour of ether, a mixture which is very dense, and through which sound in consequence travels

1 Lecture delivered at the Edinburgh meeting of the British Association by C. V. Boys, F.R.S. Continued from page 421.

researches of Mr. Scott Russell, published in the Rep of the British Association for the year 1844, in wh he gives a very full report on water waves and the properties, I found that he had made experiments 1had given a diagram showing what happens when a s tary wave meets a vertical wall. The wave, as would expected, is, under ordinary conditions, reflected perfec making an angle of reflection equal to the angle of dence, and the reflected and incident waves are alike: all respects. This continues to be the case as the a gets more and more nearly equal to a right angle, until the wave front, nearly perpendicular to the wall, along nearly parallel to it. It then at last ceases to? reflected at all. The part of the wave near the Finstead gathers strength, it gets higher, it theref." travels faster, and so causes the wave near the wall to ahead of its proper position, producing a bend in the w front, and this goes on until at last the wave near wall becomes a breaker.

In order to see if anything similar happens in the c

of air waves, I arranged the three reflecting surfaces of sheet copper seen in Fig. 11, and photographed a magazine rifle bullet when it had got to the position seen. Below the bullet two waves strike the reflector at a low angle, and they are perfectly reflected, the dark and the light lines changing places as they obviously ought to do. The left side of the V-shaped reflector was met at a nearly grazing incidence; there there is no reflection, but, as is clear on the photograph, the wave near the reflector is of greater intensity, it has bent itself ahead of its proper position as the water wave was found to do, but it cannot form a breaker, as there is no such thing in an air wave. The same photograph shows two other phenomena which are of interest. The stern wave has a piece cut out of it by the lower reflector, and bent up at the same angle. Now if a wave was a mere advancing

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flector cut, growing up to a finite sphere about the end of the reflector as a centre; beyond this there are no more centres of disturbance, the envelope of all the spheres projected upon the plate, that is, the photograph of the reflected wave, is not therefore a straight line leaving off abruptly, but it curls round, as is very clearly shown, dying gradually away to nothing. The same is the case, but it is less marked, at the end of the direct wave near the part that has been cut out.

The other point to which I would refer is the dark line between the nose of the bullet and the wire placed to receive it. This is the feeble spark due to the discharge of the small condenser which clearly must have been on the point of going off of its own accord. The feeble spark precedes--or is to all intents and purposes simultaneous with, it cannot follow-the main spark which

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thing the end of the bent-up piece would leave off suddenly, and the break in the direct wave would do the same. But according to the view of wave propagation put forward by Huygens, the wave at any epoque is the resultant of all the disturbances which may be considered to have started from all points of the wave front at any preceding epoque. The reflector, where it has cut this wave, may be considered as a series of points of disturbance arranged continuously in a line, each, however, coming into operation just after the neighbour on one side and just before the neighbour on the other. The reflected wave is the envelope of a series of spheres beginning with a point at the place where the wave and the re

makes the photograph. The feeble spark heated the air, and the light from the main spark coming through this line of heated air was dispersed, leaving a clear black shadow on the plate. One spark casts a shadow of the other. Now it is evident that if the spark at the nose of the bullet had followed instead of having preceded the main spark by even so much as a three-hundred-millionth of a second, the time that light took to travel from one to the other, it would not have been able to cast a shadow. We have the means of telling, therefore, which of two sparks actually took place first, or perhaps the order of several, even though the difference of time is so minute. Perhaps this method might be of some use in researches

now attracting so much interest in connection with the propagation of electrical waves.

On returning to the non-reflection of the air wave in the upper part of the figure we have here, I imagine, optical evidence of what goes on in a whispering gallery. The sound is probably not reflected at all, but runs round almost on the surface of the wall from one part to another.

We are now in a position to see how the reflection or non-reflection of air waves produced by a passing bullet, when they meet with some solid body, may produce a practical result which might be of importance in some cases. Suppose a bullet to be passing near and parallel to a wall. Then if the velocity of the bullet and its distance from the wall are such that the head wave meets the wall at an angle at which it can be reflected, especially, as in the case of Fig. 11, if the reflected ray can only return into the path of the bullet after it has gone, then no influence whatever can be exerted upon the bullet by its proximity to the wall. If, however, the head wave would, if undisturbed, meet the wall at such an angle

bullet has left the muzzle the imprisoned powder gases under enormous pressure, rush out, making a draug past the bullet of the most tremendous intensity tending obviously to drive it forward. While this draught doe most assuredly hurry the bullet on its forward course, does not tend to make it spin round any faster. Now f the bullet were not hurried on at all after it left the muzzle it would, travelling as in a screw of the same pit:: all the way from the breach of the rifle up to the per at which it is photographed, have turned round a certa number of times which depend upon the distance travelled and the pitch of the screw. If, however, the longitudin motion is hurried and the rotational is left unaltered the pitch will be lengthened outside the barrel and the rotation will have been less for any position than would have been if the bullet had not been accelerated in this way. If, therefore, we can find to what exter the bullet has turned actually at the place at wh it has been photographed, we can find the appare rotational lag and so working backwards get a measure of the velocity acquired after leaving the muzzle.

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that it could not be reflected, as for instance, in Fig. 12, when the head wave can be reflected by neither of the walls between which the bullet is passing, obviously the wave will become stronger and the resistance which it offers will, I imagine, become greater, and if in this case the upper plate be removed this extra resistance will be onesided and must tend to deflect the bullet. This is quite distinct from the well-known effect of a bayonet upon the path of a bullet; when a bayonet is fixed the rush of powder gases between the bullet and the bayonet is quite sufficient to account for the deflection which every practised marksman allows for.

I have devised a method by which a problem of some difficulty, about which authorities are, I believe, by no means in accord, may be solved with a fair degree of certainty. The problem is this, to find what proportion of the velocity of a bullet is given to it after it has left the barrel, or, what comes to the same thing, to find the position in front of the barrel at which the speed is a maximum. The cause of this is evident. When the

order to accomplish this I drilled a series of bee transversely through the bullet, each one at an angle the previous one, the whole series being such that whatever extent the bullet had twisted, one at least, perhaps two, would allow the light of the spark to str through it upon the photographic plate. Then from photograph it is easy to see through which hole the h shone, and knowing in what position this was in the breach, it is easy to find what fraction of half a : over or above any whole number of half turns the bu has twisted. Strictly the measure should be made different distances to eliminate all uncertainty, but only shot I have taken was sufficient to show that t was a rotational lag equivalent, according to the meas made by Mr. Barton, to something under a two p cent. acceleration outside the barrel. I do not att any importance to this figure; the experiment was with a view to see if the method was practicable and it certainly is. I would recommend, where accurac required, that having found as above about how much

bullet has turned, that a second bullet should be drilled with a series of holes at about the corresponding position differing very slightly from one another in angular position, so that several would let the light through and thus give a more accurate measure of the rotation.

There is a point of interest to sportsmen which has given rise to a controversy which the spark photographs supply the means of settling. The action of the choke bore has been disputed, some having held that the shot are made to travel more compactly altogether, while others, while they admit that the shot are less scattered laterally, as may be proved by firing at a target, assert that they are spread out longitudinally, so that if this is the case the improved target pattern is no criterion of harder hitting, especially in the case of a bird flying rapidly across the direction of aim.

shot is filled with air waves of the greatest complexity. They are not due to the cause already explained, but are, I believe, formed by the imperfect mixture of air with powder gases still accompanying the shot. The imperfect mixture of the two gases causes light to be deflected in its passage, thus producing striæ just as at the first mixing of whisky and water, striæ are seen (sometimes attributed to oil!), which disappear when the mixture is complete. I would mention, for the benefit of any one who may be tempted to continue these experiments, that a pair of wires such as are found to do so well when bullets have to be caught are not suitable, as one is sure to be shot away before such a bridge of shot is made between them as will allow a spark to pass. However, by using thick copper wires, one bent in the form of a screw, with the other along the axis, no such failurecan occur and every shot that I have taken in this way

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I was unfortunately not able, in the limited space and time that I have been able to employ, to take photographs of the shot at a reasonable distance from the gun, but I have taken comparative photographs at three or four yards only in which every shot is clearly defined, and in which it is even easy to see on the negative where the shot have been jammed into one another and dented. The difference in the scattering at this short distance is not sufficient for the results to give any information beyond this, that shot are as easily photographed as bullets, and that no difficulty need be apprehended in attempting to solve any question of the kind by this method. The photograph, Fig. 13, represents the shot from the cylindrical or right-hand barrel. The velocity now is so low that individual waves are no longer formed by each shot. The whole space, however, occupied by the

has been successful. One can of course test the action of any material mixed with the shot. For instance, in one case I mixed a few drops of liquid oil with the shot and found them more widely scattered in consequence, not, as has been stated, held together by the oil as if they were in a wire cartridge. Of course, solid grease or fat may, and no doubt does, produce such a result, but liquid oil certainly does not.

And now I wish to conclude with a series of photographs which show how completely the method is under control, how information of a kind that might seem to be outside the reach of experiment may be obtained from the electric spark photograph, and how phenomena of an unexpected nature are liable to appear when making any new experiment. The result, however, is otherwise of but little interest or importance.

I thought I should like to watch the process of the

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