two artificially-formed channels on the Ganges Canal, one of them, 168 feet wide, and the other 85 feet wide, and each having the water often about from 6 feet to 9 feet deep, he states (p. 46, article 35): "There is a constant surface motion (deviation) from the edges towards the centre, most intense at the edges and rapidly decreasing with distance from the edges."

This experimental conclusion, on the supposition of its being decidedly trustworthy, as Mr. Cunningham asserts with confidence that it is, I think may probably be satisfactorily explicable through considerations intimately connected with those which I have already given for an amended theory of the flow of water in rivers.

I wish, however, not to prolong the present paper by entering on any detailed discussion of this branch of the subject, and besides I prefer to reserve this for some further consideration before venturing to put forward the views in reference to it which at present appear to me likely to be tenable. It may be noticed, however, that Captain Cunningham's experimental result, if decidedly correct, throws additional light on the subject of the abatement of surface velocity comparatively to the velocity at some depth below the surface being found in Bazin's experiments to occur in a much greater degree near the sides of rectangular and various other channels than at middle. Bazin thought indeed from his own experiments (as I have already had occasion to mention) that the relative retardation or slowness of the surface occurred not in the middle of wide channels (that is to say, of channels wide relatively to the depth of the water) but only near the sides; but this supposition I have referred to as appearing not to be trustworthy. With these brief suggestions I will now leave for further consideration the subject of the special phenomena of the influence of the sides.

Historical Note.

Subsequently to my having formed, in all its primary or more essential features, the new view now explained of the flow of water in rivers, and before I had met with the book of Humphreys and Abbot, I happened to see in the writings of another author (paper of Mr. Gordon already referred to) the following remark in reference to their views as to the velocity at the surface being less than at some depth below. "Humphreys and Abbot attribute the fact to transmitted motion from the irregularities of the bottom; but confess themselves dissatisfied with their own explanation."

These words seemed to me to indicate a probability of Humphreys and Abbot having anticipated me in some part at least of the theory which I had been forming. On obtaining their book, however, and reading the passage referred to, not by itself alone, but with its context, it appeared to me that it involved no real anticipation,

although one clause of a sentence in it, read by itself, might be supposed to do so. The passage is to be found in their work at p. 286. They begin by saying, that their experimental observations detailed in their previous pages "prove that even in a perfectly calm day there is a strong resistance to the motion of the water at the surface as well as at the bottom," and that this resistance at the surface "is not wholly or even mainly caused by friction against the air." They go on to say:-" One important cause of this resistance is believed to be the loss of living force, arising from upward currents or transmitted motion occasioned by irregularities at the bottom. This loss is greater at the surface than near it. The experiment of transmitted motion through a series of ivory balls illustrates this effect. It is likewise illustrated on a large scale by the collision of two trains of cars on a railway, in which case it has been observed that the cars at the head of the train are the most injured and thrown the farthest from the track; those at the end of the train are next in order of injury and disturbance; while those in the middle of the train are but little injured or disturbed. Other causes may and probably do exist, but their investigation has, fortunately, more of scientific interest than practical value. For all general purposes it may be assumed that there is a resistance at the surface, of the same order or nature as that which exists at the bottom."

Now although this passage does contain the words " arising from upward currents or transmitted motion occasioned by irregularities at the bottom," yet the illustrations, by means of the series of ivory balls, and of the collision of railway trains, show that the authors attribute to those words no clear and correct meaning, but, on the contrary, I would say they put forward quite a false view of the actions going on. Besides I myself do not admit that, except from the air, there is a resistance at the surface. According to my supposition the already resisted and retarded bottom water comes to the surface and spreads out there, but receives no new resistance there, and on the contrary receives acceleration from gravity in running down hill.

II. "The Magic Mirror of Japan." Part I. By Professors W. E. AYRTON and JOHN PERRY, of the Imperial College of Engineering, Japan. Communicated by WILLIAM SPOTTISWOODE, Esq., M.A., Treas. R.S., &c., &c. Received October 2, 1878.

The Japanese mirror must, from three points of view, attract the notice of foreigners sojourning in that country-its prominence in the temples, the important feature it forms in the limited furniture of a Japanese household, and the wonderful property (which has apparently

created more interest in Europe than it has in Japan) possessed by certain Japanese and Chinese mirrors of apparently reflecting from their polished faces the raised characters on their backs.

It was for this third reason, the interest that such mirrors have long possessed for the student of science, that our attention was drawn to the subject, and it has been in this direction that our inquiry has been chiefly directed. The results of our investigation we propose giving in the present paper, reserving for a subsequent occasion* some remarks on the Japanese mirror as an object of worship, and the position it holds on the toilet table of a Japanese lady.

The mirror of the Far East is too well known to need an elaborate description; suffice it for the present to observe that it is generally more or less convex on the reflecting side, usually made of bronze, polished with a mercury amalgam, and having at its back a gracefully executed raised design, representing birds, flowers, dragons, a geometrical pattern, or some scene in Japanese mythical history. Occasionally there are in addition one or more Chinese characters (signifying long-life, happiness, or some similar idea) of polished metal, in bold relief. To the method of manufacture we shall refer further on, and especially to the mode in which the convexity of the surface is produced; which portion of the manufacture, while playing, as it does, an important part in the magical behaviour of the mirror, is, as far as we are aware, not to be found described in any of the Eastern or Western writings on the subject.

Just before leaving England, in 1873, the attention of one of the authors was directed to the so-called magic property of certain Eastern mirrors by the late Sir Charles Wheatstone, who explained to him that the Japanese had a clever trick of scratching a pattern on the surface of a bronze mirror which, after being polished, showed no traces of the scratches when looked at directly, but which, when used to reflect the sunlight on to a screen, revealed the pattern as a bright image. This opinion appears to have been shared by Sir David Brewster, since he says, in the "Philosophical Magazine" for December, 1832 :

"Like all other conjurors, the artist has contrived to make the observer deceive himself. The stamped figures on the back (of the mirror) are used for this purpose. The spectrum in the luminous area is not an image of the figures on the back. The figures are a copy of the picture which the artist has drawn on the face of the mirror, and so concealed by polishing that it is invisible in ordinary lights, and can be brought out only in the sun's rays."

As the explanation, therefore, appeared to this one of the authors to be so simple, and at the same time so complete, he practically dismissed the subject from his mind.

* A lecture at the Royal Institution.

However, he was a little astonished to find, during his residence in Japan, that, although the magic mirror was supposed in Europe to be a standard Japanese trick, and although it had been considered by Sir Charles Wheatstone as one of the best proofs of the ingenuity of the workmen of Japan, still that it formed no part of the stock-intrade of any of the numerous conjurors in this country, and was never exposed for sale in any of the curiosity-shops. He was also still more surprised when, during the visit of the "Challenger," Sir Wyville Thomson and himself were strolling about Tokio, to find that, although they asked at several mirror shops for a mirror that showed the back, a specimen of which Sir Wyville much desired to possess, the shopkeepers seemed not to have the slightest knowledge of what was wanted. At that time the author could not but regard the total apparent ignorance displayed by the Japanese mirror-vendors on this subject as the result of his limited knowledge of the language, and he had then no notion that, in Japan at any rate, the phenomenon was the result of no clever trickery, but arose from the method in which the mirrors were prepared. We have since learnt, however, by diligent inquiry, that, as is the case with many things appertaining to Japan, so with the magic mirror, the people who know least about the subject are the Japanese themselves, and we think this only furnishes another proof that teachers to instruct the Japanese about Japan itself are the greatest desideratum.

Our attention was next directed to the subject of the curious property possessed by some Japanese mirrors by a letter from Professor Atkinson, of the Tokio Dai Gaku (the Imperial University), which appeared in "Nature," May 24th, 1877, and in which he says, after referring to the phenomenon of the pattern on the back being apparently reflected when sunlight is allowed to fall on the face :

"I have since tried several mirrors, as sold in the shops, and in most cases the appearance described has been observed with more or less distinctness.*

"I have been unable to find a satisfactory explanation of this fact, but on considering the mode of manufacture I was led to suppose that the pressure to which the mirror was subjected during polishing, and which is greatest on the parts in relief, was concerned in the production of the figures. On putting this to the test by rubbing the back of the mirror with a blunt-pointed instrument, and permitting the rays of the sun to be reflected from the front surface, a bright line appeared in the image corresponding to the position of the part rubbed. This experiment is quite easy to repeat, a scratch with a knife, or with any other hard body, is sufficient. It would seem as if the pressure upon the back during polishing caused some change in

* Only a small percentage, however, of the total number of Japanese mirrors that the authors of this paper have experimented on show the phenomenon clearly.

the reflecting surface corresponding to the raised parts, whereby the amount of light reflected was greater; or supposing that, of the light which falls upon the surface, a part is absorbed and the rest reflected, those parts corresponding to the raised portions on the back are altered by the pressure in such a way that less is absorbed, and therefore a bright image appears."

Professor Atkinson cautiously adds: “This, of course, is not an explanation of the phenomenon, but I put it forward as perhaps indicating the direction in which a true explanation may be looked for."

In vol. i, p. 242, year 1832, of the " Journal of the Asiatic Society of Bengal," Mr. Prinsep gives an account of a Japanese magic mirror which he had seen in Calcutta. He does not appear to have made any direct experiments with this mirror for the purpose of elucidating which of all the possible causes is the real cause of the magic phenomena, but rather he concludes "from analogy that the thin parts or tympanum of the Japanese mirror are slightly convex with reference to the rest of the reflecting surface, which may have been caused either by the ornamental work having been stamped or partially carved with a hammer and chisel on its back; or, which is more probable, that part of the metal was by this stamping rendered in a degree harder than the rest, so that in polishing it was not worn away to the same extent." It does not seem to have occurred to him that Japanese mirrors are cast and not stamped at all.

In "Nature," June 14th, 1877, Mr. Highley refers to the exhibition of a Japanese mirror by Professor Pepper some years ago at the Polytechnic Institution, London, and to the praiseworthy attempt of an English brass worker, who saw the experiment, and who also was under the false impression that such mirrors were stamped, to solve the problem. "The workman found that taking ordinary brass and stamping upon its surface with any suitable die, not once, but three times in succession, upon exactly the same spot, grinding down and polishing between each act of stamping, a molecular difference was established between the stamped and unstamped parts, so that images of the pattern could be reflected from the finally polished surface, just as with the Japanese specula, though no difference of surface could be detected with the eye."

To people who have not been in China or Japan, and personally studied mirror-making, this idea of stamping seems very plausible, for Sir David Brewster, on p. 113 et seq. of his "Letters on Natural Magic,” published in 1842, describes fully a method, depending on the molecular change produced by stamping, by means of which the inscriptions on old coins, that have been worn quite smooth, may be deciphered. This method merely consists in heating the coin on a piece of red-hot iron, when the inscription becomes visible from the different rate of oxidation of the part of the coin that has been subjected to great