But I am forgetting Napoleon-he was able to imagine himself an emperor, and, circumstances conspiring with him, he became one. His enemies thought they were belittling him by calling him an actor, and the Pope. whom he hurled from the Papal throne, could only retort" Comediante"; but the comedian continued to play his part of emperor while the Pope was in exile. The artistic methods of the first Napoleon are brought into strong relief when contrasted with those of his less imaginative nephew. Indeed, the difference between the imaginative and the unimaginative actor is well exemplified in these two. Had Napoleon the Third possessed the true dramatic instinct, he would not have been guilty of the Boulogne fiasco. On that occasion, in order to impress the populace with a supernatural significance of his mission, he had recourse to the stagey device of a tame eagle, which, as the emblem of empire, was at a given cue to alight upon him. But the bird, which had been trained to perch upon his top-hat, disdained his crown. Here we have an illustration of the futility of unimaginative stage-management.

The imagination is the mind's eye. To him who has it not, life presents itself as a picture possessing all the merits of a photograph, and none of the blemishes of a work of art. He who does not treasure it, will lose its use. In a burst of scientific fantasy, I once propounded the theory that the soft place on the top of a baby's head was really intended by beneficent Nature to enable us, through this yet open channel, to destroy by electricity, or what not, those tissues of the brain which go to make the vicious portions of our nature. In unfolding my discovery to a scientific friend, I learned, however, that this particular part of our brain was really a primitive eye, and was no doubt used by our prehistoric ancestors for the purpose of seeing objects overhead. The Cyclops was probably a throwback of this species. In certain lower forms of animals, I am told, in lizards, for instance, this eye is infinitely more developed than it is in the higher animals, in whom, from disuse, it has become practically extinct. Even so will the imagination, this third eye of the mind, looking heavenward, lose its function unless it is exercised. The waning of the imagination is, next to the loss of his childish faith, the most tragic thing in a man's life. I can conceive no fate more terrible than that which befalls the artist in watching with still undiminished powers of selfobservation, the slow ebbing of the imaginative faculty, to see it drifting out to sea in the twilight of life. Better be deprived of sight than to feel that the world has lost its beauty-for the blind are happier than the blear-eyed. . . .

It would be interesting to know whether the cultivation of the æsthetic faculties would have strengthened or weakened in Darwin those other forces which have made him such a shining figure in the history of science. It may be that what was a loss to the man was a gain to humanity, for to every one is only vouchsafed a limited power of concentration. Nor must it be supposed that Science and Art are separate and opposing forces; they are rather two mighty

128 Mr. H. Beerbohm Tree on the Imaginative Faculty. [May 26, currents springing from one parent source. The greatest victories which mind has achieved over matter have been due to the soaring flights of the imagination rather than to a mere crawling research along the surface of facts. This hall, where Faraday, Huxley, and Tyndall have spoken, has witnessed displays of the imagination equal to those of the highest poetry. As the diver dives for pearls into the depths of the sea, so does science project itself on the wings of the imagination into the mists which shroud the vast unexplained, snatching in its flight the secrets which solve the mysteries of the universe, and which point out to mankind the invisible steppingstones connecting the known with the unknown.

It was in this hall that Professor Dewar summoned the elusive and invisible atmosphere, which since all time has enveloped the earth, and with the wand of science compelled it to appear before you in a palpable and visible form. Even so does the imagination distil from the elemental ether of thought and truth the liquid air of art. I have endeavoured to show that, just as the highest achievement of science is that which we owe to the imagination, so also is the highest achievement of art that which carries us out of the sordid surroundings of every-day life into the realms of idealised truth. Its loftiest mission is to preserve for us, amid the din and clash of life, those illusions which are its better part-to epitomise for us the aspirations of mankind, to stifle its sobs, to nurse its wounds, to requite its unrequited love, to sing its lullaby of death. It is the unwept tear of the criminal, it is the ode of the agnostic to immortality, it is the toy of childhood, the fairyland of the mature, and gilds old age with the afterglow of youth.

[H. B. T.]

WEEKLY EVENING MEETING,

Friday, June 2, 1893.

SIR DOUGLAS GALTON, K.C.B. D.C.L. LL.D. F.R.S. Vice-President, in the Chair.

PROFESSOR OSBORNE REYNOLDS, M.A. LL.D. F.R.S.

Study of Fluid Motion by means of Coloured Bands.

In his charming story of The Purloined Letter,' Edgar Allan Poe tells how all the efforts and artifices of the Paris police to obtain possession of a certain letter, known to be in a particular room, were completely baffled for months by the simple plan of leaving the letter in an unsealed envelope in a letter-rack, and so destroying all curiosity as to its contents; and how the letter was at last found there by a young man who was not a professional member of the force. Closely analogous to this is the story I have to set before you tonight-how certain mysteries of fluid motion, which have resisted all attempts to penetrate them are at last explained by the simplest means and in the most obvious manner.

This indeed is no new story in science. The method adopted by the minister, D., to secrete his letter appears to be the favourite of Nature in keeping her secrets, and the history of science teems with instances in which keys, after being long sought amongst the grander phenomena, have been found at last not hidden with care, but scattered about, almost openly, in the most commonplace incidents of every-day life which have excited no curiosity.

This was the case in physical astronomy-to which I shall return after having reminded you that the motion of matter in the universe naturally divides itself into three classes.

1. The motion of bodies as a whole as a grand illustration of which we have the heavenly bodies, or more humble, but not less effective, the motion of a pendulum, or a falling body.

2. The relative motion of the different parts of the same fluid or elastic body-for the illustration of which we may go to the grand phenomena presented by the tide, the whirlwind, or the transmission of sound, but which is equally well illustrated by the oscillatory motion of the wave, as shown by the motion of its surface, and by the motion of this jelly, which, although the most homely illustration, affords by far the best illustration of the properties of an elastic solid.

3. The inter-motions of a number of bodies amongst each other -to which class belong the motions of the molecules of matter VOL. XIV. (No. 87.)

K

resulting from heat, as the motions of the molecules of a gas, in illustration of which I may mention the motions of individuals in a crowd, and illustrate by the motion of the grains in this bottle when it is shaken, during which the white grains at the top gradually mingle with the black ones at the bottom-which interdiffusion takes an important part in the method of coloured bands.

Now of these three classes of motion that of the individual body is incomparably the simplest. Yet, as presented in the phenomena of the heavens, which have ever excited the greatest curiosity of mankind, it defied the attempts of all philosophers for thousands of years, until Galileo discovered the laws of motion of mundane matter. It was not until he had done this and applied these laws to the heavenly bodies that their motions received a rational explanation. Then Newton, taking up Galileo's parable and completing it, found that its strict application to the heavenly bodies revealed the law of gravitation, and developed the theory of dynamics.

Next to the motions of the heavenly bodies, the wave, the whirlwinds, and the motions of clouds, had excited the philosophical curiosity of mankind from the earliest time. Both Galileo and Newton, as well as their followers, attempted to explain these by the laws of motion, but although the results so obtained have been of the utmost importance in the development of the theory of dynamics it was not till this century that any considerable advance was made in the application of this theory to the explanation of fluid phenomena, and although during the last fifty years splendid work has been done, work which, in respect of the mental effort involved, or the scientific importance of the results, goes beyond that which resulted in the discovery of Neptune, yet the circumstances of fluid motion are so obscure and complex that the theory has yet been interpreted only in the simplest cases.

To illustrate the difference between the interpretation of the theory of the heavenly bodies and that of fluid motion, I would call your attention to the fact that solid bodies, on the behaviour of which the theory of the motion of the planets is founded, move as one piece, so that their motion is exactly represented by the motion of their surfaces; that they are not affected with any internal disorder which may affect their general motion. So surely is this the case, that even those who have never heard of dynamics can predict with certainty how any ordinary body will behave under any ordinary circumstances, so much so that any departure is a matter of surprise. Thus I have here a cube of wood, to one side of which a string is attached. Now hold it on one side, and holding the string you naturally suppose that when I let go it will turn down so as to hang with the string vertical; it does not do so, that is a matter of surprise; I place it on the other side and it still remains as I place it. If I swing it as a pendulum it does not behave like one.

Would Galileo have discovered the laws of motion had his pendulum behaved like this? Why is its motion peculiar? There is

internal motion. Of what sort? Well, I think my illustration may carry more weight if I do not tell you; you can all, I have no doubt, form a good idea. It is not fluid motion or I should feel bound to explain it. You have here an ordinary looking object which behaves in an extraordinary manner, which is yet very decided and clear, to judge by the motion of its surface, and from the manner of the motion I wish you to judge of the cause of the observed motion.

This is the problem presented by fluids, in which there may be internal motion which has to be taken into account before the motion of the surface can be explained. You can see no more of what the motion is within a homogeneous fluid, however opaque or clear, than you can see what is going on within the box. Thus, without colour bands the only visual clue to what is going on within the fluids is the motion of their bounding surfaces. Nor is this all; in most cases the surfaces which bound the fluid are immovable.

In the case of the wave on water the motion of the surface shows that there is motion, but because the surface shows no wave it does not do to infer that the fluid is at rest.

The only surfaces of the air within this room are the surfaces of the floor, walls, and objects within it. By moving the objects we move the air, but how far the air is at rest you cannot tell unless it is something familiar to you.

Now I will ask you to look at these balloons. They are familiar objects enough, and yet they are most sensitive anemometers, more sensitive than anything else in the room; but even they do not show any motion; each of them forms an internal bounding surface of the air. I send an aerial messenger to them, and a small but energetic motion is seen by which it acknowledges the message, and the same message travels through the rest, as if a ghost touched them. It is a wave that moves them. You do not feel it, and, but for the surfaces of the air formed by the balloons, would have no notion of its existence.

In this tank of beautifully clear distilled water, I project a heavy ball in from the end, and it shows the existence of the water by stopping almost dead within two feet. The fact that it is stopped by the water, being familiar, docs not raise the question, Why does it stop?-a question to which, even at the present day, a complete answer is not forthcoming. The question is, however, suggested, and forcibly suggested, when it appears that with no greater or other evidence of its existence, I can project a disturbance through the water which will drive this small disc the whole length of the tank.

I have now shown instances of fluid motion of which the manner is in no way evident without colour bands, and were revealed by colour bands, as I showed in this room sixteen years ago. At that time I was occupied in setting before you the manners of motion revealed, and ] could only incidentally notice the means by which this revelation was accomplished.