Sir J. A. Fleming

“ LEAVING the consideration of waves and ripples on a water-surface, we pass on to discuss the subject of waves and ripples in the air. Nearly every one is aware, in a general way, that sound is due to a disturbance created in the atmosphere. Few, however, are fully acquainted with the nature of the movements in the air which excite our sense of hearing, and to which we owe, not only the pleasures of conversation and the enjoyment of all the sounds in nature, but those delights of music which are amongst the purest forms of pleasure we possess. ”

“ IT is impossible for the most careless spectator to look at a steam-vessel making her way along a lake, a boy’s boat skimming across a pond, or even a duck paddling on a stream, without noticing that the moving body is accompanied in all cases by a trail of waves or ripples, which diverge from it and extend behind. In the case of a steamer there is an additional irregular wave-motion of the water caused by the paddle-wheels or screw, which churn it up, and leave a line of rough water in the steamer’s wake. ”

“ It is that all space is filled with what we may call an ocean of æther, which can be tossed into waves and ripples just as the air we breathe is traversed in all directions by aerial vibrations, and the restless sea by waves and ripples on the water-surface. ”

“ A careful observer can in this school of Nature instruct himself in all the laws of wave-motion, and gather a fund of knowledge on this subject during an hour’s dalliance at low tide on some sandy coast, or in the quiet study of seaside pools, the surface of which is corrugated with trains of ripples by the breeze. ”

“ Matter in its various forms serves as the vehicle of Energy. We have no experience of Energy apart from Matter of some kind, nor of Matter altogether devoid of Energy. We do not even know whether these two things can exist separately, and we can give no definition of the one which does not in some way presuppose the existence of the other. Returning, then, to the subject of waves, we may say that a true wave can only exist when Energy is capable of being associated with a medium in two forms, and the wave is a means by which that Energy is transferred from place to place. ”

“ When anything takes time to travel from one place to another, it can only be one of two things. It must either be an actual object which is transferred bodily from place to place, like a letter sent by post or a bullet fired from a gun, or else it must be a wave-motion created in a medium of some kind which fills all space. The illustrious Newton suggested an hypothesis or supposition as to the nature of light, viz. that it consists of small corpuscles shot out violently from every luminous body. ”

“ If a wave is made and travels away over water, it carries with it energy, and hence it can only be created if we have a store of energy to draw upon. If we suppose that skin friction is absent, and that the ship floats upon a perfect fluid, it would nevertheless be true that, if the moving object creates waves, it will thereby reduce its own movement and require the application of force to it to keep it going. We may say therefore that if any floating object creates waves on a liquid over which it moves, these waves rob the floating body of some of its energy of motion. ”

“ One essential element in Maxwell’s theory of electricity is that an electric strain or displacement, whilst it is being made or whilst it is disappearing, is in effect an electric current, and it is for that reason sometimes spoken of as a [222] displacement current. We have seen that every electric circuit possesses a quality analogous to inertia, that is to say, when a current is produced in it it tends to persist, and it cannot be created at its full value instantly by any electromotive force. ”

“ If an atom is struck by colliding with another, and then left to itself, it appears as if the electrons which compose it and form part of it are set in vibration, and each executes its oscillation in some definite period of time. An atom has, therefore, been compared to a “collection of small tuning-forks,” which, if rudely struck, would result in the emission of a set of air-wave trains, each one corresponding in wave-length to one particular tuning-fork which emitted it. ”

“ It is not quite so easy to see at once that if the solid body is not symmetrical in shape the same thing is true, but it can be established by a strict line of reasoning. The result is to show that when a solid of any shape is immersed in a perfect liquid, it cannot be moved by the liquid flowing past it, and correspondingly would not require any force to move it against and through the liquid. In short, there is no resistance to the motion of a solid of any shape when pulled through a perfect or frictionless liquid. ”

“ The whole theory of the trail of waves made by a canal-boat is only comprehensible if it is clearly seen that a water-surface wave has a certain velocity determined by its wave-length. If the wave-speed is small, the waves are short. As the speed increases the waves get longer. Or the matter may be put in another way. We may say that just as a pendulum has a certain rate of vibration depending on its length, so a water wave has a certain frequency, and therefore, a certain speed of propagation dependent upon the wave-length, or shortest distance from one wave-crest to the next. ”

“ The fact that there is such a space-filling æther is, therefore, only to be deduced by reasoning from experiments and observations, but it is not directly the object of our sense-perceptions in the same way that water or air can be. Nevertheless, there is abundant proof that it is not merely a convenient scientific fiction, but is as much an actuality as ordinary gross, tangible, and ponderable substances. It is, so to speak, matter of a higher order, and occupies a rank in the hierarchy of created things which places it above the materials we can see and touch. ”

“ It is hardly necessary to occupy time with experiments in showing that a ray of light can be reflected and refracted by mirrors and prisms, and converged or diverged by transparent lenses. These are simple optical facts, and if you are not familiar with them it will be easy for you to make their acquaintance by studying any simple book upon optics; but I should like to draw your attention to the fact that, in addition to rays of light and electric radiation, we are acquainted with another kind of radiation, which is also susceptible of being refracted, and that is commonly called dark heat. ”

“ We are led, therefore, to infer that, even when agitated by waves, each particle of water never moves far from its position when at rest, and that the real movement of the water is something very different from its apparent motion. If we place on the surface of water a number of corks or pieces of paper, and then watch them as a wave passes over them, we shall notice that the corks or bits of paper rise and fall successively, that is, one after the other, and not all together. ”

“ Every electric lamp that is lighted, every electric tram-car that glides along, is drawing its supply of energy through the æther. The wire or conductor, as we call it, serves to guide and direct the path of the energy transferred; but the energy is not in but around the wire. We have lately learnt to make what we may best describe as billows in the æther, and these are the long waves we employ in wireless telegraphy. But in telegraphy, whether with wires or [285] without, we are merely manipulating the æther as a medium of communication, just as in speech or hearing we use the air. ”

“ The air in the tube is an elastic substance, and it resists compression. At constant temperature the volume into which the air is squeezed is inversely as the pressure applied.
The air, therefore, possesses elasticity of bulk, as it is called, and it resists being made to occupy a smaller volume. Again, the air possesses inertia, and when it is set in motion it continues to move like any other heavy body, after the moving force is withdrawn. We have, therefore, present in it the two essential qualities for the production of a wave-motion, as explained in the first lecture. ”

“ It has already been explained that a true wave can only be created in a medium which elastically resists some kind of deformation, and persists in motion in virtue of inertia. When any material possesses such a quality of resistance to some kind of strain or deformation of such a character that the deformation disappears when the force creating it is withdrawn, it is called an elastic material. ”

“ Whenever we can produce interference in this manner between two sets of sound-rays, or light-rays, or rays of any other kind, we have the strongest possible proof that we are concerned with a wave-motion; because in no other way that we can understand is it possible that a destruction of sound by sound can take place by, so to speak, superimposing two sound-rays, or a destruction of light by bringing together two rays of light. ”

“ We have constant occasion in mechanical work to notice that whereas one pull or push of great vigour will not create some desired displacement of an object, a number of very small hits, or properly timed pushes or pulls, will achieve the requisite result. We might summarize the foregoing facts by saying that it is a maxim in dealing with bodies capable of any kind of free vibration that impulses, however small, will create oscillations of any required magnitude, if only applied at intervals equal to the natural free period of vibration of the body in question. ”

“ A study of breaking waves shows us that the cause of their great power to effect damage to coast structures, such as piers, harbour works, and shipping in harbours, is really due to the forward motion of the water as the wave is breaking. Every cubic foot of water weighs 631⁄2 lbs., so that a cubic yard of water weighs about three-quarters of a ton. If this water is moving with a speed of many feet per second in a forward direction, the energy of motion stored up in it is tremendous, and fully sufficient to account for the destructive power of storm waves on a coast. ”

“ The peculiar charm of the violin is the quality of the sound which a skilled player can elicit from it. That wonderful pleading, sympathetic, voice-like tone, which conveys so much emotional meaning to the trained musical ear, is due to the proper admixture of the harmonics, or overtones, with the fundamental notes. The string vibrates not merely as a whole, but in sections. Hence the place at which the bow touches must always be an anti-node, or ventral point, and the smallest change in this position greatly affects the quality of the tone. ”

“ Hence as the velocity of visible light is 186,500 miles per second, or 1000 million feet, or 12,000 million inches per second, whilst the wave-length is something like 1⁄50000 inch, it is clear that the frequency, or number of light waves which enter the eye per second, must be reckoned in millions of millions. In fact, it ranges from 400 to 700 billions. There is a certain difference of opinion as to what is meant by a billion. We here use the word to signify a million times a million, a million being a thousand times a thousand. ”

“ It is somewhat difficult to realize at first that, outside of ourselves, there is no such thing as colour. Colour is a sensation produced when æther waves of a certain wave-length enter the eye and fall on the retina. If the retina is stimulated 400 billions of times per second, we experience a sensation of redness, and if it is stimulated 700 billion times per second, we experience a sensation of blueness; but externally, there is no such thing as red and blue, there is only a difference in wave-frequency. ”

“ The sea-surface acts somewhat like an optical reflector or mirror, and the electric waves glide along its surface. The rotundity of the earth within certain limits hardly makes any perceptible effect upon the ease of communication. The waves sent out by the transmitter of a long-distance wireless station are from 3000 to 20,000 feet in length, and there is, therefore, a considerable amount of bending or diffraction. It is a familiar fact, as already explained, that a wave-motion, whether on water or in air, spreads round an obstacle to a certain extent. ”

“ If light is a substance, no possible explanation can be given which will enable us to account for the combination of two rays of light producing darkness at their meeting-point. If, on the other hand, rays of light consist of waves of some kind in a medium, then, as we have seen in the case of water ripples and air waves, it is quite possible for two wave-trains to annihilate each other’s effect at a certain point, if a hollow of one wave-train reaches [191] that place coincidently with a hump belonging to the other. ”

“ As a wave progresses over the surface, wave-energy is continually being imparted to portions of the water in front, and it is transferred away from others in the rear. In the very act of setting a fresh particle of water in oscillation, the portions already vibrating must diminish their own motion. They may hand on the whole of their energy or only a part of it to their neighbours. This distinction is a very important one, and it determines whether a single act of disturbance shall create a solitary wave or wave-train in a medium. ”

“ The fish, progressing through the water wholly under the surface, makes no waves, but the water adheres to its skin, and there is friction between them as he moves. Also he creates eddies in the water, which require energy to produce them, and whenever mechanical work has to be done, as energy drawn off from a moving body, this implies the existence of a resistance to its motion which has to be overcome. ”

“ Every one knows that it is not instantaneous. We see the flash of a gun at a distance, and a second or so afterwards we hear the bang. We notice that the thunder is heard often long after the lightning flash is seen. It would take too long to describe the experiments which have been made to determine precisely the speed of sound waves. Suffice it to say that all the best experiments show that the velocity of a sound wave in air, at the temperature of melting ice, or at 0° C. = 32° Fahr., is very nearly 1087 feet per second, or 33,136 centimetres [116] per second. ”

“ We cannot produce an air wave by moving any object such as a fan slowly to and fro through the air. In order to produce an air wave we must strike the air a very sudden blow, or, which comes to the same thing, we must apply and remove a very sudden pressure to the air; and under these circumstances we start into existence an air wave, which travels away from the vibrating or rapidly moving body, and continues its journey out into surrounding space. ”

“ It is a wonderful testimony to Newton’s exalted powers of thought, that the most recent investigations show that hot and luminous bodies, such as the sun and a lamp, are in fact projecting small bodies called corpuscles into space, but there is abundant proof that these are not the cause of light. Subsequently to the date of Newton’s speculations on the nature of light, the alternative hypothesis was developed, viz. that it consists in a wave-motion in a universally diffused medium called the æther. ”

“ Consider the case of a loud sound made near the surface of the earth. If the air were all at rest, and everywhere at the same temperature, the sound waves should spread out in hemispherical form. But if, as is generally the case, the temperature near the ground is higher than it is up above, then the part of the wave near the earth travels more quickly than that in the higher regions of the air. ”

“ It has been explained that in a wave-motion each one of a series of contiguous objects executes some identical movement in turn. We have all seen the wind blowing on a breezy day across a cornfield, and producing a sort of dark shadow which sweeps along the field. This is clearly caused by the wind bending down, in turn, each row of cornstalks, and as row after row bows itself and springs up again, we are presented with the appearance of a wave-motion in the form of a rift rushing across the field. ”

“ A little examination of wave-motion shows us, therefore, that we always have at any instant associated with the material in which the wave is being propagated, both Energy of Strain and Energy of Motion. It can be shown that in a true wave of permanent type, the whole energy at any one moment is half energy of strain and half energy of motion, or, as it is called, half potential and half kinetic. ”

“ In all departments of natural science we find ourselves confronted by the phenomena of wave-motion. In the study of earthquakes and tides, telegraphs and telephones, as well as terrestrial temperature, no less than in the examination of water waves and ripples, sound, music, or light and heat, we are bound to consider waves of some particular kind. ”

“ If you think what the action must be, you will easily see that the operation of the sound wave is to throw the particles of the gas, just as they escape from the hole in the jet, into vibration in a direction transverse or at right angles to the direction of their movement in the flame. The gas molecules are, when unacted upon by the sound wave, rushing out of the jet, in an upward direction. When the sound wave impinges on them they are, so to speak, caught, and caused to rock to and fro in a direction across the flame. ”