It should be noted, however, that whilst there are more or less definite limits to the wave-lengths of the eye-affecting radiation, and probably also to the actinic, or photographic radiation (radiation of some wave-lengths being both visible and actinic), rays of every wave-length are in some degree thermal, or heat-producing. The term dark-heat radiation is, however, generally restricted to radiation of that wave-length which is non-visible and non-actinic. This mode of presenting the facts will call your attention again to the narrow limits of sensibility of the human eye as compared with those of the ear.

The above-mentioned range of wave-lengths does not, however, exhaust our powers of æther-wave production. If we skip over six octaves lying below the limits of the longest dark-heat wave with which we are acquainted, we should arrive at a wave whose wave-length is about 4000μ, or 4 millimetres. At this point we encounter the shortest æther waves which have yet been made by means of electrical oscillations in the fashion first discovered by Hertz.

It is not possible to define exactly the wave-length limits of radiation as yet made by means of electrical oscillations. Lampa has experimented with æther waves made by the Hertz method, the wave-length of which was not more than 4 millimetres. Professors Lodge, Rhigi, Bose, Trouton, the author, and many others, have carried out quasi-optical experiments with electrically made æther waves, the wave-length of which ranged from a few millimetres to several inches. Hertz’s own work was chiefly done with æther waves from 1 or 2 feet to 30 or 40 feet in wave-length. More recently, æther waves of 800 to 1000 feet in wave-length have been employed in wireless telegraphy. Perhaps we shall not be wrong in saying that we are acquainted with sixteen or seventeen octaves of æther-wave radiation which is made electrically, and is usually called the Hertz radiation.

Between the radiation of greatest wave-length which proceeds from hot or incandescent bodies such as the sun, the electric arc, or a hot ball, and that of the shortest wave-length which has been created by means of electrical oscillations set up in some form of Hertz oscillator, there is a range of six octaves of æther waves which, so far as we know, have not yet been manufactured or detected. Herein lies an opportunity for much future scientific work. We have to discover how to create and recognize these interconnecting wave-lengths. From the fact that all Hertz waves travel with the same speed as light, and from our ability to imitate, as you have seen, the well-known optical phenomena with Hertz radiation of short wave-length, the great induction has been made that all æther waves have the same essential nature, and that invisible actinic rays, light rays, dark-heat rays, and Hertz rays are all of them æther waves of various wave-lengths and amplitudes. Thus we see, as Maxwell long ago predicted, that light in all probability is an electro-magnetic phenomenon, and therefore all optical effects must be capable of receiving an electro-magnetic explanation. The inclusion thus made of the whole science of Optics within the domain of Electricity and Magnetism is one of the grandest achievements of Physical Science. It stands second only to Newton’s great discovery of universal gravitation, which reduced all Physical Astronomy to pure Dynamics, and showed that the force concerned in the falling of a stone is identical with that which holds the planets in their orbits, and controls the motions of galaxies of suns.

At the end of the last chapter it was explained that these Hertz radiations are created in the æther by the suddenly starting, stopping, or reversing the motion of crowds of electrons, which are, as it were, instantly released from a state of pressure or tension, and set moving inside a straight insulated conductor, which forms an open electric circuit. The radiations we call light and dark heat are probably, therefore, started in a similar manner by vibrations of the electrons which form parts of, or which build up, atoms. There are many physical phenomena which seem to show that the electrons which we can detach from atoms in a high vacuum tube are capable of vibrating freely in definite periods when in connection with their atom. If the atoms are able to move freely, and if each is practically independent, as is the case in a gas, and if they are then caused to radiate by any means, the radiation emitted by the vibration of these electrons consists of certain definite wave-lengths. Hence, when we form the spectrum of an incandescent gas, we find it to consist of several detached bright lines, each corresponding to one particular wave-length, and we do not obtain a uniformly graduated band of coloured light. 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. Hence, if we could administer a blow to such a congeries of tuning-forks, and then analyze the compound sound, we should obtain a sound spectrum consisting of separated tones—in other words, a bright line spectrum of the complex sound. Supposing, however, that we have a mass of atoms much more closely in contact, as in the case of a solid body, the continual collisions between the atoms and the closer contact between them cause the vibrations of the electrons to be “forced,” and not “free.” Hence the electrons are compelled to execute all varieties of irregular motion, and these predominate over their regular free natural vibrations. Accordingly, the waves emitted are of a large variety of wave-length, and when the radiation is analyzed by a prism, we obtain a continuous spectrum, or band of many-coloured light, as the result of the separation of the rays of different wave-lengths present in it.

It is this fact which renders our present method of creating artificial light so excessively uneconomical.

All our practical methods for making light consist in heating a solid body in one way or another. In the case of the electric light we heat electrically a carbon rod or filament, or else, as in the Nernst lamp, a rod composed of magnesia and the rare earths. In the case of the lime-light we heat a cylinder of lime. In an ordinary gas or candle flame we heat small particles of carbon, and the same is the case even in the sun itself.

But this process manufactures not only the single octave of radiation which can affect our eyes as light, but a dozen other octaves of radiation to which they are insensible. Hence it follows that of the whole radiation from a gas flame, only about 3 per cent. is eye-affecting light, the remainder is dark heat. In the case of an incandescent electric lamp, this luminous efficiency may amount to 5 per cent., and in the electric arc to 10 or 15 per cent. There is, however, always a great dilution of the useful light by useless dark heat.

The proportion of the light or eye-affecting radiation to the dark heat in the total radiation from any source of light increases with the temperature, but it is not always merely a question of temperature. Thus the electric arc is hotter than a candle flame, and the sun is hotter than the electric arc. Hence, whilst the luminous rays only form 3 parts out of 100, or 3 per cent. of the radiation of a candle, they constitute 10 to 15 per cent. of those of the electric arc, and more than 30 per cent. of those of the sun. On the other hand, the glow-worm and the fire-fly seem to have possession of a knowledge and an art which is as yet denied to man. It has been shown by Professor Langley and Mr. Very that nearly the whole of the radiation from the natural torch of the fire-fly is useful light, and none of it is useless dark heat. Hence these photogenic (light-producing) insects have the art, which we have not, of creating cold light, or unadulterated luminous radiation.

At the present moment in ordinary incandescent or glow-lamp electric lighting we require to expend an amount of power, called one horse-power, to produce illumination equal to that of 600 candles. Supposing, however, that all our power could be utilized in generating merely the rays useful for vision, or which can impress our eyes, we might be able to create by the expenditure of one horse-power more than twenty times as much illumination, that is, a light equal to 12,000 candles.

These figures show us what rewards await the inventor who can discover a means of generating æther waves having wave-lengths strictly limited to the range lying between the limits 0·4μ and 0·7μ without, at the same time, being obliged to create radiation comprising longer waves which are not useful for the purpose of rendering objects visible to us. For the purposes of artificial illumination we require only the æther waves in this one particular octave, and nothing else.

This increase in the efficiency of our sources of artificial illumination is only likely to be brought about when we abandon the process of heating a solid substance to make it give out light, and adopt some other means of setting the electrons in vibration.

It is almost impossible to discuss the subject of æther waves without some reference to the most modern utilization of them in the so-called wireless telegraphy. Without entering upon the vexed questions of priority, or on the historical development of the art, we shall simply confine our attention here to a consideration of the methods employed by Mr. Marconi, who has accomplished such wonderful feats in this department of invention.

We have already seen that when two insulated conductors are placed with their ends very near together, and are then electrified, one positively and the other negatively, and then allowed to be suddenly connected by an electric spark, they constitute an arrangement called an electrical oscillator. If the conductors take the form of two long rods placed in one line, and if their contiguous ends are provided with spark-balls separated by a small gap, we have seen that we have shown that, under the above-mentioned conditions, electric currents of very high frequency are set up in these rods. For creating these oscillations, an instrument called an induction coil or spark-coil is generally employed. You will understand the arrangements better if a brief description is given first of the spark-coil itself as used in wireless telegraphy.

The appliance consists of a large bundle of fine iron wires, which are wound over with a long coil of insulated wire. This forms the primary coil. It is enclosed entirely in a tube of ebonite. One end of this coil is a contact-breaker, which automatically interrupts an electric current flowing from a battery through the primary coil (see Fig. 81). A hand-key is also placed in the circuit to stop and start the primary current as desired. Over the primary coil is a very long coil of much finer silk-covered copper wire, called the secondary coil. The length of this coil is very considerable, and may amount to many miles. The secondary coil is divided into sections all carefully insulated from each other. Another important part is the condenser. This consists of sheets of tinfoil laid between sheets of waxed paper, alternate tinfoil sheets being connected. The arrangement forms virtually a Leyden jar, and one set of tinfoils is connected to one side of the automatic break, and the other to the adjacent side. When, therefore, the primary circuit is interrupted by the break, the condenser is at that moment thrown into series with the primary coil. The rapid interruption of the primary current causes a secondary current in the fine-wire coil. The automatic contact-breaker makes from ten to fifty such interruptions per second. At every “break” of the primary a very high electromotive force is generated in the secondary circuit, which may amount to many hundreds of thousands of volts. This very high secondary electromotive force is able to cause an electric discharge in the form of a spark between brass balls connected to the secondary circuit terminals. Coils are generally rated by the length (in inches) of the spark they can produce between brass balls about ¹⁄₂ inch in diameter. The coil most commonly used in wireless telegraphy is thus technically termed a “10-inch induction coil,” from the length of the spark this particular type of coil can produce.

If the insulated brass balls, called the spark-balls, connected to the secondary terminals, are placed an inch or so apart, and the hand-key in the primary circuit is closed, a battery connected to the primary circuit will send a rapidly interrupted current through the primary coil, and a torrent of sparks will pass between the spark-balls. The primary current of the 10-inch coil is usually a current of 10 ampères, supplied at a pressure of 10 volts.

If the hand-key is raised or pressed, it is possible to make long or short torrents of secondary sparks.

Suppose, then, that we connect to the secondary spark-balls two long insulated rods, and place the spark-balls about ¹⁄₄ inch apart. On pressing the hand-key, we obtain a peculiarly bright crackling spark between the balls, which is an oscillatory spark, and at the same time, as already described, electrical oscillations are set up in the rods and electric waves given off. We may represent to ourselves these electrical oscillations in the rods as similar to the mechanical vibrations which would be set up in a long elastic wooden rod, clamped at the middle and set in vibration at the ends. Or we may consider them similar to the fundamental vibrations of an open organ-pipe, the middle of the pipe corresponding with the middle of the rod. In comparing the mechanical vibrations of the rod or the acoustic vibration of the air in the organ-pipe with the electrical oscillations in the long rods, we must bear in mind that the displacement of the rod or the air in the organ-pipe at any point corresponds with electrical pressure, or potential, as it is called, at any point in the long oscillator. Hence, bearing in mind the remarks in the fourth lecture, it will be evident to you that just as the length of the air wave emitted by the open organ-pipe is double the length of the pipe, so the length of the electric wave thrown off from the pair of long rods is double their total length.

Instead of using a pair of rods for the electrical oscillator, it was found by Mr. Marconi to be an improvement to employ only one insulated rod, held vertically, and to connect it to one spark-ball of the coil, and to connect the opposite spark-ball to a metal plate buried in the earth. Then, when the spark-balls are placed a little apart and the hand-key pressed, we have a torrent of oscillatory sparks between the “earthed ball” and the insulated rod ball. This sets up in the rod electrical oscillations, which run up and down the rod. It is easy to show that there is a strong electric current passing into and out of the rod by connecting it to the spark-ball by means of a piece of fine wire. When the sparks are taken, we find this wire will become hot, it may be red hot, or sometimes it may be melted.

By applying the principles already explained, it is not difficult to demonstrate that in the case of an oscillator consisting of a single rod connected to one spark-ball the electric waves thrown off are in wave-length four times the length of the rod.

The electrical actions taking place, therefore, are as follows: At each interruption of the primary current of the spark-coil there is an electromotive force created in the secondary circuit, which gradually charges up the insulated rod until it attains a state in which it is said to be at a potential or electrical pressure of several thousand volts. The spark then happens between the balls, and the rod begins to discharge.

This process consists, so to speak, in draining the electric charge out of the rod, and it takes the form of an electric current in the rod, which has a zero value at the top insulated end, and has its maximum value at the spark-ball end.

Also, when the oscillations take place, we have variations of electric pressure, or potential, which are at a maximum at the upper or insulated end, and have a zero value at the spark-ball end. From the rod we have a hemispherical electric wave radiated. In the language of wireless telegraphists, such a simple insulated rod is called an insulated aerial, or an insulated antenna.

A simple insulated aerial has, however, a very small electrical capacity, and it can store up so little electric energy that the whole of it is radiated in the first oscillation. Hence, strictly speaking, we have no train of electric waves radiated, but merely a solitary wave or electric impulse. The effect on the æther thus produced corresponds to the effect on the air caused by the crack of a whip or an explosion, and not to a musical note or tone as produced by an organ-pipe.

We can, however, make an arrangement which is superior in electric wave-making power, as follows:⁠—

The vertical rod, or antenna, A, is not insulated, but is connected by its lower end with one end of a coil of insulated wire, S, wound on a wooden frame (see Fig. 82). The other end of this last coil is connected to a metal plate, e, buried in the earth. Around the wooden frame is wound a second insulated wire, P, one end of which is connected to one spark-ball of the induction coil, and the other end to the outside of a Leyden jar, L, or collection of jars. This double coil on a frame is called an oscillation transformer. The inside of this condenser is connected to the second spark-ball of the induction coil I. When these spark-balls S are placed a short distance apart, and the coil set in action, we have a torrent of oscillatory electric sparks between these balls, and powerful oscillations set up in one circuit of the oscillation-transformer. These oscillations induce other oscillations in the second circuit of the oscillation-transformer, viz. in the one connected to the aerial. The oscillations produced in the air-wire, or aerial, are therefore induced, or secondary oscillations. The aerial wire, or antenna, has therefore a much larger store of electric energy to draw upon, viz. that stored up in the Leyden jars, than if it was itself directly charged by the coil.

In order, however, to obtain the best results certain adjustments have to be made. It has already been explained that every open electrical circuit has a certain natural time-period for the electrical oscillations which can be set up in it. This is technically called its tune.

If we administer a blow to a suspended pendulum we have seen that, if left to itself, it vibrates in a definite period of time, called its natural period. In the same manner, if we have a condenser or Leyden jar having electrical capacity which is joined in series with a coil of wire having electrical inertia or inductance, and apply to the circuit so formed a sudden electromotive force or impulse, and then leave the circuit to itself, the electric charge in it vibrates in a certain definite period, called its natural electrical periodic time.

The aerial, or antenna, is simply a rod connected to the earth, but it has a certain inductance, and also a certain electrical capacity, and hence any metal rod merely stuck at one end in the earth has a perfectly definite periodic time for the electrical oscillations which can be produced in it. We may compare the rod in this respect with a piece of steel spring held at one end in a vice. If we pull the spring on one side, and let it vibrate, it does so in accordance with its natural time-period for mechanical vibrations. The sound waves given out by it have a wave-length equal to four times the length of the spring. In the same manner the fundamental wave-length of the electric waves emitted by an “earthed aerial,” or rod stuck in the earth, when an electric impulse is applied to its lower end, and electrical oscillations are set up in it, have a wave-length equal to four times that of the rod. Hence to obtain the best result the circuit, including the aerial A, must be “tuned” electrically to the circuit including the Leyden jar L.[27]

A consideration of these arrangements will show you that if the hand-key in the primary circuit of the induction coil is pressed for a long or short time, we have long or short torrents of sparks produced between the secondary balls, and long or short trains of electric waves emitted from the aerial, or earthed vertical wire.

Whenever we have any two different signals, we can always make an alphabet with them by suitable combinations of the two. In the well-known Morse alphabet, with which every telegraphist is as familiar as we all are with the printed alphabet, the sign for each of the letters of the alphabet is composed of groups of long and short symbols, called dots and dashes, as follows: Each letter is made by selecting some arrangements of dots or dashes, these being the technical names for the two signs. The Morse code, as used all over the world, is given in the table below⁠—

The process of sending a wireless message consists in so manipulating the key in the primary circuit of the induction coils that a rapid stream of sparks passes between the secondary balls for a shorter or for a longer time. This gives rise to a corresponding series of electric waves, radiated from the aerial. The dash is equal in duration to about three dots, and a space equal to three dots is left between each letter, and one equal to five dots between each word. Thus, in Morse alphabet the sentence “How are you?” is written⁠—

We have, in the next place, to explain how the signals sent out are recorded.

At the receiving station is erected a second insulated aerial, antenna, or long vertical rod, A (see Fig. 83), and the lower end is connected to the earth through a coil of fine insulated wire, P, which forms one circuit of an oscillation-transformer. The secondary circuit, S, of this oscillation-transformer, which is called a jigger, is cut in the middle and has a small condenser, C₁, inserted, consisting of two sheets of tinfoil separated by waxed paper (see Fig. 83), and to the ends of this circuit is connected the coherer, or metallic filings tube, T, which acts as a sensitive receiver. The Marconi sensitive tube (see Fig. 84) is made as follows. A glass tube about ¹⁄₄ inch in diameter and 2 inches long has two silver plugs put in it, and these are soldered to two platinum wires which are sealed into the closed ends of the tube. The ends of the plugs are cut in a slanting fashion and made very smooth. These ends very nearly touch each other. A very small quantity of very fine metallic powder consisting of nineteen parts nickel and one part silver is then placed between the plugs. The quantity of this powder is scarcely more than could be taken up on the head of a large pin. The glass tube is then exhausted of its air and sealed. The tube is attached to a bone rod by means of which it is held in a clip.

To the two sides of the above-mentioned condenser are connected two wires which lead to a circuit including a single voltaic cell, V, and a relay, E. The relay is connected to another circuit which includes a battery, B, and a piece of apparatus called a Morse printer, M, for marking dots and dashes on a strip of paper.

The working details of the above rather complicated system of apparatus devised by Mr. Marconi would require for its full elucidation a large amount of explanation of a technical character. The general reader may, however, form a sufficiently clear idea of its performance as follows:⁠—

When the electrical waves from the distant transmitting station reach the aerial at the receiving station, they set up in it sympathetic electrical oscillations. The most favourable conditions are when the two aerials at the distant stations are exactly similar. These electrical oscillations, or rapid electric currents, set up an electromotive force in the secondary circuit of the oscillation-transformer, and this acts, as already explained, upon the metallic filings in the coherer-tube and causes it to become an electrical conductor. The cell attached to the relay then sends a current through the conductive circuit so formed and operates the relay. This last contrivance is merely a very delicate switch or circuit-closer which is set in action by a small current sent through one of its circuits, and it then closes a second circuit and so enables another much larger battery to send a current through the Morse printer. The printer then prints a dot upon a moving strip of paper and records a signal. One other element in this rather complicated arrangement remains to be noticed, and that is the tapper. Underneath the coherer-tube is a little hammer worked by an electro-magnet like an electric bell. This tapper is set vibrating by the same current which passes through the Morse printer, and hence almost as soon as the latter has begun to print, the sensitive tube receives a little tap which causes the metallic filings to become again a non-conductor, and so arrests the whole of the electric currency. If it were not for this tapper, the arrival of the electric wave would cause the printer to begin printing a line which would continue. The dot is, so to speak, an arrested line. If, however, trains of electric waves continue to arrive, then dots continue to be printed in close order, and form a dash on the paper strip. It will thus be seen that the whole arrangements constitute an exceedingly ingenious device of such a nature that a single touch on the hand-key at one station causing a spark or two to take place between the spark-balls makes a dot appear upon a band of paper at the distant station; whilst, if the hand-key is held down so that a stream of sparks takes place at the transmitting station, a dash is recorded at the receiving station. The means by which this distant effect is produced is the train of electric waves moving over the earth’s surface setting out from one aerial and arriving at the other.

The reader who has difficulty in following the above explanations may perhaps gather a sufficiently clear notion of the processes at work by considering a reduced, or simplified, arrangement. Imagine two long insulated rods, A, A′ (see Fig. 85), like lightning-conductors set up at distant places. Suppose each rod cut near the bottom, and let a pair of spark-balls, S, be inserted in one gap and a coherer or sensitive tube, C, in the other. At one station let an electrical machine have its positive and negative terminals connected to the two spark-balls, and at the other let a battery and electric bell be connected to the ends of the coherer. Then, as long as the coherer remains in a non-conductive condition, the electric bell does not ring. If, however, a spark is made between the balls, in virtue of all that has been explained, the reader will understand that the coherer-tube becomes at once conductive by the action of the electric wave sent out from the transmitter-rod. The battery at the receiver-rod then sends a current through the coherer, and rings the bell.

All the other complicated details of the receiver are for making the process of stopping the bell and beginning over again self-acting, and also for the production of two kinds of signals, a long and a short, by means of which an alphabet is made. In order that we may have telegraphy in any proper sense of the word, we must be able to transmit any intelligence at pleasure, and not merely one single arbitrary signal. This transmission of intelligence involves the command of an alphabet, and that in turn requires the power of production of two kinds of signals.

It remains to notice a few of the special details which characterize Mr. Marconi’s system of wireless telegraphy. In establishing wireless communication between two places, the first thing to be done is to equip them both with aerials. If one station is on land, it is usual to erect a strong mast about 150 feet high, and to the top of this is attached a sprit. From this sprit a stranded copper wire is suspended by means of an insulator of ebonite, so that the upper end of the wire is insulated. The lower end of the wire is led into a little hut or into some room near the foot of the mast in which is the receiving and transmitting apparatus.

If the apparatus is to be installed on board ship, then a similar insulated wire is suspended from a yardarm or from a sprit attached to a mast. Each station is provided with the transmitting apparatus and the receiving apparatus, and the attendant changes over the aerial from one connection to the other so as to receive or send at pleasure.

In the case of long-distance wireless telegraphy, the aerial is not a single wire, but a collection of wires, suspended so as to space them a little from each other. Thus in the case of the first experiments made by M. Marconi across the Atlantic, the aerial erected on the coast of Cornwall consisted of fifty stranded copper wires each 150 feet in length suspended in a fan-shaped fashion from a long transverse stay upheld between two masts. The wires were spaced out at the top and gathered in together at the bottom.

The question which almost immediately occurs to most people to ask is how far it is possible to prevent the electric waves emanating from one station affecting all receiving instruments alike within a certain radius. The answer to this is that considerable progress has been made in effecting what is called “tuning” the various stations. In speaking of acoustic resonance it has been pointed out that a train of air waves can set up vibration in other bodies which have the same natural period of vibration. Thus, if we open a piano so as to expose the strings, and if a singer with a strong voice sings a loud true note and then stops suddenly, it will be found that one particular string of the piano is vibrating, viz. that which would give out if struck the note which was sung, but all the rest of the strings are silent. It has been pointed out that every open electric circuit has a natural electrical time-period of vibration in which its electric charge oscillates if it is disturbed by a sudden electromotive force and then left to itself. If the two aerials at two stations are exactly alike, and if the various circuits constituting the oscillation-transformers in the transmitting and receiving appliances are all adjusted to have the same electrical period, then it is found that the stations so tuned are sympathetic at distances vastly greater than they would be if not so tuned. Hence it is possible to arrange wireless telegraph apparatus so that it is not affected by any electric waves arriving from a distance which have not a particular time-period.

Mr. Marconi has also proved that it is possible to receive on the same aerial, at the same time, two different messages on separate receiving instruments from two distant but properly tuned transmitting stations.

Since the date of these pioneer inventions many different forms of wave detector have been discovered, and wireless telegraphy has shown itself to be of the greatest utility in effecting communication between ship and ship, and ship and shore. Its value in enabling intelligence to be transmitted from lightships or lighthouses to coast stations cannot be over estimated. One very remarkable feature of the apparatus as arranged by Mr. Marconi is the small space it occupies. It is in this respect most admirably adapted for use on board ship. It only requires a long, insulated, vertical wire which can easily be suspended from a mast, and the whole receiving and transmitting apparatus can be placed on board ship in a small cabin. Employing the sensitive tube and Marconi receiving arrangements, messages can easily be sent 150 miles over the sea-surface by means of an aerial 150 feet high and a 10-inch induction coil.

It is a curious fact that better results are obtained over a water-surface than over land. Two similar stations with the same appliances can communicate at two or three times greater distance if they are separated by sea than if they are on land and have no water between. This is connected with the fact that electric waves are not able to pass through sea-water, but can diffuse through dry earth. 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. Thus an interposing rock or wall does not form a sharply marked sound-shadow, but there is some deflection of the air waves by the edge of the obstacle. The amount of bending which takes place depends on the length of the wave.

If we take two places on the sea-surface 200 miles apart, the surface of the sea at the halfway distance is just 1¹⁄₄ miles above the straight line joining the places. In other words, the rotundity of the earth interposes a mountain of water 1¹⁄₄ miles high between the places. The electric waves used in wireless telegraphy have a wave-length of about 600 to 1000 feet, or say five or six to the mile. Hence the interposition of an object, the height of which is one-fortieth of the distance, is not sufficient to make a complete electric shadow. If we were, for instance, blowing a trumpet creating air waves 5 feet long, the interposition of a cliff between two places a mile apart, but so situated that the cliff protruded to the extent of 40 yards across the line joining them, would not cut off all sound. There would be diffraction or diffusion enough of the air waves to enable the sound to be heard round the corner. In the same manner the electric waves are, so to speak, propagated round the corner of the earth. More remarkable still, they have been detected, when sufficiently powerful, at a distance of 6000 miles from the generating station, and in this case they must have travelled a quarter of the way round the earth.

A good conception of the relative speeds of water waves, air waves, and æther waves can be gained by considering the time each of these would take to cross the Atlantic Ocean, travelling in its own medium. Suppose we could, at the same moment, create a splash in the sea near England sufficiently great to cause a wave which would travel over the surface of the Atlantic at the speed of many ocean waves, say at 30 miles an hour. To cover a distance of 3000 miles this water wave would then require 100 hours. Imagine that we could, at the same moment, make a sound loud enough to be heard across the same ocean, travelling at the rate of 1100 feet a second, or about 700 miles an hour, the sound wave would cross from England to the coast of the United States in about four hours. If, however, we were to make an æther wave it would flit across the same distance in about the sixtieth part of a second.

If you have been able to follow me in these descriptions, you will see that the progress of scientific investigation has led us from simple beginnings to a wonderful conclusion. 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. We cannot feel or handle this imponderable æther, but we have indubitable proof that we can create waves in it by suddenly applying or reversing something we call electric force, just as we are able to produce air or water waves by the very sudden application of mechanical force or pressure. These æther waves, when started, not only travel through the ocean of æther with astonishing speed, but they are the means by which enormous quantities of energy are transferred through space.

From every square yard of the sun’s surface energy is cast forth at a rate equal to that produced by the combustion of eleven tons of best Welsh coal per hour, and conveyed away into surrounding space by æther ripples, to warm and light the sun’s family of planets. Every plant that grows upon the earth’s surface is nourished into maturity by the energy delivered to it in this way. Every animal that basks in the sunlight is kept warm by the impact of these æther waves upon the earth. All the coal we possess buried in the earth’s crust, and in this age of steam forming the life-blood of the world, has been manufactured originally by æther ripples beating in their millions, in long-past ages, upon the vegetation of the primeval world.

But in another way the æther serves as a vehicle of energy—in the form of an electric current. 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 without, we are merely manipulating the æther as a medium of communication, just as in speech or hearing we use the air.

We therefore find our physical investigations lead us to three great final inquiries, when we ask—What is the nature of electricity, æther, and energy? Already, it seems possible, we may obtain some clue to an answer to the first question, and find it in a study of the electrons, or tiny corpuscles which build up atoms. Concerning the structure of æther, physical investigation, which has revealed its existence, may be able to analyze a little more deeply its operations. But the question, What is Energy? seems to take us to the very confines of physical inquiry, where problems concerning the structure of the material universe seem to merge into questions concerning its origin and mystery. In its ultimate essence, energy may be incomprehensible by us, except as an exhibition of the direct operation of that which we call Mind and Will. In these final inquiries into the nature of things, the wisest of us can merely speculate, and the majority but dimly apprehend.

We must not, however, travel beyond the limits of thought proper for these elementary lectures. Their chief object has been to show you that the swiftly moving ocean waves, which dash and roll unceasingly against the coast-lines of our island home, are only instances of one form of wave-motion, of which we find other varieties in other media, giving rise to all the entrancing effects of sound and light. In these expositions we have been able to do no more than touch the fringe of a great subject. Their object will have been fulfilled if they have stimulated in you a desire to know more about these interesting things. Every star and flower, every wave or bird that hovers over it, can tell us a marvellous story, if only we have eyes to see, and ears to hear. We may find in the commonest of surrounding things a limitless opportunity for intelligent study and delight. When, therefore, you next sail your boat upon a pond, or watch ducks or swans swimming, or throw stones into a pool, or visit the seaside, may I hope that some of the matters here discussed will recur to your minds, and that you will find a fresh meaning and new interest in these everyday objects. Yon may thus, perhaps, receive an impulse attracting you to the study of some chapters in the “Fairy Tale of Science,” more wonderful than any romance woven by the imaginations of men, and open to yourselves a source of elevating pleasure, which time will neither diminish nor destroy.