our task is completed. On this point we find some recent remarks of Prof. Silvanus Thompson so appropriate that we quote them in full, as being more authoritative than anything we could ourselves say. Prof. Thompson has thoroughly studied the subject, and therefore "speaks by the card." "It has been shown," he says, "that there are three general methods of transmitting electric signals across space. All of them require base lines or base areas. The firstconduction requires moist earth or water as a medium, and is for distances under three miles the most effective of the three. The second-induction-is not dependent upon earth or water, but will equally well cross air or dry rock. The third-electric wave propagation-requires no medium beyond that of the ether of space, but is interfered with by interposed things such as masts or trees. Given proper base lines or base areas, given adequate methods of throwing electric energy into the transmitting system, and sufficiently sensitive instruments to pick up and translate the signals, it is possible, in my opinion, so to develop each of the three methods that by any one of them it will be possible to establish electric communication between England and America across the intervening space. It is certainly possible, either by conduction or by induction; whether by waves I am somewhat less certain. Conduction might very seriously interfere with other electric agencies, since the waste currents in the neighbourhood of the primary base line would be very great. It is certainly possible either by conduction or induction to establish direct communication across space with either the Cape, or India, or Australia (under the same assumptions as before), and at a far less cost than that of a connecting submarine cable. "Instruments which operate by means of alternating currents of high frequency, like Mr Langdon-Davies's phonophore, are peculiarly liable to set up disturbance in other circuits. A single phonophore circuit can be heard in lines a hundred miles away. When this first came to my notice it impressed me greatly, and coupled in my mind with the Ferranti incident mentioned above" (see note, p. 144, supra), "caused me to offer to one of my financial friends in the City, some eight years ago, to undertake seriously to establish telegraphic communication with the Cape, provided £10,000 were forthcoming to establish the necessary basal circuits in the two countries, and the instruments for creating the currents. My offer was deemed too visionary for acceptance. The thing, however, is quite feasible. The one necessary thing is the adequate base line or area. All the rest is detail." 1 One word more. A press telegram of April 12, 1899, says: "The Wireless Telegraph Company have been approached by the representative of a proposed syndicate which desires to acquire the sole rights of establishing wireless telegraphic communication between England and America. The directors of the Company will consider the matter at their first meeting, which is fixed for an early date." 2 Thus I end my task as I began it, with a dream-the self-same dream! As to its realisation in the distant future who can say nay? "There are more things in heaven and earth, Horatio, 1 'Journal, Society of Arts,' April 1, 1898. 2 The syndicate must hurry up, as Mr Nikola Tesla is now on their track with a wireless telegraph that will " stagger humanity." We read (Electrician,' January 19, 1900) that he is convinced he will soon be able to communicate, not only with Paris, but with every city in the world, and that at a speed of from 1500 to 2000 words per minute! See also p. 239, supra, for Prof. Fessenden's great hopes. 262 APPENDIX A. THE RELATION BETWEEN ELECTRICITY AND LIGHT Before Hertz. SUBSTANCE of a lecture by Prof. Oliver Lodge, London Institution, December 16, 1880.1 Ever since the subject on which I have to speak to-night was arranged, I have been astonished at my own audacity in proposing to deal, in the course of sixty minutes, with a subject so gigantic and so profound that a course of sixty lectures would be inadequate for its thorough and exhaustive treatment. I must, therefore, confine myself to some few of the most salient points in the relation between electricity and light, and I must economise time by plunging at once into the middle of the matter without further preliminary. What is electricity? We do not know. We cannot assert that it is a form of matter; neither can we deny it. On the other hand, we cannot certainly assert that it is a form of energy; and I should be disposed to deny it. It may be that electricity is an entity per se, just as matter is an entity per se. Nevertheless, I can tell you what I mean by electricity by appealing to its known behaviour. Here is a voltaic battery. I want you to regard it, and all electrical machines and batteries, as kinds of electricitypumps, which drive the electricity along through the wire very much as a water-pump can drive water along pipes. 1 Based on a report in 'Design and Work,' February 5, 1881. While this is going on, the wire manifests a whole series of properties, which are called the properties of the current. [Here were shown an ignited platinum wire, the electric arc between two carbons, an electric machine spark, an induction coil spark, and a vacuum tube glow. Also a large nail was magnetised by being wrapped in the current, and two helices were suspended and seen to direct and attract each other.] To make a magnet, then, we only need a current of electricity flowing round and round in a whirl. A vortex or whirlpool of electricity is in fact a magnet, and vice versû. And these whirls have the power of directing and attracting other previously existing whirls according to certain laws, called the laws of magnetism. And, moreover, they have the power of exciting fresh whirls in neighbouring conductors, and of repelling them according to the laws of diamagnetism. The theory of the actions is known, though the nature of the whirls, as of the simple streams of electricity, is at present unknown. [Here was shown a large electro-magnet and an inductioncoil vacuum discharge spinning round and round when placed in its field.] So much for what happens when electricity is made to travel along conductors-i.e., when it travels along like a stream of water in a pipe, or spins round and round like a whirlpool. But there is another set of phenomena, usually regarded as distinct and of another order, but which are not so distinct as they appear, which manifest themselves when you join the pump to a piece of glass or any non-conductor and try to force the electricity through that. You succeed in driving some through, but the flow is no longer like that of water in an open pipe; it is as if the pipe were completely obstructed by a number of elastic partitions or diaphragms. The water cannot move without straining and bending these diaphragms, and if you allow it, these strained partitions will recover themselves and drive the water back again. [Here was explained the process of charging a Leyden jar.] The essential thing to remember is that we may have electrical energy in two forms, the static and the kinetic; and it is therefore also possible to have the rapid alternation from one of these forms to the other, called vibration. Now we will pass to the second question: What do you mean by light? And the first and obvious answer is, Every-. body knows. And everybody that is not blind does know to a certain extent. We have a special sense-organ for appreciating light, whereas we have none for electricity. Nevertheless, we must admit that we really know very little about the intimate nature of light-very little more than about electricity. But we do know this, that light is a form of energy; and, moreover, that it is energy rapidly alternating between the static and the kinetic forms-that it is, in fact, a special kind of energy of vibration. We are absolutely certain that light is a periodic disturbance in some medium, periodic both in space and time-that is to say, the same appearances regularly recur at certain equal intervals of distance at the same time, and also present themselves at equal intervals of time at the same place; that, in fact, it belongs to the class of motions called by mathematicians undulatory or wave motions. Now how much connection between electricity and light have we perceived in this glance into their natures? Not much truly. It amounts to about this: That on the one hand electrical energy may exist in either of two forms— the static form, when insulators are electrically strained by having had electricity driven partially through them (as in the Leyden jar), which strain is a form of energy, because of the tendency to discharge and do work; and the kinetic form, where electricity is moving bodily along through conductors, or whirling round and round inside them, which motion of electricity is a form of energy, because the conductors and whirls can attract or repel each other and thereby do work. On the other hand, light is the rapid alternation of energy from one of these forms to the other-the static form where the medium is strained, to the kinetic form when it moves. It is just conceivable then that the static form of the energy of light is electro-static—that is, that the medium is electrically strained-and that the kinetic form of the energy of light is electro-kinetic—that is, that the motion is not ordinary motion, |