It has been remarked for some time since | cially light, it might be best explained by a change in the position of the earth's axis; but such a change was also consid. ered until now as highly improbable. that Pulkova and Berlin change from year to year their geographical position. Their latitudes decrease; every year the two observatories seem to move away from the North Pole by a few inches; and as they do not move in reality, there is no alternative but to conclude (after having tried all possible explanations) that the North Pole itself changes its position, although such a movement had been hitherto considered as most improbable by all scientists. We all know were it only from observations upon a spinning-top-that if a solid body is rotating, its axis may change its position in space, but that relatively to the rotating body itself it remains unchanged. A spinning-top may incline towards the floor, and its axis of rotation may describe a conical surface, but it does not alter its position within the top; each of the particles of the top describes the same circle round the same spot of the axis. The same was considered to be true as regards the earth. Its axis of rotation slowly changes its position in space; but within the earth itself, we were told, it remains unaltered. So that if two Arctic travellers attained the North and the South Poles, and erected two cairns upon these spots, the cairns would always represent the position of the axis of rotation of the earth. And yet recent observations tend to overthrow this view; we learn that the cairns must continually be shifted in order to represent the true position of the Poles. The importance of this discovery for the physical geographer is self-evident. The geologist has no means to explain by terrestrial causes alone two great geolog. ical facts of primary importance; the glaciation of the earth, and the extension, during the Tertiary epoch, of a very rich flowering and fruit-bearing vegetation, now characteristic of southern Europe, over a wide continent which embraced Greenland, Spitzbergen, the Arctic islands of Siberia, and North America. If the simultaneous glaciations of both hemispheres be proved - and some specialists are of this opinion, while those who oppose it will confess that the whole question has not been studied sufficiently it could not be explained by astronomical hypotheses implying the alternate glaciation of the two hemispheres. Nothing short of a decrease in the amount of heat received from the sun would give the explanation; but few astronomers would be prepared to make such an admission. As to the prevalence of a rich flora in Arctic regions which receive but a limited amount of heat, and espe Schiaparelli, the great Italian astronomer, fully grasped these weighty considerations, and they induced him to revise, a few years ago, the whole question as to the supposed invariability of the axis of rotation of the earth. He calculated the effects which slight displacements of matter on the earth's surface might have upon the position of the axis, and he demonstrated by mathematical analysis that slight but prolonged geological changes "may give origin to great displacements of the poles of rotation, provided the earth's spheroid is not of absolute rigidity." The same position was taken by George C. Comstock,† who examined the available and sufficiently reliable determinations of latitudes at several observatories, and concluded that they give some support to the hypothesis of a secular shifting of the axis of the earth. Thus, the latitude of Greenwich has pretty regularly decreased from 51° 28′ 38′′:59 in 1826 to 51° 28′ 37"95 in 1889. The Pulkova observations (especially reliable for this subject) show a decrease of latitude of o"33 during the years 1843 to 1882, which (taking into ac count the probable errors) corresponds to a shifting of nearly six inches every year (o".005). Another quite independent Pulkova series gives much the same result. Königsberg moves away from the Pole by o"003 every year, while Washburn, in Wisconsin, approaches the Pole by o"043 in the twelve months. The four would well agree together if the Pole were shifting every year by over four feet (o"044) along the meridian of 69° west of Greenwich. Several other observations (Cambridge, Prague, Potsdam) also speak in favor of a shifting of the Pole. The whole question is so important that the Geodetical Association decided, at the end of 1890, to send an astronomical expedition to Honolulu (189° east of Berlin), in order to make there consecutive deter minations of latitudes which might be compared with those of Pulkova and Berlin. The expedition began its observations in June last, and the measurements of the first three months, now fully computed, prove that the changes were en light or radiant heat is transmitted through the interstellar space, or through the vacuum obtained in a glass tube - that is, through space in which we detect almost no traces of ponderous matter (matter acted upon by gravitation) - we explain the transmission of the luminous and heat tirely accordant in magnitude with the European ones, but, as foreseen, they were in the opposite direction. However, a new explanation has been proposed in the mean time by S. S. Chandler, namely, that the variation is merely periodical, and will be completed in fourteen months. Fourteen months hence the axis will re-energy by making a plausible supposition; turn to its present position. But this we assume that besides the matter which explanation does not account for the constitutes the solid, liquid, and gaseous above-mentioned secular variations, so bodies, there is some other matter, or that we must wait now for further observa- rather some other still more attenuated tions. One thing is, however, certain: condition of matter, inseparable from the the axis of the earth is not so immutable former, which we call ether; and we asas it was supposed to be, and it is possible sume that the displacements of the partithat the study now being pursued by Mr. cles of ether (vibrations, or, maybe, other Lockyer of old Egyptian monuments, changes of state) are the medium for the which used to be astronomical observa- transmission of luminous and heat energy. tories as well, may give some indications It was quite natural, therefore, to suppose as to the changes of latitude since that and it was supposed that the transremote period. III. THE interest awakened some three years ago by the novel and startling experiments in electricity made by the Karlsruhe Professor Hertz is still maintained. They not only confirmed the long since suspected connection between electricity, magnetism, light, and radiant heat; they also gave a new impulse to speculations as to the structure of matter altogether, and the modes of transmission of energy. Numerous works on these subjects, all more or less connected with the Karlsruhe researches, are continually appearing, and in order to appreciate them we are bound to revert to the starting-point-Hertz's experiments themselves. The best means for mastering a new branch of science, it has been remarked, is to study it in its nascent state. When a moving body - say, a billiard ball strikes another body at rest, and, imparting to it part of its energy, sets it in motion; when the waves, originated on the surface of a pond by a falling stone, spread in wider and wider circles, and finally begin to rock a piece of wood that was quietly floating in a corner of the pond; or when a tuning-fork communicates its vibrations to another fork at a certain distance - we may not be able to trace all the complicated movements of the two balls, the water of the pond, and the air; but our mind is satisfied to some extent as to the manner of transmission of energy from one ball to the other, from the stone to the piece of wood, and from the sounding fork to the other fork. Again, when Astronomical Journal, Nos. 248-251; American Journal of Science, February, 1892. mission of electro-magnetic disturbances is effected in the same way; that they also produce vibrations, or some other changes in the usual conditions of the particles of the surrounding ether; and that these changes, or vibrations, are transmitted in all directions from one particle of ether to the next, at some measurable speed - the speed of transmission probably being not much different from the speed of transmission of light and radiant heat, which is about one hundred and eighty thousand miles in a second. However probable this hypothesis, physicists had hitherto failed to confirm it. Maxwell advocated it chiefly on theoretical grounds, but decisive experiments were wanted; and although Siemens had once measured the speed of transmission of electricity, and found it not very differ. ent from that of light,* his measurements were still considered as uncertain. Now came Hertz with his ingenious experi ments. He applied a method which had proved most successful in studying sound. When a tuning-fork is set vibrating, its vibrations alternately condense and rarefy the surrounding air, and both rarefactions and condensations are transmitted by the air in all directions; we may call them, by analogy, waves. Now, if these waves meet anywhere a reflecting board, they are sent back, in the same way as the waves of the sea are reflected by the wall of a quay. But they may be sent back so that each reflected condensation meets on its back journey with a new condensation coming from the fork, and in that case the sound is reinforced; or, each reflected Two hundred thousand to two hundred and sixty thousand kilomètres in a second; the velocity of light being about three hundred thousand kilomètres. three instruments the vibrator, the screen, and the detector- the experi ments could be carried on, and they proved at once the close connection existing between the phenomena of electricity and light. condensation meets with a rarefaction, and in this case both actions neutralize each other the sound is weakened. So that, if we slowly approach our reflecting board to the fork, there will be places where the board reinforces the sound (condensations meeting with condensations), then weak- As soon as sparking began in the ens it, and then makes it louder again, vibrator, and the detector was approached although the board is moved all the time to it, sparks began to jerk between the in one direction, towards the tuning-fork. knobs of the latter; but they disappeared Of course, things are not so easy with as soon as the screen was interposed beelectricity. There is no great difficulty tween the two-the "waves being interin producing alternate electrifications of rupted in this case. On the contrary, the surrounding ether which would corre- when the screen was placed immediately spond to the alternate condensations of behind the detector, strong sparking fol. the air, but they must follow each other lowed; if it was removed about eighteen with a tremendous rapidity. In fact, if feet, the sparking ceased; the direct and the tuning-fork makes, say, one thousand the reflected waves extinguishing each vibrations in the second-the speed of other; but when the screen was moved sound in dry air being but eleven hundred away for another eighteen feet, sparking feet in the same time-a condensation reappeared the two waves reinforcing will only have travelled a little over one each other, and so on. In short, the phefoot before a new condensation follows it. nomena were exactly like those which The "waves " of sound will be II foot would be noticed if a tuning-fork, a relong. But if our electrical discharges flecting board, and a resonator were used. also succeeded each other with a fre- It was thus proved that each electrical quency of no more than one thousand dis-discharge produces some disturbance in charges in a second, the electric wave the surrounding space; that the disturb. (supposing that it spreads at the rate of ance is transmitted, through the "nonone hundred and eighty thousand miles in conductive" air, exactly as luminous or a second, like light) would have travelled sound vibrations are transmitted; and that one hundred and eighty miles before a electricity is propagated, like heat and new wave would be originated by the next light, at some finite and measurable speed. discharge. And waves of that length are Of course it would not be possible to give not easy to deal with. So that, in order here the tedious processes by which the to obtain waves of a reasonable length measurements were made, nor to tell the following each other at a distance of, say, difficulties, the doubts, and the seemingly thirty-five or forty feet- Hertz had to contradictory facts which were met with produce discharges alternating thirty mil-in the way; although dating from yester lion times in a second. So he did. He day, "Hertz's experiments" have already obtained such rapid discharges for very a whole history. Suffice it to say, that the short intervals of time, and thus he could measure the distances at which the electrical "waves" followed each other. A reflecting board, and some means for detecting the "loops and nodes," i.e., the places where the waves reinforce or extinguish each other, were the next requisites. A reflecting board was readily made out of a sheet of zinc, ten to twelve feet 'square. As to the "detector," Hertz chose, out of the various means at his disposal, a brass wire, provided with two knobs and bent into a ring, which could give sparks when it received electrical waves of a certain length. With these Thirty million times thirty-five feet would make one hundred and eighty thousand miles. To attain a very rapid succession of alternate electrifications, Hertz used two brass plates, twelve inches square, to each of which was attached a thick wire, about two inches long, terminated by a brass knob. The distance between the two knobs was very small-less velocity of electricity, both in the air and the conductive wires, proved to be very near to that of light, namely, about one hundred and eighty thousand miles in a second. than one-tenth of an inch. When the plates were electrified by an induction coil, a series of sparks jerked from one knob to the other, the charge rapidly passing forwards and backwards, and giving very rapid alternative discharges. This was the "vibrator." As to the "detector," or resonator," it consisted of a thick wire, the two ends of which were provided with brass knobs, and the length of which was taken so as to suit the oscillations in the vibrators. The wire being bent into a circle, its two knobs were brought very near to each other, so as to show sparks at the reception of the feeblest electric waves (Sitzungsberichte der Berliner Acad. der Wissenschaften, February 9, 1888). It hardly needs adding that during the experiments the reflecting board, or the apparatus used instead, remained stationary, and that the resonator was moved instead of it. For more details see an excellent résumé in the last chapter of Th. Preston's "Theory of Light," London, 1890. The general reader may consult the very good papers in Nature, March 5 and 14, 1890. with them. First of all, it was necessary to verify the experiments; and so they were verified by several physicists-in this country by Professor Fitzgerald and Fr. Trutton at Dublin, and by Professor Lodge and Mr. Dragoumis at Liverpool.† In fact, Professor Lodge had nearly discovered the same phenomena simultaneously with Hertz, as he was making in 1887 and 1888 his experiments on the rapid discharges obtained from Leyden jars.‡ Blondlot, in France, slightly modifying the primitive experiments, finally settled the velocity of electricity in the air at from two hundred and ninety-one thousand to three hundred and four thousand kilomètres in the second, thus very nearly approaching to the velocities of light.§ Then, Hertz himself having been brought by his earlier measurements to admit that the speed of the electrical disturbances is much smaller in wires than in the surrounding air, more careful measurements were required, and they were made in Geneva and in Germany, and proved that the velocity, as foreseen by theory, is equal in both cases.|| This may be considered as the first part | and, in fact, nearly all that is now written of the experiments. The second part is upon electricity is in some way connected even more interesting, as it disclosed further analogies between electro-magnetism and light. Light is transmitted by some bodies, and is reflected by other bodies. Electro-magnetic waves behave in the same way; a plate of zinc acts upon them as a mirror and sends them back, but they pass through a wooden door just as light passes through a window plate. Hertz could send them into the next room through a shut door. If we put a red-hot iron ball in the focus of a parabolic mirror, we may make it light a match adjusted in the focus of another parabolic mirror which is placed at the other end of a room. Electricity behaves in the same way; we can send beams of electrical oscillations by means of a parabolic mirror, and intercept them at a distance by another mirror and send them into its focus. If we interrupt the initial discharges in a certain way as they are interrupted in the Morse alphabet - we shall transmit electrical signals and have a telegraph without connecting wires. Light is refracted by transparent bodies if they have the shape of a prism or a lens; and by means of a big prism of pitch Hertz refracted the electro-magnetic "rays;" he could bend them, and send them under a right angle into another room. Reflected light can be polarized, and electro-magnetic "rays" are polarized, too. In short, Maxwell's hypothesis as to the identity of light and electricity is fully confirmed. Both are disturbances (vibrations, or whatever they might be) in the usual state of ether which are transmitted like all other kinds of energy-like the energy of the billiard ball, the stone, and the tuning-fork, of which we spoke at the beginning of this chapter, that is, from one particle to the next. So we finally part with the mysterious "electric fluid" just as we parted, thirty years ago, with the "caloric fluid," and we simply have before us a separate mode of energy. When the waves of ether have lengths of from '000012 to 000016 parts of an inch, we have chemical energy; when they follow each other at distances of from 000016 to 00003 parts of the inch, our eye sees them as light; when they grew to 00012 parts of the inch, we see them no more, but we feel them as radiant heat; and when they attain lengths which are measured by yards and miles, they give the electrical phenomena. A wide series of researches was evidently called into life by these researches, Another important matter was to study the magnetic part of the same electric disturbances. In Maxwell's theory the magnetic disturbances ought to be nothing but transversal rotations of the particles of ether in a plane perpendicular to the line of transmission of light and electricity "molecular vortices,' as he used to say. And Hertz succeeded in proving by a new series of experiments—or, at least, in rendering it most probable - that the magnetic force obeys in its transmis sion the same laws as electricity, but that the direction of its vibrations is perpendicular to the line of transmission of the electric waves; and he made at the same time an attempt at measuring the mechan ical effects of the electric disturbances.** • Nature, vol. xxxix., p. 391, vol. xli., p. 295. Prof. Lodge writes, in the Proceedings of the Royal Society (vol. l., No. 302, August 28, 1891): "This same discovery (Hertz's) would have been made by the audience at the Royal Institution on the evening of March 8, 1889, if it had not been made before; for, during a lecture on Leyden jars, every time one was discharged through a considerable length of wire, the heavily gilt wall paper sparkled brightly by reason of the incident radiation. $ Comptes Rendus, 1891, t. 112, p, 1058; t. 113, p. 628. Sarasin et L. de la Rive in Comptes Rendus, 1891, t. 112, Nos. 12 et 13; Rubens and Ritter in Wiede mann's Annalen der Physik, 1890, vol. xl. "Ueber die mechanischen Wirkungen electrischer regards the very structure of matter, and some others opening new fields for experimental work, like J. J. Thomson's researches into the speed of propagation of the luminous discharge of electricity through a rarefied gas,† and Hertz's new experiments upon the transmission of the same discharges through various screens, transparent or not for light might be mentioned in connection with the above. But we must say, at least, a few words about the quite new lines of research indicated by Mr. Crookes's experiments on what he names "electrical evaporation." It was already known that an induction current, when passing through the platinum electrodes of a vacuum tube, tears off the molecules of platinum from the sphere of attraction of the wire, and transports them to a certain distance. Now, Mr. Crookes, comparing these phenomena with those of evaporation of liquids, made various experiments in order to determine the "evaporating power of the electric At the same time a further confirmation | mathematical and highly suggestive as of the light theory of electricity was given by Arons and Rubens, who proved that the relation which, according to Maxwell, ought to exist between the isolating power of various substances and their powers of refracting the rays of light, exists in reality. The resistance offered to the passage of light and that offered to the passage of electricity are connected by a simple relation. On the other side, Sir William Thomson read before the Royal Society a most interesting paper on the screens, and their efficiency against waves of different lengths. He demonstrated that if the electric sparks have a frequency of four or five per second, a clean white paper screen is sufficient to stop them; but when the frequency of the sparks is fifty, or more, the white paper screen makes no perceptible difference. If the paper is thoroughly blackened with ink on both sides, some moderate frequency of a few hundreds per second is, no doubt, sufficient to practically annul the effect of the interposition of the screen. For discharges following each other with frequencies up to one thousand millions in a second, a screen of blackened paper is perfectly transparent," but if we raise the frequency to five hundred million millions, the influence to be transmitted is light, and the blackened paper becomes an almost perfect screen."t As to the wonderful electrical effects produced by means of currents alternating with very high frequency, such as they are produced by the Montenegrin professor, Nikola Tesla, the readers of this review have already been familiarized with them in a preceding number (LIVING AGE, No. 2496, p. 309). Many more researches - some of them Drahtwellen," in Wiedemann's Annalen der Physik, 1891, vol. xlii., p. 405. Ritter and Rubens in same periodical, vol. xl. 1890. MM. Sarasin and De la Rive having come to the conclusion that the vibrators send out a great number of undulations of various periods, new researches were undertaken by Bjerkness (Archives des Sciences physiques et naturelles, 1891, t. 27, p. 229), and they brought to light the so-called "dampening" of electrical undulations - a question which also was discussed mathematically by Poincaré (Archives, t. 25, p. 609), and Perot (Comptes Rendus. January 25, 1892). - offer a All the gases, many liquids, and many solids (glass, sage of electricity, and the relative amounts of this stress under different circumstances and with different substances. He caused water to be transported in this way by the electric current; in order to increase the power of electricity upon metals, he diminished the cohesion of their molecules by heating the metals; and he studied also the relations between the transport of the molecules by electric stress, and the phenomena of phosphorescence.§ One feels, especially when remembering the speculations of the first half of this century (chiefly those of Séguin), that a new and most promising field is opened by these researches; they raise a host of questions relative to the most difficult parts of molecular mechanics. The same must be said as regards modern research in chemistry. The work now done is of two different kinds. While a numerous army of laboratory workers accumulate heaps and heaps of minute facts, and study the properties of separate chem ical compounds without being guided by any general idea, a few chemists devote themselves to the most intricate questions reactions and molecular structure. They relative to the very substance of chemical endeavor to bridge over the gulf between molecular physics and chemistry, and to * On some Test Cases for the Maxwell-Boltzmann Doctrine regarding Distribution of Energy, by Sir William Thomson, in Proceedings of the Royal Society, vol. 1., No. 302, p. 79. ↑ Philosophical Magazine, 1890, vol. xxix.; Proceedings of the Royal Society, January 15, 1891. Annalen der Physik, 1892, Bd. 45, p. 28. § Proceedings of the Royal Society, vol. 1., p. 87. |