From The Nineteenth Century. I. A BREATH of youthful energy and youthful hopes inspires modern astronomical work. "Astronomy, the oldest of the sciences, has more than renewed her youth," as William Huggins said at the end of the inaugural address he delivered before the last meeting of the British Association. Since the spectroscope, formerly used but to study and reveal the chemical composition of the celestial bodies, has become an instrument for measuring their unseen movements and for penetrating into the secrets of their history, and since photography has been taken as a necessary auxiliary by astron omers, a new chapter of astro-physics has been opened. The proper movements of the stars have acquired a new meaning; of the group, and on examining the whole, one cannot refrain from concluding that the stars are simply spots upon which the and condensed to make new suns. The diffuse nebulous matter has agglomerated same is also seen in the photographs of the nebulæ in Orion - the more so as the spectroscope reveals the unity of compowhich surround them and link them together. sition of both the stars and the nebulæ Still more interesting results have been obtained by H. C. Russell with his photographs of nebulæ in the constellation of Argus. His earlier photographs, obtained been referred to with admiration by Wilby a three-hours' exposure, have already liam Huggins in his address. But when the photographic film was exposed for eight hours to the faint light of the nebula, not only shows that the nebulous matter new facts were revealed. The photograph the faint masses of nebulous matter, scatextends far beyond the limits assigned to tered round and amidst the stars, have become animated indications of the gene-servations at the Cape, while confirming it by Herschel during his memorable obsis of solar systems; and the great prob- at the same time the great accuracy of the lems relative to the life of the stellar worlds—their origin, their growth, their description of what he did see; it also decay, and their rejuvenescence have come again to the front, supported by renewed hopes as to the proximity of their ultimate solution. It is not possible, indeed, to examine the splendid photographs, made by Mr. Roberts, of the nebula in Andromeda, and to see this whirlpool of luminous matter, divided into dark and bright rings surrounding a large, undefined central mass, without perceiving in it a gigantic solar system in the way of formation, and without concluding in favor of a similar origin, on a much smaller scale, of our own solar system. The best drawings of the same nebula, which were made by Bond and John Herschel with the aid of the best telescopes, told nothing of the kind; the complicated structure of the nebula, its life, were missing in what was reproduced by the pen of a cautious observer. Again, in another part of the sky- the the photographs of the Brothers Henry show at once that this cluster of suns is not an occasional gathering. Streaks of nebulous matter, revealed by photography, connect together the stars proves that the nebula has lived since 1837, and has altered considerably its aspect during the last fifty years. At the very same place where Herschel saw one of its brightest and most conspicuous parts, we have now a dark oval space, upon which no trace of luminous matter can be detected. The matter either has been drawn elsewhere, or is luminous no more; may be, it is passing through some stage preparatory to the appearance of a new star. We are thus convinced that these tic their dimensions, are living at a much accumulations of matter, however giganmore rapid speed than we were prepared to admit. Changes occur in them, even within the short limits of one man's life; and as the new star in Auriga, rapidly passing through a series of transformabirth of new suns, so also we may hope tions, reveals to us the secrets of the that the study of the modifications of the nebula will initiate us into the secrets of the earlier stages of development of the stellar worlds. In the movements of those • See an article by Mr. Norman Lockyer in LIVING AGE, No. 2497, p. 323. remote agglomerations we learn to feel | The spectra of the stars, the nebulæ, the the continuous life of nature, its continu- corona, and the protuberances of the sun, ous change, its evolution. are now photographed; and by this means When the great photographic map of the powers of the astronomer are considerthe whole sky is ready, many a change in ably extended. He can study the specthe stellar worlds and nebulæ which es- trum in its ultra-violet part, which is not, capes now our attention will be recorded visible to the eye, as it hardly acts upon forever. The preparatory work is already our retina, while its chemical rays act completed; the instruments are chosen, very well upon the photographic sensitive and the uniformity of methods is secured. plate; he obtains greater enlargements of The sky is apportioned between the eigh- the spectrum, and he can study the spectra teen observatories which will perform the at his leisure and measure the positions of whole of this immense work, each of them the bright or dark lines which intersect having to make from one thousand to fif-them- the more so as the spectrum of teen hundred separate photographs in some well-known body (incandescent hy order to map all stars down to the sixteenth drogen or iron) is photographed on the magnitude; and the first specimens already same plate for the sake of comparison. published satisfy the most severe exigen. This method has already given some excies of the astronomers. Many new facts cellent results. It has permitted us to are sure to be revealed by this grand sur-measure the movements of the stars in vey of the sky, because even now, when a the line of vision with a quite unexpected simple preliminary exploration is being accuracy. The proper movements of the made, we can already mention some dis- stars offer an immense interest; but while coveries due to photography. Thus, when we always could ascertain their movethe amateur astronomer, Dr. Anderson ments north and south, or west and east, (equipped with but a small pocket tele- on the celestial sphere, we formerly had scope and the little atlas of the sky by no means of telling whether a star is apKlein), discovered on the 31st of January proaching us, or going away, during its the new star in Auriga, it appeared that displacements in space. The spectrothe newcomer had already been photo-scope gives those means. graphed without astronomers being aware of the fact. Professor Pickering found its portrait on photographs taken on three different occasions since the Ist of December, and the indefatigable Heidelberg astronomer, Max Wolf, also had it on his photographs since the 8th of the same month. The appearance of the new star thus would have been recorded, even if nobody had remarked its appearance. Another photographic discovery is due to the same Max Wolf. Having photo-left. William Huggins long ago explained graphed one part of the sky on two consecutive nights in December, he sent his negatives to Dr. Berberich, who at once noticed that two minute spots had changed their positions in the twenty-four hours. One of them proved to be a new addition to the list of minor planets, while the other was a previously known small planet of the same group. The spectrum of a star usually consists of a band of faint light, intersected by several bright (or dark) lines, corresponding to the lines appearing in the spectra of hydrogen, calcium, iron, magnesium, natrium, and so on. But if we reproduce under the spectrum of the star the spectrum of, say, hydrogen, we often see that the hydrogen lines in the former do not quite coincide with the same lines of the latter; they are slightly displaced to the right or to the that this displacement is due to the proper movements of the stars and gives a means of measuring them, and Mr. Christie even measured in this way, several years ago, the otherwise invisible movements of several stars. In fact, the blue and violet light of the spectrum is due to very quick, luminous vibrations, while its red light is due to much slower vibrations, just as the high pitch of a sound depends on much quicker vibrations of the air than the low pitch. But if a star approaches us with a the Earth at a speed of 7.4 miles in a second; and when we determine the same speed with the aid of the spectroscope, we find 7.8 miles. The spectroscope errs by but four-tenths of a mile- by less than seven hundred yards!* great rapidity, our eye will receive from it | tance. We may calculate beforehand tha more vibrations in a second, and its light at a given moment Venus will approach will appear bluer, so to say; in other words, its spectral bright lines will be shifted towards the blue end of its spectrum; and they will be shifted towards the red end if the star goes away with the same rapidity. In our century of railways many of us must have witnessed an analogous fact when looking at an express train passing by a station. When the rapidly running engine sounds its whistle, the pitch of the whistle seems to become higher as the train approaches us, and it seems to become lower when it goes away - the ear receiving in a second of time more and more vibrations in the former case, and less vibrations in the second case. So it is also with the stars, and the advantages of having the spectrum of the star and the comparison spectrum photographed on the same plate are self-evident. If we examine, for instance, the photographed spectra of Sirius we see that their hydrogen lines are always shifted towards the blue end of the spectrum, and from this we may safely conclude that the star is approaching us. And if we calculate the speed of its approach, we find it (after having taken into account the movement of the earth in its orbit) to be about seven miles in a second. The measurements may be made at different observatories and at different seasons of the year; the final results will not differ from each other by more than one mile, or even a fraction of a mile. We do not know the immense distance which separates us from Sirius, we only gauge it by saying that its light takes nearly sixteen and a half years to reach us; but a change of seven miles per second in that enormous distance is revealed by the spectrum. These results seem almost incredible, and they could not be relied upon had they not been submitted to severe tests. Thus we know the movements of the earth in its orbit, and we conclude that they must be reflected in our measurements, if these measurements are sufficiently accurate; and they are reflected with perfect accuracy. Again, we know the distance which separates us from Venus, and how the movements of both the Earth and Venus affect this dis We may thus place full confidence in our new auxiliaries. When Mrs. Flemming and Miss Maury, on examining the spectrum of 6 Lyræ, remarked that it consists in reality of two spectra periodically superposed, and Professor Pickering concluded therefrom that the star must consist of two luminous bodies which rotate around a common centre of gravity at a very great speed,† or when we are told that the new Auriga star consists of at least three separate agglomerations of incandescent gases, we can safely rely upon these conclusions. And, finally, the spectroscope, combined with photography, enables us to explore the ultra-violet part of the spectrum quite invisible to the eye. By using this method, Hale at Chicago, and Deslandres at Paris, obtain day by day the positions of those solar emissions of incandescent gas, or protuberances, which consist chiefly of incandescent hydrogen, and the light of which is so feeble that they escape observation, even during the eclipses of the sun, when its light is screened by the moon. The movements of these invisible clouds are now studied like the movements of our own atmosphere, and we learn that the laws of cyclonic storms which prevail on the earth hold good for the hot vapors of hydrogen and calcium on the surface of the sun. The unity of Nature and her laws thus receives a further brilliant confirmation. II. ANOTHER question which, although it has a direct bearing upon our own terrestrial affairs, preoccupies astronomers considerably, is the variation of latitudes. • Prof. Vogel at the Astronomical Society (Observa tory, January, 1892). ↑ Observatory, October, 1891. Comptes Rendus de l'Académie des Sciences, 1891, t. 113, p. 307. 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 considered 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 studjed 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 account 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 (0"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 determinations 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 * Annales of the Pulkova Observatory, 50th anni versary volume, St. Petersburg; 1889; Italian text in Il Nuova Cimento. † Pisa, October, 1891, fasc. 7 and 8; American Fournal of Science, December, 1891. tirely accordant in magnitude with the light or radiant heat is transmitted through European ones, but, as foreseen, they the interstellar space, or through the were in the opposite direction. However, vacuum obtained in a glass tube — that is, a new explanation has been proposed in through space in which we detect almost the mean time by S. S. Chandler, namely, no traces of ponderous matter (matter that the variation is merely periodical, and acted upon by gravitation)- we explain will be completed in fourteen months. the transmission of the luminous and heat Fourteen months hence the axis will re-energy by making a plausible supposition; turn to its present position. But this explanation does not account for the above-mentioned secular variations, so that we must wait now for further observations. One thing is, however, certain: the axis of the earth is not so immutable as it was supposed to be, and it is possible that the study now being pursued by Mr. Lockyer of old Egyptian monuments, which used to be astronomical observatories as well, may give some indications as to the changes of latitude since that remote period. III. we assume that besides the matter which constitutes the solid, liquid, and gaseous bodies, there is some other matter, or rather some other still more attenuated condition of matter, inseparable from the former, which we call ether; and we assume that the displacements of the particles of ether (vibrations, or, maybe, other changes of state) are the medium for the transmission of luminous and heat energy. It was quite natural, therefore, to suppose -and it was supposed that the transmission 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. 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. 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 different from that of light,* his measurements were still considered as uncertain. Now came Hertz with his ingenious experiments. 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 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 sound-sound is reinforced; or, each reflected ing fork to the other fork. Again, when • Astronomical Journal, Nos. 248-251 ; American Journal of Science, February, 1892. • Two hundred thousand to two hundred and sixty thousand kilomètres in a second; the velocity of light being about three hundred thousand kilomètres. |