remote agglomerations we learn to feel | The spectra of the stars, the nebulæ, the corona, and the protuberances of the sun, The spectrum that this displacement is due to the proper movements of the stars and gives a means of measuring them, and Mr. Christie ever 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 However, the chief progress recently high pitch of a sound depends on much achieved in physical astronomy is due to quicker vibrations of the air than the low the spectroscope aided by photography. 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 ẞ 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 (Observatory, January, 1892). Observatory, October, 1891. Comptes Rendus de l'Académie des Sciences, 1891, t. 113, p. 307. 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." It has been remarked for some time since | cially light, it might be best explained by 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 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 (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 light or radiant heat is transmitted through tirely accordant in magnitude with the 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. we assume that besides the matter which However probable this hypothesis, phys- Two hundred thousand to two hundred and sixty thousand kilomètres in a second; the velocity of light being about three hundred thousand kilomètres. D condensation meets with a rarefaction, and | three instruments As soon as sparking began in the vibrator, and the detector was approached to it, sparks began to jerk between the knobs of the latter; but they disappeared as soon as the screen was interposed between the two- the "waves being interrupted in this case. On the contrary, when the screen was placed immediately behind the detector, strong sparking fol lowed; if it was removed about eighteen feet, the sparking ceased; the direct and the reflected waves extinguishing each in this case both actions neutralize each screen, and the detector - the experiother — the sound is weakened. So that, ments could be carried on, and they if we slowly approach our reflecting board proved at once the close connection exto the fork, there will be places where the isting between the phenomena of elecboard reinforces the sound (condensations tricity and light. meeting with condensations), then weakens it, and then makes it louder again, although the board is moved all the time in one direction, towards the tuning-fork. Of course, things are not so easy with electricity. There is no great difficulty in producing alternate electrifications of the surrounding ether which would correspond to the alternate condensations of the air, but they must follow each other with a tremendous rapidity. In fact, if the tuning-fork makes, say, one thousand 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 I'I 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 velocity of electricity, both in the air and measure the distances at which the elec- the conductive wires, proved to be very trical "waves" followed each other. A near to that of light, namely, about one reflecting board, and some means for de- hundred and eighty thousand miles in a tecting the "loops and nodes," i.e., the second. 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 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," each other, so as to show sparks at the reception of the It 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 feeblest electric waves (Sitzungsberichte der Berliner Acad. der Wissenschaften, February 9, 1888). 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. |