Chapter 9

Besides its more direct use in the chemical analysis of the heavenly bodies, the spectroscope had given to us a great and unexpected power of advance along the lines of the older astronomy. In the future a higher value might, indeed, be placed upon this indirect use of the spectroscope than upon its chemical revelations. By no direct astronomical methods could motions of approach or of recession of the stars be even detected, much less could they be measured. A body coming directly toward us or going directly from us appeared to stand still. In the case of the stars we could receive no assistance from change of size or of brightness. The stars showed no true disks in our instruments, and the nearest of them was so far off that if it were approaching us at the rate of a hundred miles in a second of time, a whole century of such rapid approach would not do more than increase its brightness by the one-fortieth part. Still it was formerly only too clear that, so long as we were unable to ascertain directly those components of the stars' motions which lay in the line of sight, the speed and direction of the solar motion in space, and many of the great problems of the constitution of the heavens must have remained more or less imperfectly known. Now the spectroscope had placed in our hands this power, which, though so essential, had previously appeared almost in the nature of things to lie forever beyond our grasp; it enabled us to measure directly, and, under favorable circumstances, to within a mile per second, or even less, the speed of approach or of recession of a heavenly body. This method of observation had the great advantage for the astronomer of being independent of the distance of the moving body, and was, therefore, as applicable and as certain in the case of a body on the extreme confines of the visible universe, so long as it was bright enough, as in the case of a neighboring planet.

ALGOL AND SPICA.

By observations with the Potsdam spectograph, Professor Vogel found that the bright star of Algol pulsated backward and forward in the visual direction in a period corresponding to the known variation of its light. The explanation which had been suggested for the star's variability, that it was partially eclipsed at regular intervals of 68.8 hours by a dark companion large enough to cut off nearly five-sixths of its light, was, therefore, the true one. The dark companion, no longer able to hide itself by its obscureness, was brought out into the light of direct observation by means of its gravitational effects. Seventeen hours before minimum Algol was receding at the rate of about 24½ miles a second, while seventeen hours after minimum it was found to be approaching with a speed of about 28½ miles. From these data, together with those of the variation of its light, Vogel found, on the assumption that both stars have the same density, that the companion, nearly as large as the sun, but with about one-fourth his mass, revolved with a velocity of about fifty-five miles a second. The bright star of about twice the size and mass moved about the common center of gravity with the speed of about 26 miles a second. The system of the two stars, which were about 3¼ millions of miles apart, considered as a whole, was approaching us with a velocity of 2.4 miles a second. The great difference in luminosity of the two stars, not less than fifty times, suggested rather that they were in different stages of condensation, and dissimilar in density. It was obvious that if the orbit of a star with an obscure companion was inclined to the line of sight, the companion would pass above or below the bright star and produce no variation of its light. Such systems might be numerous in the heavens. In Vogel's photographs, Spica, which was not variable, by a small shifting of its lines revealed a backward and forward periodical pulsation due to orbital motion. As the pair whirled round their common center of gravity, the bright star was sometimes advancing, at others receding. They revolved in about four days, each star moving with a velocity of about 56 miles a second in an orbit probably nearly circular, and possessed a combined mass of rather more than two and one-half times that of the sun. Taking the most probable value for the star's parallax, the greatest angular separation of the stars would be far too small to be detected with the most powerful telescopes.

THE VALUE OF PHOTOGRAPHY.

Referring to the new and great power which modern photography had put into the hands of the astronomer, the president said that the modern silver bromide gelatine plate, except for its grained texture, met his needs at all points. It possessed extreme sensitiveness, it was always ready for use, it could be placed in any position, it could be exposed for hours, lastly it did not need immediate development, and for this reason could be exposed again to the same object on succeeding nights, so as to make up by several installments, as the weather might permit, the total time of exposure which was deemed necessary. Without the assistance of photography, however greatly the resources of genius might overcome the optical and mechanical difficulties of constructing large telescopes, the astronomer would have to depend in the last resource upon his eye. Now, we could not by the force of continued looking bring into view an object too feebly luminous to be seen at the first and keenest moment of vision. But the feeblest light which fell upon the plate was not lost, but taken in and stored up continuously. Each hour the plate gathered up 3,600 times the light energy which it received during the first second. It was by this power of accumulation that the photographic plate might be said to increase, almost without limit, though not in separating power, the optical means at the disposal of the astronomer for the discovery or the observation of faint objects.

TWO EXAMPLES.

Two principal directions might be pointed out in which photography was of great service to the astronomer. It enabled him within the comparatively short time of a single exposure to secure permanently with great exactness the relative positions of hundreds or even of thousands of stars, or the minute features of nebulæ or other objects, or the phenomena of a passing eclipse, a task which by means of the eye and hand could only be accomplished, if done at all, after a very great expenditure of time and labor. Photography put it in the power of the astronomer to accomplish in the short span of his own life, and so enter into their fruition, great works which otherwise must have been passed on by him as a heritage of labor to succeeding generations. The second great service which photography rendered was not simply an aid to the powers the astronomer already possessed. On the contrary, the plate, by recording light waves which were both too small and too large to excite vision in the eye, brought him into a new region of knowledge, such as the infra-red and the ultra-violet parts of the spectrum, which must have remained forever unknown but for artificial help.

A PHOTOGRAPHIC CHART.

The present year would be memorable in astronomical history for the practical beginning of the photographic chart and catalogue of the heavens which took their origin in an international conference which met in Paris in 1887. The decisions of the conference in their final form provided for the construction of a great chart with exposures corresponding to forty minutes' exposure at Paris, which it was expected would reach down to stars of about the fourteenth magnitude. As each plate was to be limited to four square degrees, and as each star, to avoid possible errors, was to appear on two plates, over 22,000 photographs would be required. A second set of plates for a catalogue was to be taken, with a shorter exposure, which would give stars to the eleventh magnitude only. The plans were to be pushed on as actively a possible, though as far as might be practicable plates for the chart were to be taken concurrently. Photographing the plates for the catalogue was but the first step in this work, and only supplied the data for the elaborate measurements which would have to be made, which were, however, less laborious than would be required for a similar catalogue without the aid of photography.

A DELICATE OPERATION.

The determination of the distances of the fixed stars from the small apparent shift of their positions when viewed from widely separated positions of the earth in its orbit was one of the most refined operations of the observatory. The great precision with which this minute angular quantity, a fraction of a second only, had to be measured, was so delicate an operation with the ordinary micrometer, though, indeed, it was with this instrument that the classical observations of Sir Robert Ball were made, that a special instrument, in which the measures were made by moving the two halves of a divided object glass, known as a heliometer, had been pressed into this service, and quite recently, in the skillful hands of Dr. Gill and Dr. Elkin, had largely increased our knowledge in this direction. It was obvious that photography might be here of great service, if we could rely upon measurements of photographs of the same stars taken at suitable intervals of time. Professor Pritchard, to whom was due the honor of having opened this new path, aided by his assistants, had proved by elaborate investigations that measures for parallax might be safely made upon photographic plates, with, of course, the advantages of leisure and repetition; and he had already by this method determined the parallax for twenty-one stars with an accuracy not inferior to that of values previously obtained by purely astronomical methods.

PHOTOGRAPHIC REVELATIONS.

The remarkable successes of astronomical photography, which depended upon the plate's power of accumulation of a very feeble light acting continuously through an exposure of several hours, were worthy to be regarded as a new revelation. The first chapter opened when, in 1880, Dr. Henry Draper obtained a picture of the nebula of Orion; but a more important advance was made in 1883, when Dr. Common, by his photographs, brought to our knowledge details and extensions of this nebula hitherto unknown. A further disclosure took place in 1885, when the Brothers Henry showed for the first time in great detail the spiral nebulosity issuing from the bright star Maia of the Pleiades, and shortly afterward nebulous streams about the other stars of this group. In 1886 Mr. Roberts, by means of a photograph to which three hours' exposure had been given, showed the whole background of this group to be nebulous.

In the following year Mr. Roberts more than doubled for us the great extension of the nebular region which surrounds the trapezium in the constellation of Orion. By his photographs of the great nebula in Andromeda, he had shown the true significance of the dark canals which had been seen by the eye. They were in reality spaces between successive rings of bright matter, which appeared nearly straight, owing to the inclination in which they lay relatively to us. These bright rings surrounded an undefined central luminous mass. Recent photographs by Mr. Russell showed that the great rift in the Milky Way in Argus, which to the eye was void of stars, was in reality uniformly covered with them.

THE STORY OF THE HEAVENS.

The heavens were richly but very irregularly inwrought with stars. The brighter stars clustered into well known groups upon a background formed of an enlacement of streams and convoluted windings and intertwined spirals of fainter stars, which became richer and more intricate in the irregularly rifted zone of the Milky Way. We, who formed part of the emblazonry, could only see the design distorted and confused; here crowded, there scattered, at another place superposed. The groupings due to our position were mixed up with those which were real. Could we suppose that each luminous point had no relation to the others near it than the accidental neighborship of grains of sand upon the shore, or of particles of the wind-blown dust of the desert? Surely every star from Sirius and Vega down to each grain of the light dust of the Milky Way had its present place in the heavenly pattern from the slow evolving of its past. We saw a system of systems, for the broad features of clusters and streams and spiral windings marking the general design were reproduced in every part. The whole was in motion, each point shifting its position by miles every second, though from the august magnitude of their distances from us and from each other, it was only by the accumulated movements of years or of generations that some small changes of relative position revealed themselves.

THE WORK OF THE FUTURE.

The deciphering of this wonderfully intricate constitution of the heavens would be undoubtedly one of the chief astronomical works of the coming century. The primary task of the sun's motion in space, together with the motions of the brighter stars, had been already put well within our reach by the spectroscopic method of the measurement of star motions in the line of sight. Astronomy, the oldest of the sciences, had more than renewed her youth. At no time in the past had she been so bright with unbounded aspirations and hopes. Never were her temples so numerous, nor the crowd of her votaries so great.

The British Astronomical Association formed within the year numbered already about 600 members. Happy was the lot of those who were still on the eastern side of life's meridian! Already, alas! the original founders of the newer methods were falling out--Kirchhoff, Angstrom, D'Arrest, Secchi, Draper, Becquerel; but their places were more than filled; the pace of the race was gaining, but the goal was not and never would be in sight. Since the time of Newton our knowledge of the phenomena of nature had wonderfully increased, but man asked perhaps more earnestly now than in his days, what was the ultimate reality behind the reality of the perceptions? Were they only the pebbles of the beach with which we had been playing? Did not the ocean of ultimate reality and truth lie beyond?

* * * * *

CLIMATIC CHANGES IN THE SOUTHERN HEMISPHERE.

By C.A.M. TABER.

Having had occasion to cruise a considerable time over the Southern Ocean, I have had my attention directed to its prevailing winds and currents, and the way in which they affect its temperature, and also to the ice-worn appearance of its isolated lands.

It is now generally conceded that the lands situated in the high latitudes of the southern hemisphere have in the remote past been covered with ice sheets, similar to the lands which lie within the antarctic circle. The shores of Southern Chile, from latitude 40° to Cape Horn, show convincing evidence of having been overrun by heavy glaciers, which scoured out the numerous deep channels that separate the Patagonian coast from its islands. The Falkland Islands and South Georgia abound with deep friths; New Zealand and Kerguelen Land also exhibit the same evidence of having been ice-laden regions; and it is said that the southern lands of Africa and Australia show that ice accumulated at one time to a considerable extent on their shores. At this date we find the southern ice sheets mostly confined to regions within the antarctic circle; still the lands of Chile, South Georgia, and New Zealand possess glaciers reaching the low lands, which are probably growing in bulk; for it appears that the antarctic cold is slowly on the increase, and the reasons for its increase are the same as the causes which brought about the frigid period which overran with ice all lands situated in the high southern latitudes.

Why there should be a slow increase of cold on this portion of the globe is because of the independent circulation of the waters of the Southern Ocean. The strong westerly winds of the southern latitudes are constantly blowing the surface waters of the sea from west to east around the globe. This causes an effectual barrier, which the warm tropical currents cannot penetrate to any great extent. For instance, the tropical waters of the high ocean levels, which lie abreast Brazil in the Atlantic and the east coast of Africa in the Indian Ocean, are not attracted far into the southern sea, because the surface waters of the latter sea are blown by the westerly winds from west to east around the globe. Consequently the tropical waters moving southward are turned away by the prevailing winds and currents from entering the Southern Ocean. Thus the ice is accumulating on its lands, and the temperature of its waters slowly falling through their contact with the increasing ice; and such conditions will continue until the lands of the high southern latitudes are again covered with glaciers, and a southern ice period perfected. But while this gathering of ice is being brought about, the antarctic continent, now nearly covered with an ice sheet, will, through the extension of glaciers out into its shallow waters, cover a larger area than now; for where the waters are shoal the growing glaciers, resting on a firm bottom, will advance into the sea, and this advancement will continue wherever the shallow waters extend. Especially will this be the case where the snowfall is great.

Under such conditions, it appears that the only extensive body of shallow water extending from the ice-clad southern continent is the shoal channel which separates the South Shetlands from Cape Horn, which is a region of great snowfall. Therefore, should the antarctic ice gain sufficient thickness to rest on the bottom of this shallow sea, it would move into the Cape Horn channel, and eventually close it. The ice growth would not be entirely from the southern continent, but also from lands in the region of Cape Horn. Thus the antarctic continent and South America would be connected by an isthmus of ice, and consequently the independent circulation of the Southern Ocean arrested. Hence it will be seen that the westerly winds, instead of blowing the surface waters of the Southern Ocean constantly around the globe, as they are known to do to-day, would instead blow the surface waters away from the easterly side of the ice-formed isthmus, which would cause a low sea level along its Atlantic side, and this low sea level would attract the tropical waters from their high level against Brazil well into the southern seas, and so wash the antarctic continent to the eastward of the South Shetlands.

The tropical waters thus attracted southward would be cooler than the tropical waters of to-day, owing to the great extension of cold in the southern latitudes. Still they would begin the slow process of raising the temperature of the Southern Ocean, and would in time melt the ice in all southern lands. Not only the Brazil currents would penetrate the southern seas, as we have shown, but also the waters from the high level of the tropical Indian Ocean which now pass down the Mozambique Channel would reach a much higher latitude than now.

The ice-made isthmus uniting South America to the antarctic continent would on account of its location be the last body of ice to melt from the southern hemisphere, it being situated to windward of the tropical currents and also in a region where the fall of snow is great; yet it would eventually melt away, and the independent circulation of the Southern Ocean again be established. But it would require a long time for ice sheets to again form on southern lands, because of the lack of icebergs to cool the southern waters. Still, their temperature would gradually lower with the exclusion of the tropical waters, and consequently ice would slowly gather on the antarctic lands.

The above theory thus briefly presented to account for the climatic changes of the high southern latitudes is in full accord with the simple workings of nature as carried on to-day; and it is probable that the formation of continents and oceans, as well as the earth's motions in its path around the sun, have met with little change since the cold era iced the lands of the high latitudes.

At an early age, previous to the appearance of frigid periods, the ocean waters of the high latitudes probably did not possess an independent circulation sufficient to lower the temperature so that glaciers could form. This may have been owing to the shallow sea bottom south of Cape Horn having been above the surface of the water, the channel having since been formed by a comparatively small change in the ocean's level. For, while considering this subject, it is well to keep in mind that whenever the western continent extended to the antarctic circle it prevented the independent circulation of the Southern Ocean waters, consequently during such times ice periods could not have occurred in the southern hemisphere.

It will be noticed that according to the views given above, the several theories which have been published to account for great climatic changes neglect to set forth the only efficacious methods through which nature works for conveying and withdrawing tropical heat sufficient to cause temperate and frigid periods in the high latitudes. While lack of space forbids an explanation of the causes which would perfect an ice period in the northern hemisphere, I will say that it could be mainly brought about through the independent circulation of the arctic waters, which now largely prevent the tropical waters of the North Atlantic from entering the arctic seas, thus causing the accumulation of ice sheets on Greenland. But before a northern ice period can be perfected, it seems that it will need to co-operate with a cold period in the southern hemisphere; and in order to have the ice of a northern frigid period melt away, it would require the assistance of a mild climate in the high southern latitudes.--_Science_.

* * * * *

AMMONIA.

In the majority of refrigerating and ice machines ammonia gas is the substance used for producing the refrigeration, although there are other machines in which other material is employed, one of these being anhydrous sulphurous acid, which is also a gas. Ammonia of itself is a colorless gas, but little more than one half as heavy as air. In its composition ammonia consists of two gases, nitrogen and hydrogen, in the proportion by weight of one part nitrogen and three parts hydrogen. The gas hydrogen is one of the constituents of water and is highly inflammable in the presence of air or oxygen, while the other component of ammonia, nitrogen, forms the bulk or about four-fifths of the atmosphere. Nitrogen by itself is an inert gas, colorless and uninflammable. Ammonia, although composed of more than three-fourths its weight of hydrogen, is not inflammable in air, on account of its combination with the nitrogen. This combination, it will be understood, is not a simple mixture, but the two gases are chemically combined, forming a new substance which has characteristics and properties entirely different from either of the gases entering into its composition when taken alone or when simply mixed together without chemical combustion. Ammonia cannot be produced by the direct combination of these elements, but it has been found that it is sometimes made or produced in a very extraordinary manner, which goes to show that there is yet considerable to be learned in regard to the chemistry of ammonia. Animal or vegetable substances when putrefying or suffering destructive distillation almost invariably give rise to an abundant production of this substance.