THE SUN.*

NUMEROUS and startling as are the revelations which modern science has made with regard to the functions and physical features of the Sun, no one can study Mr. Proctor's book without seeing that much remains to be revealed. Yet, doubtless, the time had arrived when the numerous observations and hypotheses of astronomers and the experiments of physicists should be arranged and digested. For such work (which consists partly of the due appreciation of existing theories, and partly of the devisal of new), and for the popular interpretation of the wonders' of astronomical science, no writer seems better adapted than the author of the book before us. He possesses remarkable powers of clear reasoning, extensive knowledge, and a facile and almost too fertile pen. We wish Mr. Proctor would give us time to read his books thoroughly, not to speak of his numerous contributions to periodical literature. For before we had finished his volume on Saturn, our attention was drawn to several of his interesting and instructive papers on Mars, Jupiter, and recent spectroscopic discoveries, contributed to various monthly and weekly journals. Afterwards we met with his account of " Other Worlds than Ours," and then, just nine months later, we were presented with 500 pages on the Sun. As we cannot here convey anything like a complete idea of this last store of astronomical learning, we must content ourselves with stating and briefly illustrating the contents of the nine chapters of the work.

The sun's distance and the various methods available and most effective for ascertaiuing it are discussed in chapter i. The his- torical growth of our knowledge of this important subject is given, not in mere description of such methods as the determina- tion of the solar parallax and the observation of the trausits of Venus, but in simple but sufficient diagrammatic illustrations of the phenomena in question. Then we are led to the consideration of the recent revision of the number representing the earth's dis- tance from the sun, which has been reduced to about 91,730,000 miles, with a probable error of about 300,000 miles. If the sun's mean distance be roughly placed at 92,000,000 of miles, the sun's diameter is fairly represented by 850,000 miles, corresponding to a volume not less than 1,250,00) times that of the earth. These numbers are confessedly approximate only, but though they are nearer the truth than earlier results, and will doubtless be in time submitted to further corrections, yet astronomers have abundant reasons to be proud of the advance made in this direction. For, as Sir J. Herschel has said, this recent correction in the value of the sun's parallax corresponds to the apparent breadth of a human hair at a distance of 125 feet, or of a sovereign at eight miles off.

The next aspect in which the author teaches us to regard the sun is as the Ruler of the motions of his family of planets. The

* The Sun, Ruler of the Planetary System. By R. A. Proctor. 1571.

overmastering attraction of the sun produces the stability of their orbital motions, and permits them to suffer no sudden, great, or permanent change. Though it is far from being his sole office of rule, yet the gravitating or attracting power of the sun is that which first attracts our notice. This may be ascertained by com- paring the amount of velocity which the earth's attraction com- municates to falling bodies, with the actual motion towards him- self which the sun causes in any planet ; or by comparing the motion of any planet round the sun with the moon's motion round the eart‘). Thus we are led to some very startling results, amongst which we may name the following :—The sun exerts 315,000 times as much force on any body, at equal distances, as the earth. If the earth's mass (not volume) were as great as the sun's, a pound weight would weigh more than fourteen and a half tons, and a small object raised a single inch and let fall would strike the ground with a velocity three times as great as that of the swiftest express train (p. 74). Our author applies the ascertained proportion between terrestrial and solar attractive energy to the controlling influence exerted by the sun upon the earth, which, in spite of the inconceivably great velocity with which she moves and tends to travel onward in a right line and thus free herself for ever from the sun's control, compels her to circle around him. The earth's mean distance from the sun is the measure of her partial freedom from solar control, while the shortness of the period in which she completes her circuit measures the amount of the sun's power over her. At the distance at which the earth moves about the sun, the mean velocity which she exhibits is 18.2 miles per second, and the interesting question arises by how much must this velocity be increased to free the earth from solar service, and dismiss her for ever to a cold and desolate wandering through space ? The in- crease requisite to produce this unhappy result is by no means large. "If the earth, when at her mean distance, had her velocity suddenly increased from 18.2 miles per second to 25.7 miles, we should be carried thenceforth continually farther and farther away from the light and life of the planetary scheme." Mr. Proctor traces the results of this alteration of the earth's career in the destruction of the sequence of the seasons, and the continued diminution of heat and light. But a similar fate would befall us, and the other planets as well, if the sun's mass were suddenly re- duced in the proportion of about 1,414 to 1,000. From a table on page 82 we learn that the planet Uranus would be just freed from solar allegiance if his velocity could be suddenly increased to that naturally belonging to the next inner planet, Saturn ; and that Venus would be similarly released by acquiring a velocity one mile per second greater than that of Mercury. At the surface of the sun a body travelling around him must have a velocity of 268 miles per second.

The analysis of the solar beams is next discussed. With great propriety Mr. Proctor lays great stress on a thorough understand- ing of spectroscopic analysis, and says, truly enough, that the whole history of many recent scientific researches is a sealed book to those who do not clearly understand the nature of the instru- ment by which the researches have been effected. Some fifty pages and twenty illustrative figures are devoted to a history of spectrum researches, especially as applied to the sun's rays. We are led from the earlier observations of Newton, Wollaston, and Fraun- hofer, to the great and final discovery by Kirchkoff of the meaning of the thousands of black linen which cross the solar spectrum. Identifying these lines as the reversed bright lines of certain metallic spectra, he showed, by imitating the solar phenomena, bow complete a proof was at hand of the existence of such metals as iron, sodium, barium, &c., in the sun. Angstrom and other physicists have amply confirmed these results,—identifying, for instance, 450 bright lines furnished by iron and 200 by titanium, with corresponding black spaces in the sun's spectrum. It might be thought that these spaces were black just because these very metals were absent from the sun. But no one can read Mr. Proctor's account of the absorptive properties of various vapours for particular luminous rays without seeing that the above-named black lines are due to the opacity of certain vapours, and to their consequent suppression or reversal of the bright lines which they themselves form. Hitherto the spectroscope has in this and other manners revealed the presence of no element in sun or stars which is not found upon the earth, just as the chemical analysis of meteorites, bodies which reach the earth from space, has added no new member to our list. But there is a considerable amount of coincidence in the results of these two methods of investigation into the chemistry of other worlds, hydrogen and iron having been, for example, abundantly recognized by both methods. For the beautiful way in which the spectroscope indicates the approach or recession of a star we refer our readers to page 146.

Chapter iv. comprises a study of the sun's surface, especially in relation to the solar spots, their movements, and zones of dis- tribution, their periods of maximum and minimum recurrence, their connection with magnetic storms, and their vast dimensions. We have presented to us clear and beautiful drawings of these singular appearances, amongst which we may especially call atten- tion to that figure (on page 232) representing a spot having the appearance of cyclonic motion, and conveying the notion of a terrible solar tornado; and to the several figures on page 230, where the progress of one of the sun's spots is recorded by the marvellously accurate and patient pencil of the Rev. Mr. Howlett. The divergent, yet perhaps not irreconcilable, views as to the solar surface announced by Nasmyth, Secchi, and others, are wel illustrated by drawings of the appearances known as " willow- leaves," "granules," and " corrugations."

The most magnificent yet most distinct of recent solar dis- coveries is reserved for separate discussion in the next chapter, which is devoted to the red prominences and chromosphere of the sun. The red clouds seem to have been first actually observed and noted in 1733 by Vassenius of Gottenburg, yet it was not till more than a century later that they were fully recognized. During the great eclipse of 1842 the red prominences were particularly scrutinized by Airy, Arago, Struve, Biela, and other skilful astronomers. On subsequent occasions they were again and again examined, their appearance, movements, colours, and distribu- tion and disappearance being carefully recorded. But it was not until photography and spectroscopy combined were brought to bear upon these red prominences and the lumin- ous layer from which they rise, the chromosphere, that their real nature became known. Exact observation had proved that these appearances were an appanage of the sun, but photographs produced an image of their shape taken by means of the actinic or radiant chemical energy which accompanies their luminous rays. Further than this, the spectroscope analysed these luminous and visible rays, and discovered that they were in part if not chiefly due to glowing hydrogen ! Then it was found that these pro- minences may be studied (by a sufficient dispersive power in the spectroscope and extreme narrowness of the slit admitting the light to be analysed) even when the sun is not eclipsed. By sub- sequent study the red promiuences have been observed to undergo the most extraordinary changes, and present the most remarkable forms. We are forced to believe that out of the red envelope of the sun there continually rise mountains of glowing hydrogen 100,000 miles, or perhaps much more, in height, and of the most fantastic shapes,—eruptive pillars of flame, with spiral and floating clouds.

The corona of the sun and zodiacal light are probably connected, according to Mr. Proctor, with a vast zone of meteoric and perhaps cometic matter revolving about the sun. There are difficulties in the acceptance of this theory as an adequate explanation of the very complex and singular phenomena of the corona. Our author's chain of argument is, at all events, well worthy of the closest attention, while the series of pictures of the coronal light visible during eclipses is of great completeness and interest. Speci- ally we direct attention to the beautiful coloured lithograph (opposite page 343), which represents the quadrangular form of the corona, its general violet hue, and its inexplicable dark radial bars.

We must not linger any more amongst these fascinating sub- jects, nor amongst those which are discussed in the remaining chapters of the volume under review. We may be permitted to doubt whether the seventh chapter, on the physical condi- tion of the sun, and parts of the sixth chapter, are quite as satisfactory as they might have been. We feel some doubts as to the validity of our author's reasoning when he speaks of the immense repulsive forces of the sun ; of considerable variations in the density of different layers of the sun's envelopes; of the segregation, spite of motion and diffusion, of several of the constituents of these layers ; of very low pressures combined with the existence about the sun in a liquid and even solid form of gases incondeneable on the earth, and of the occurrence of the vapour of iron within the limits of the terrestrial atmosphere. In the succeeding chapter, the sun, as originating nearly all the force

treasured up in the earth and daily reaching it, or added to it, is

treated in a clear and accurate manner. Here come in the ques- tions of the future fuel of the earth, the direct assimilation of solar energies by mechanical contrivances, and the absolute and direct dependence of all vegetable life, and consequently of all animal life, upon the solar force.

Chapter ix. is entitled " The Sun among his Peers," and includes, with other important matter, the author's views as to the real

nature of the Milky Way, as a spiral system of small stars, and the journey of the sun and the solar system through space.

The appendices on the Transits of Venus in 1874 and 1882 and on Eclipses will be of particular value to those who wish to pursue these important subjects of astronomical study.

It will be evident enough that the subject-matter of Mr. Proctor's volume, and the mode in which he has treated it, are calculated to attract and instruct the general reader. Other features of the book conduce to the same end. Such features are—the clearness of the type, the appropriateness, beauty, and in some instances, the weird splendour of the illustrations, as well as the ease of dic- tion and the lucidity of argument and explanation which charac- terize the book. Wherever popular explanation of scientific reason- ing is possible, the author does not shrink from the task of giving it, and he accomplishes this task without fatiguing the attention of the reader. Of course it is possible to find a few mistakes and misprints in the pages under review. Thus, we are told (page 264) that the red prominences on the sun's chromosphere " extend to some 800,000 miles from his surface," while the received estimates are usually nearer 80,000 miles, and never exceed double that length,—which is surely sufficiently vast ! Then, again, we read (page 115) of "chlorine of tungsten," "perchlorine of manganese," names which a tyro in chemistry will be able to revise. But such errors as these are not only rare, but of very trifling moment, com- pared with a blemish of a different sort which obtrudes itself too often as we read the book. Our enjoyment of an otherwise wholly pleasant treatise is ever and anon marred by the intrusion of some personal dispute which the author has had with a brother astro- nomer. Why should we be annoyed by such petty matters, when we are wrapt in the study of the intricate magnificence of the sun ? Whyshould occasion be given to the outside world of scoffers to say, " See how little these disciples of science love one another "? 1Ve will not direct further attention to this subject by giving a list of the passages to which we allude, yet there is one sentence of Mr. Proctor's (note to page 206) which is so curious as to demand remark. The author writes, "The account I gave has been cavilled at by the Cavilian Professor of Astronomy ;" are we to understand the substitution of Cavilian for Savilian to be a very small joke of Mr. Proctor's, or a most singular and unfortunate mistake of the printer ?