Part 3

Now, the difficulty of the problem will be recognised when we remember that the strongest tints of the corona's light--the green tint classified as 1474 Kirchhoff--has been specially but ineffectually searched for in the sun's neighbourhood with the most powerful spectroscopic appliances yet employed in the study of the coloured prominences. In other words, when the light of our own air over the region occupied by the corona has been diluted as far as possible by spectroscopic contrivances, the strongest of the special coronal tints has yet failed to show through the diluted spectrum of the sky. Again, we have even stronger evidence of the difficulty of the task in the spectroscopic observations made by Respighi during the eclipse of 1871. The instrument, or I should rather, perhaps, say the arrangement, which during mid totality showed the green image of the corona to a height of about 280,000 miles, did not show any green ring at all at the beginning of totality. In other words, so faint is the light of the gaseous corona, even at its brightest part, close to the sun, that the faint residual atmospheric light which illuminates the sky over the eclipsed sun at the beginning of totality sufficed to obliterate this part of the coronal light.

Whether with any combination specially directed to meet the difficulties of this observation, the gaseous corona can be rendered discernible, remains to be seen. I must confess my own hopes that the problem will ever be successfully dealt with are very slight, though not absolutely evanescent. It seems to me barely possible that the problem might be successfully attacked in the following way. Using a telescope of small size, for the larger the telescope the fainter is the image (because of greater loss of light by absorption), let the image of the sun be received in a small, perfectly darkened camera attached to the eye-end of the telescope. Now if the image of the sun were received on a smooth white surface we know that the prominences and the corona would not be visible. And again, if the part of such a surface on which the image of the sun itself fell were exactly removed, we know (the experiment has been tried by Airy) that the prominences would not be seen on the ring of white surface left after such excision. Still less, then, would the much fainter image of the corona be seen. But if this ring of white surface, illuminated in reality by the sky, by the ring of prominences and sierra, and by the corona, were examined through a battery of prisms (used without a slit) adjusted to any one of the known prominence tints, the ring of prominences and sierra would be seen in that special tint. If the battery of prisms were sufficiently effective, and the tint were one of the hydrogen tints--preferably, perhaps, the red--we might possibly be able to trace the faint image of the corona in that tint. But we should have a better chance with the green tint corresponding to the spectral line 1474 Kirchhoff. If the ring of white surface were replaced by a ring of green surface, the tint being as nearly that of 1474 Kirchhoff as possible, the chance of seeing the coronal ring in that tint would be somewhat increased; and, still further, perhaps, if the field of view were examined through green glass of the same tint. It seems just possible that if prisms of triple height were used, through which the rays were carried three times, by an obvious modification of the usual arrangement for altering the level of the rays, thus giving a power of eighteen flint glass prisms of sixty degrees each, evidence, though slight perhaps, might be obtained of the presence of the substance which produces the green line. Thus variations in the condition of the corona might be recognised, and any law affecting such variations might be detected. I must confess, however, that a consideration of the optical relations involved in the problem leads me to regard the attempt to recognise any traces of the corona when the sun is not eclipsed as almost hopeless.

It is clear that until some method for thus observing the corona has been devised, future eclipse observations will acquire a new interest from the light which they may throw on the coronal variations, and their possible association in some way, not as yet detected, with the sun-spot period. Even when a method has been devised for observing the gaseous corona, the corona whose light comes either directly or by reflection from solid or liquid matter will still remain undiscernible save only during total eclipses of the sun. Many years must doubtless pass, then, before the relation of the corona to the prominences and the sun-spots shall be fully recognised. But there can be no question that the solution of this problem will be well worth waiting for, even though it should not lead up (as it most probably will) to the solution of the mystery of the periodic changes which affect the surface of the sun.

FOOTNOTES:

[Footnote 1: The actual condition of the sun in 1842 may be inferred from the following table, showing the number of spots observed in 1837 the preceding year of maximum disturbance, in 1842, and in 1844 the following year of minimum disturbance; the observer was Schwabe of Dessau:

Days of Days without New groups observation spots observed 1837 168 0 333 1842 307 64 68 1844 321 111 52

Only it should be noticed that nearly all the spots seen in the year 1844 belonged to the next period, the time of actual minimum occurring early in 1844.]

[Footnote 2: The following table shows the position occupied by the years 1851 and 1860 in this report, as compared with the year 1848 (maximum next preceding 1851), 1856 (minimum next following 1851) and 1867, minimum next following 1860:--

Days of Days without New groups observation spots observed 1848 278 0 930 1851 308 0 141 1856 321 193 34 1860 332 0 211 1867 312 195 25

A comparison of the three tables given in these notes and the text will afford some idea of the irregularities existing in the various waves of sun-spots.]

_SUN-SPOTS AND COMMERCIAL PANICS._

We are not only, it would seem, to regard the sun as the ultimate source of all forms of terrestrial energy, existent or potential, but as regulating in a much more special manner the progress of mundane events. Many years have passed since Sabine, Wolf, and Gauthier asserted that variations in the daily oscillations of the magnetic needle appear to synchronise with the changes taking place in the sun's condition, the oscillations attaining their _maximum_ average range in years when the sun shows most spots, and their _minimum_ range when there are fewest spots. And although it is well known that the Astronomer Royal in England and the President of the Academy of Sciences in France reject this doctrine, it still remains in vogue. True, the average magnetic period appears to be about 10.45 years, while Wolf obtains for the sun-spot period 11.11 years; but believers in the connection between terrestrial magnetic disturbances and sun-spots consider that among the imperfect records of the past condition of the sun Wolf must have lost sight of one particular wave of sun-spots, so to speak. If there have been 24 such waves between 1611 and 1877, when sun-spots were fewest, we get Wolf's period of 11.11 years; if there have been 25 such waves then, taking an admissible estimate for the earliest epoch, we get 10.45 years, the period required to synchronise with the period of terrestrial magnetic changes. The matter must be regarded as still _sub judice_. This, however, is only one relation out of many now suggested. Displays of the aurora, being unquestionably dependent on the magnetic condition of the earth, would of course be associated with the sun spot period, if the magnetic period is so; and certainly the most remarkable displays of the aurora in recent times have occurred when the sun has shown many spots. Yet this of itself proves nothing more than had been already known--namely, that the last few magnetic periods have nearly synchronised with the last few sun-spot periods. It is rather strange, too, that no auroras are mentioned in the English records for 80 years preceding the aurora of 1716, and in the records of the Paris Academy of Sciences one only--that of 1666, which occurred when sun-spots were fewest. The great aurora of 1723, seen as far south as Bologna, also occurred at the time of _minimum_ solar activity. Here we are not depending on either Wolf's period of 11 years or Brown's of 10-1/2 years; from records of actual observation it is known that in 1666 and 1713 there were no sun-spots. In fact it is worth mentioning that Cassini, writing in 1671, says, 'It is now about 20 years since astronomers have seen any considerable spots on the sun,' a circumstance which throws grave doubt on the law of sun-spot periodicity itself. It is at least certain that the interval from _maximum_, spot-frequency to _maximum_, or from _minimum_ to _minimum_, has sometimes fallen far short of 9 years, and has at others exceeded 18 years.

It appears again that certain meteorological phenomena show a tendency, more or less marked, to run through a ten-year cycle. Thus, from the records of rainfall kept at Oxford it appears that more rain fell under west and south-west winds when sun-spots were largest and most numerous than under south and south-east winds, these last being the more rainy winds when sun-spots were least in size and fewest in number. This is a somewhat recondite relation, and at least proves that earnest search has been made for such cyclic relations as we are considering. But this is not all. When other records were examined, the striking circumstance was discovered that elsewhere, as at St. Petersburg, the state of things observed at Oxford was precisely reversed. At some intermediate point between Oxford and St. Petersburg, no doubt the rainfall under the winds named was equally distributed throughout the spot period. Moreover, as the conditions thus differ at different places, we may assume that they differ also at different times. Such relations appear then to be not only recondite, but complicated.

When we learn that during nearly two entire sun-spot periods cyclones have been somewhat more numerous in the Indian Seas when spots are most numerous than when the sun is without spots, and _vice versâ_, we recognise the possible existence of cyclic relations better worth knowing than those heretofore mentioned. The evidence is not absolutely decisive; some, indeed, regard it as scarcely trustworthy. Yet there does seem to have been an excess of cyclonic disturbance during the last two periods of great solar disturbance, precisely as there was also an excess of magnetic disturbance during those periods. The excess was scarcely sufficient, however, to justify the rather daring statement made by one observer, that 'the whole question of cyclones is merely a question of solar activity.' We had records of some very remarkable cyclonic disturbances during the years 1876 and 1877, when the sun showed very few spots, the actual _minimum_ of disturbance having probably been reached late in 1877. A prediction that 1877 would be a year of few and slight storms would have proved disastrous if implicit reliance had been placed on it by seamen and travellers.

Rainfall and atmospheric pressure in India have been found to vary in a cyclic manner, of late years at any rate, the periods being generally about 10 or 11 years. The activity of the sun, as shown by the existence of many spots, apparently makes more rainfall at Madras, Najpore, and some other places; while at Calcutta, Bombay, Mysore, and elsewhere it produces a contrary effect. Yet these effects are produced in a somewhat capricious way: for sometimes the year of actual _maximum_ spot frequency is one in which rainfall is below the average (instead of above) at the former stations, and above the average (instead of below) at the latter. It is only by taking averages--and in a somewhat artificial manner--that the relation seems to be indicated on which stress has been laid.

Since Indian famines are directly dependent on defective rainfall, it is natural that during the years over which observation has hitherto extended the connection apparently existing between sun-spots and Indian rainfall should seem also to extend itself to Indian famines. It was equally to be expected that since cyclones have been rather more numerous, for some time past, in years when sun-spots have been most numerous, shipwrecks should also have been somewhat more frequent in such years. Two years ago Mr. Jeula gave some evidence which, in his opinion, indicated such a connection between sun-spots and shipwrecks. He showed that in the four years of fewest spots the mean percentage of losses was 8.64; in four intermediate years the mean percentage was 9.21; in three remaining years of the eleven-year cycle--that is, in three years of greatest spot frequency the mean percentage was 9.53. Some suggested that possibly such events as the American war, which included two of the three years of greatest spot frequency, may have had more effect than sun-spots in increasing the percentage of ships lost; while perhaps, the depression following the commercial panic of 1866 (at a time of fewest sun-spots) may have been almost as effective in reducing the percentage of losses as the diminished area of solar maculation. But others consider that we ought rather to regard the American war as yet another product of the sun's increased activity in 1860-61, and the great commercial panic of 1866 as directly resulting from diminished sun-spots at that time, thus obtaining fresh evidence of the sun's specific influence on terrestrial phenomena instead of explaining away the evidence derived from Lloyd's list of losses.

This leads us to the last, and, in some respects, the most singular suggestion respecting solar influence on mundane events--the idea, namely, that commercial crises synchronise with the sun-spot period, occurring near the time when spots are least in size and fewest in number; or, as Professor Jevons (to whom the definite enunciation of this theory is due) poetically presents the matter, that from 'the sun, which is truly "of this great world both eye and soul," we derive our strength and our weakness, our success and our failure, our elation in commercial mania, and our despondency and ruin in commercial collapse.' We have better opportunities of dealing with this theory than with the others, for we have records of commercial matters extending as far back as the beginning of the eighteenth century. In fact, we have better evidence than Professor Jevons seems to have supposed, for whereas in his discussion of the matter he considers only the probable average of the sun-spot period, we know approximately the epochs themselves at which the _maxima_ and _minima_ of sun spots have occurred since the year 1700. The evidence as presented by Professor Jevons is very striking, though when examined in detail it is rather disappointing. He presents the whole series of decennial crises as follows:--1701? (such query marks are his own), 1711, 1721, 1731-32, 1742 (?), 1752 (?), 1763, 1772-73, 1783, 1793, 1804-5 (?), 1815, 1825, 1836-9 (1837 in the United States), 1847, 1857, 1866 and 1878. The average interval comes out 10.466 years, showing, as Jevons points out, 'almost perfect coincidence with Brown's estimate of the average sun-spot period.' Let us see, however, whether these dates correspond so closely with the years of _minimum_ spot-frequency as to remove all doubt. Taking 5-1/4 years as the average interval between _maximum_ and _minimum_ sun-spot frequency, we should like to find every crisis occurring within a year or so on either side of the _minimum_ though we should prefer perhaps to find the crisis always following the time of fewest sun-spots, as this would more directly show the depressing effect of a spotless sun. No crisis ought to occur within a year or so of _maximum_ solar disturbance; for that, it should seem, would be fatal to the suggested theory. Taking the commercial crises in order, and comparing them with the known (or approximately known) epochs of _maximum_ and _minimum_ spot frequency, we obtain the following results (we italicize numbers or results unfavourable to the theory):--The doubtful crisis of 1701 followed a spot _minimum_ by _three_ years; the crisis of 1711 preceded a _minimum_ by one year; that of 1721 preceded a _minimum_ by two years; 1731-32, preceded a _minimum_ by one year; 1742 preceded a _minimum_ by _three_ years; 1752 followed a _maximum_ by _two_ years; 1763 followed a _maximum_ by _a year and a half_; 1772-73 came _midway_ between a _maximum_ and a _minimum_; 1783 preceded a _minimum_ by nearly two years; 1793 came nearly midway between a _maximum_ and a _minimum_; 1804-5 coincided with a _maximum_; 1815 preceded a _maximum_ by two years; 1825 followed a _minimum_ by _two_ years; 1836-39 _included_ the year 1837 of _maximum_ solar activity (that year being the time also when a commercial crisis occurred in the United States); 1847 preceded a _maximum_ by a _year and a half_; 1866 preceded a _minimum_ by a year; and 1878 followed a _minimum_ by a year. Four favourable cases out of 17 can hardly be considered convincing. If we include cases lying within two years of a _minimum_, the favourable cases mount up to seven, leaving ten unfavourable ones. It must be remembered, too, that a single decidedly unfavourable case (as 1804, 1815, 1837) does more to disprove such a theory than 20 favourable cases would do towards establishing it. The American panic of 1873, by the way, which occurred when spots were very numerous, decidedly impairs the evidence derived from the crises of 1866 and 1878.

_NEW PLANETS NEAR THE SUN._

Perhaps no scientific achievement during the present century has been deemed more marvellous than the discovery of the outermost member (so far as is known) of the sun's family of planets. In many respects, apart from the great difficulty of the mathematical problem involved, the discovery appealed strongly to the imagination. A planet seventeen hundred millions of miles from the sun had been discovered in March, 1781, by a mere accident, though the accident was not one likely to occur to any one but an astronomer constantly studying the star-depths. Engaged in such observation, but with no idea of enlarging the known domain of the sun, Sir W. Herschel perceived the distant planet Uranus. His experienced eye at once recognised the fact that the stranger was not a fixed star. He judged it to be a comet. It was not until several weeks had elapsed that the newly discovered body was proved to be a planet, travelling nearly twice as far away from the sun as Saturn, the remotest planet before known. A century only had elapsed since the theory of gravitation had been established. Yet it was at once perceived how greatly this theory had increased the power of the astronomer to deal with planetary motions. Before a year had passed more was known about the motions of Uranus than had been learned about the motion of any of the old planets during the two thousand years preceding the time of Copernicus. It was possible to calculate in advance the position of the newly discovered planet, to calculate retrogressively the path along which it had been travelling, unseen and unsuspected, during the century preceding its discovery. And now observations which many might have judged to be of little value, came in most usefully. Astronomers since the discovery of the telescope had formed catalogues of the places of many hundreds of stars invisible to the naked eye. Search among the observations by which such catalogues had been formed, revealed the fact that Uranus had been seen and catalogued as a fixed star twenty-one several times! Flamsteed had seen it five times, each time recording it as a star of the sixth magnitude, so that five of Flamsteed's stars had to be cancelled from his lists. Lemonnier had actually seen Uranus twelve times, and only escaped the honour of discovering the planet (as such) through the most marvellous carelessness, his astronomical papers being, as Arago said, 'a very picture of chaos.' Bradley saw Uranus three times.[3] Mayer saw the planet once only.

It was from the study of the movements of Uranus as thus seen, combined with the planet's progress after its discovery, that mathematicians first began to suspect the existence of some unknown disturbing body. The observations preceding the discovery of the planet range over an interval of ninety years and a few months, the earliest observation used being one made by Flamsteed on December 23, 1690. There is something very strange in the thought that science was able thus to deal with the motions of a planet for nearly a century before the planet was known. Astronomy calculated in the first place where the planet had been during that time; and then, from records made by departed observers, who had had no suspicion of the real nature of the body they were observing, Astronomy corrected her calculations, and deduced more rigorously the true nature of the new planet's motions.

But still stranger and more impressive is the thought that from researches such as these, Astronomy should be able to infer the existence of a planet a thousand million miles further away than Uranus itself. How amazing it would have seemed to Flamsteed, for example, if on that winter evening in 1693, when he first observed Uranus, he had been told that the orb which he was entering in his lists as a star of the sixth magnitude was not a star at all, and that the observation he was then making would help astronomers a century and a half later to discover an orb a hundred times larger than the earth, and travelling thirty times farther away from the sun.

Even more surprising, however, than any of the incidents which preceded the discovery of Neptune was this achievement itself. That a planet so remote as to be quite invisible to the naked eye, never approaching our own earth within less than twenty-six hundred millions of miles, never even approaching Uranus within less than nine hundred and fifty millions of miles, should be detected by means of those particular perturbations (among many others) which it produced upon a planet not yet known for three-quarters of a century, seemed indeed surprising. Yet even this was not all. As if to turn a wonderful achievement into a miracle of combined skill and good fortune, came the announcement that, after all, the planet discovered in the spot to which Adams and Leverrier pointed was not the planet of their calculations, but travelled in an orbit four or five hundred millions of miles nearer to the sun than the orbit which had been assigned to the unknown body. Many were led to suppose that nothing but a most marvellous accident had rewarded with such singular success the calculations of Adams and Leverrier. Others were even more surprised to learn that the new planet departed strangely from the law of distances which all the other planets of the solar system seemed to obey. For according to that law (called Bode's law) the distance of Neptune, instead of being about thirty times, should have been thirty-nine times the earth's distance from the sun.