Although the evidence favouring Schiaparelli's theory was now strong, yet it was well that at this stage still more convincing evidence was forthcoming. The date of the November display has changed since the Leonides were first recognised, in such sort as to show that the position of their path has changed. The change is due to the disturbing attractions of the planets. It occurred to our great astronomer Adams, discoverer with Leverrier of distant Neptune, to inquire whether the observed change accorded with the calculated effects of planetary attraction, if the Leonides are supposed to travel in any of the smaller paths suggested by astronomers, or could be explained only by the assumption that the meteors travel on the widely-extending path corresponding to the 33¼ years period. The problem was worthy of his powers—in other words, it was a problem of exceeding difficulty. By solving it, Adams made that certain which Schiaparelli and his followers had merely assumed. He showed beyond all possibility of doubt or question that of all the paths by which the periodic meteoric displays could be accounted for, the wide path carrying the November meteors far beyond the track of Uranus was the only one which accorded with the observed effects of planetary perturbation.
It was in the confidence resulting from this masterly achievement that in 1872 some astronomers (among them Professor Alex. Herschel, one of Sir J. Herschel's sons) announced the probable occurrence of a display of meteors when the earth crossed the track of Biela's missing comet. An occurrence of this sort was alone wanting to complete the evidence for the meteoric theory. It had been found that the August Perseids move as if they followed in the track of a known comet; the path of the November Leonides had been shown to be identical with that of another comet; if astronomers could predict the appearance of meteors at the time when the earth should pass through the track of a known comet, even those who could not appreciate the force of the mathematical evidence for the new theory would be convinced by the meteoric display. Possibly such observers would have been satisfied with a meteor shower which would not have contented astronomers. The display must have special characteristics to satisfy scientific observers. The path of a body following Biela's comet being known, and its exact rate of motion, the direction in which it must enter our earth's atmosphere (if at all) is determined. Calculation showed this direction to be such that every meteor would appear to travel directly from the constellation Andromeda,—from a point near the feet of the Chained Lady. A meteor might appear in any part of the sky, but its course must be directed from that point, otherwise it could not possibly be travelling in the track of Biela's comet.
The event corresponded exactly with the anticipations of astronomers. On the evening of November 27, 1872, many thousands of small meteors were seen. In England between 40,000 and 50,000 were counted. In Italy the meteors were so numerous that at one time there seemed to be a cloud of light around the region near the feet of Andromeda whence all the meteor-tracks seemed to radiate. The meteors were unmistakably travelling on the track of Biela's comet. They overtook the earth on a path slanting downwards somewhat from the north—precisely in the direction in which Biela's comet would itself have descended upon the earth if at any time the earth had chanced to reach the part of her path crossed by the comet's when the comet was passing that way.
Strangely enough, a German astronomer, Klinkerfues, seems to have regarded the meteoric display of November 27, 1872, as an actual visit from Biela's comet. He telegraphed to Pogson, Government Astronomer at Madras, 'Biela touched earth on November 27, look out for it near Theta Centauri;' which, being interpreted, means, Biela's comet then grazed the earth, coming from the feet of Andromeda, look for it where it is travelling onwards in the opposite direction—that is towards the shoulder of the Centaur. As Biela's comet had in reality passed that way twelve weeks earlier, the instructions of Klinkerfues were somewhat wide of the mark. However, Pogson followed them, and near the spot indicated he saw two faint cloud-like objects, slowly moving athwart the heavens. These he supposed to be the two comets into which the missing comet had divided. It so happens, strangely enough, that these objects, though moving parallel to the track of the missing comets, were neither those comets themselves, nor the meteor flight through which the earth had passed a few hours before. They were probably somewhat richer meteor clouds, fragments (like the cloud through which our earth had passed) of this most mysterious of all known comets.
To-night, or perhaps to-morrow or next night (for the position of the meteor flights is not certainly known) we shall probably see meteors travelling in advance of the main body. For the earth passes during the next three days across the orbit of Biela's comet, about as far in front of the head as she passed behind the head in 1872. Now, there is no known reason for supposing (on à priori grounds) that meteors get strewn behind a comet's nucleus more readily than in front of it. The disturbing forces which would tend to delay some meteoric attendants would be balanced by forces which would tend to hasten others. As a matter of fact it would seem that the meteor flights which follow a comet's nucleus are commonly denser than those which precede the nucleus. Yet in 1865 many thousands of Leonides were seen which were in advance of the main body forming the comet of 1866. In 1859, 1860, and 1861, many Perseids were seen, which were in advance of the comet of 1862. So that we might fairly expect to see a great number of Andromeds to-night (or on the following nights) even if we had none but the probabilities thus suggested to guide us. But since many were seen on November 27 last, when the head of the comet, now some four months' journey from us, was a whole year's journey further away, it seems probable that on the present occasion a display well worth observing will be seen should fine weather prevail. It will be specially interesting to astronomers, as showing how meteors are strewn in front of a comet. How meteors are strewn behind a comet we already know tolerably well from observations made on the Perseids since 1862 and on the Leonides since 1865.
During the cold weather of last December (1878) we heard much about old-fashioned winters. It was generally assumed that some thirty or forty years ago the winters were colder than they now are. Some began to speculate on the probability that we may be about to have a cycle of cold winters, continuing perhaps for thirty or forty years, as the cycle of mild winters is commonly supposed to have done. If any doubts were expressed as to the greater severity of winter weather thirty or forty years ago, evidence was forthcoming to show that at that time our smaller rivers were commonly frozen over during the winter, and the larger rivers always encumbered with masses of ice, and not unfrequently frozen from source to estuary. Skating was spoken of as a half-forgotten pastime in these days, as compared with what it was when the seniors of our time were lads. Nor were dismal stories wanting of villages snowed up for months, of men and women who had been lost amid snowdrifts, and of other troubles such as we now associate rather with Siberian than with British winters.
Turning over recently the volume of the 'Penny Magazine' for the year 1837, I came across a passage which shows that these ideas about winter weather forty years ago were entertained forty years ago about winter weather eighty or ninety years ago. It occurs in an article on the 'Peculiarities of the Climate of Canada and the United States.' Discussing the theory whether the clearing away of forests has any influence in mitigating the severity of winter weather, the writer of the article says, 'Many persons assert, and I believe with some degree of accuracy, that the seasons in Europe, and in our own island particularly, have undergone a remarkable change within the memory of many persons now living; and if such really be the case, how few attempts have been made to account for this change, since no great natural phenomenon, like that of clearing away millions of acres of forest timber, and thereby exposing the cold and moist soil to the action of the sun's rays, has recently taken place here; so that if the climate of Great Britain has actually undergone a change, the cause, whatever that may be, must be of a different nature from that generally supposed to affect the climate of North America.' It must be explained that, though in this passage the writer does not speak of a diminution in the severity of the winters, it is a change of that sort that he is really referring to. He had said, a few lines before, that 'some of the older inhabitants of North America will declare to you that the winters are much less severe "now" than they were forty or fifty years ago,' and in the passage quoted he is discussing the possibility of a similar change in Europe, where, however, as he points out, the cause assigned to the supposed change in America has certainly no existence. Since 1830, by the way, the theory has been advanced that the supposed mildness of recent winters may have been caused by the large increase in the consumption of coal, owing to the use of steam machinery, gas for lighting purposes, and so forth.