But perhaps the most striking way to exhibit the sympathy is to combine different variations of the needle into one picture. And first we must remark that there is another important variation of the earth’s magnetic action which we have not yet considered. We have mentioned the swing of the needle to and fro, and the swing up and down, and these correspond to changes in the direction of the force of attraction on the needle. But there may be also changes in intensity of this action; the pull may be a little stronger or a little weaker than before, and these variations are not represented by any actual movement of the needle, though they can be measured by proper experiments. We can, however, imagine them represented by a movement of the end of the needle if we suppose it made of elastic material,Daily curves. so that it would lengthen when the force was greater and contract slightly when the force was less. If a pencil were attached to the end of such an elastic needle so as to make a mark on a sheet of paper, and if for a moment we exclude the up and down movements, the pencil would describe during the day a curve on the paper, as the end of the needle swung backwards and forwards with the change in direction, and moved across the direction of swing with the change in intensity. Now when curves of this kind are described for a day in each month of the year, there is a striking difference between the forms of them.Difference between summer and winter, and between sun-spot maximum and minimum.
Cause unknown. During the summer months they are, generally speaking, comparatively large and open, and during the winter months they are small and close. This change in form is seen by a glance at Plate [XIII.], which gives the curves throughout the whole of one year. Let us now, instead of forming a curve of this kind for each month, form a single average curve for the whole year; and let us further do this for a series of years. (Plate [XIV.]) We see that the curves change from year to year in a manner very similar to that in which they change from month to month in any particular year, and the law of change is such that in years when there are many sun-spots we get a large open curve similar to those found in the summer, while for years when there are few sun-spots we get small close curves very like those in the winter. Hence we have two definite conclusions suggested: firstly, that the changes of force are sympathetic with the changes in the sun-spots; and secondly, that times of maximum sun-spots correspond to summer, and times of minimum to winter. And here I must admit that this is about as far as we have got at present in the investigation of this relationship. Why the needle behaves in this way we have as yet only the very vaguest ideas; suggestions of different kinds have certainly been put forward, but none of them as yet can be said to have much evidence in favour of its being the true one. For our present purpose, however, it is sufficient to note that there is this very real connection, and that consequently Schwabe’s sun-spot period may have a very real importance with regard to changes in our earth itself.

Plate XIII.

Plate XIV.

But I may perhaps repeat the word of caution already uttered against extending without sufficient evidence this notion of the influence of sun-spots to other phenomena, such as weather. A simple illustration will perhaps serve better than a long argument to show both the way in which mistakes have been made and the way in which they can be seen to be mistakes. There is at the Royal Observatory at Greenwich an instrument for noting the direction of the wind, the essential part being an ordinary wind-vane, the movements of which are automatically recorded on a sheet of paper.Illustration of spurious connection. As the wind shifts from north to east the pencil moves in one direction, and when it shifts back again towards the north the pencil moves in the reverse way. But sometimes the wind shifts continuously from north to east, south, west, and back to north again, the vane making a complete revolution; and this causes the pencil to move continuously in one direction, until when the vane has come to north again, the pencil is far away from the convenient place of record; on such occasions it is often necessary to replace it by hand. Then again, the vane may turn in the opposite direction, sending the pencil inconveniently to the other side of the record. During the year it is easy to count the number of complete changes of wind in either direction, and subtracting one number from the other, we get the excess of complete revolutions of the vane in one direction over that in the other. Now if these rather arbitrary numbers are set down year by year, or plotted in the shape of a diagram, we get a curve which may be compared with the sun-spot curve, and during a period of no less than sixteen years—from 1858 to 1874—there was a remarkable similarity between the two diagrams. From this evidence alone it might fairly be inferred that the sun-spots had some curious effect upon the weather at Greenwich, traceable in this extraordinary way in the changes of the wind. But the particular way in which these changes are recorded is so arbitrary that we should naturally feel surprise if there was a real connection between the two phenomena; and fortunately there were other records preceding these years and following them which enabled us to test the connection further, and it was found, as we might naturally expect, that it was not confirmed. On looking at diagrams (Plate [XV.]) for the periods before and after, no similarity can be traced between the sun-spot curve and the wind-vane curve, and we infer that the similarity during the period first mentioned was entirely accidental. This shows that we must be cautious in accepting, from a limited amount of evidence, a connection between two phenomena as real and established; for it may be purely fortuitous. We may particularly remark that it is desirable to have repetitions through several complete periods instead of one alone. It is possible to reduce to mathematical laws the rules for caution in this matter; and much useful work has already been done in this direction by Professor Schuster of Manchester and others, though as yet too little attention has been paid to their rules by investigators naturally eager to discover some hitherto unthought-of connection between phenomena.

Faculæ follow spots

With this example of the need for caution, we may return to phenomena of which we can certainly say that they vary sympathetically with the sun-spots. Roughly speaking, the whole history of the sun seems to be bound up with them. Besides these dark patches which we call spots (which, by the way, are not really dark but only less bright than the surrounding part of the disc), there are patches brighter than the rest which have been called faculæ. With ordinary telescopes, either visual or photographic, these can generally only be detected near the edge of the sun’s disc; but even with this limitation it can easily be established that the faculæ vary in number and size from year to year much in the same way as the spots, and this conclusion is amply confirmed by the beautiful method of observing the faculæ with the new instrument designed by Professor Hale of the Yerkes Observatory. With this instrument, called a spectroheliograph, it is possible to photograph the faculæ in all parts of the sun’s disc, and thus to obtain a much more complete history of them, and there is no doubt whatever of their variation sympathetically with the spots.and the chromosphere. Nor is there any doubt about similar variations in other parts of the sun which we cannot see at all with ordinary telescopes, except on the occasions when the sun is totally eclipsed. Roughly speaking, these outlying portions of the sun consist of two kinds, the chromosphere and the corona, the former looking like an irregular close coating of the ordinary sun, and the latter like a pearly halo of light extending to many diameters of the sun’s disc, but not with any very regular form.