penumbra, is a phenomenon which takes place beneath the level of the brighter part of the Sun's photosphere.' Summing up the results of this portion of their researches, they express their belief that,

1. The umbra of a spot is nearer the Sun's centre than the penumbra, or, in other words, it is at a lower level.

2. Solar faculæ, and probably also the whole photosphere, consist of solid or liquid bodies of greater or less magnitude, either slowly sinking or suspended in equilibrium in a gaseous medium.

3. A spot including both umbra and penumbra is a phenomenon which takes place beneath the level of the Sun's photosphere.

As respects the sequent series of researches by which Messrs. De La Rue, Stewart, and Loewy have endeavoured to estimate the influence of the planets upon the solar spots, it is to be remarked that the evidence adduced seems as yet not wholly decisive. They believe that it has been rendered probable that Venus exerts a special influence on the solar spots, and that the conjunctions of the planets also affect importantly the condition of the solar photosphere. There is room, in my judgment, for some doubt as to the justice of either conclusion. It should not be forgotten that the planetary system presents so many periodic relations as to render it almost certain that any observed periodic changes in the Sun's condition may be associated statistically with some period of planetary motion-sidereal, synodical, nodical, or other

wise.*

There is a remark towards the close of Carrington's volume on the solar spots, which bears very significantly on this subject. After exhibiting the relation between the phenomena of the solar spots (as tabulated by Professor Wolf) and Jupiter's variations of distance, he says, that from the year 1770, there is a very fair agreement between maxima of frequency and maxima of Jupiter's radius vector, and between minima and minima;' . . . but in the two periods which precede that date there appears to be a total disagreement.' It is important,' he then adds, 'to see before us an instance in which eight consecutive cases of general, but imperfect agreement, between the variations of two physical phenomena, are shown to be insufficient to base any conclusion upon, at the same time that they powerfully stimulate further inquiry with the view of ascertaining whether the discrepancy may admit of future explanation.'

I would by no means be understood to imply, however, that I regard the conclusions of Messrs. De La Rue, Stewart, and Loewy, respecting the influences exerted by the planets on solar phenomena, as inadmissible. On the contrary, I regard them as, on the whole, the most probable yet advanced. Based as they are on observed facts, and on statistical relations, they are worthy of the most attentive consideration.

* I was much struck with this fact when perusing a valuable contribution by Professor Kirkwood to the subject of planetary influences on solar phenomena. His long experience in dealing with such matters enables him to exhibit relation after relation, each showing a remarkably close agreement, as respects period, with periodic solar phenomena.

They do not seem to me, however, to be by any means demonstrated, nor are they so regarded (it is proper to add) by their propounders.

It remains only that I should indicate in a general, and necessarily brief manner, those features of the Sun's surface which recent observations have revealed to us.

Let me begin with the famous willow-leaves.' It was announced by Mr. Nasmyth in 1862 that the pores seen in the solar photosphere are 'polygonal interstices' (I quote Sir John Herschel's account) between certain luminous objects of an exceedingly definite shape and general uniformity of size, whose form (at least as seen in projection in the central portions of the discs) is that of the oblong leaves of a willow-tree. These cover the whole disc of the Sun (excepting the space occupied by the spots) in countless millions, and lie crossing each other in every imaginable direction.' The appearance of the Sun, according to this view, is exhibited in figs. 47 and 48, both of which are from drawings by Mr. Nasmyth.

This announcement led to a controversy which still remains undecided. Mr. Dawes asserted his belief that no such interlacing as Mr. Nasmyth described is ever observable among the small bright spots which lie scattered over the general ground of the photosphere; that these spots can in no sense be said to resemble willow-leaves, though they present every variety of figure and size; and, lastly, that they had been long known to solar observers, and are, in fact, no other than the nodules of Sir William Herschel. The only

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situation,' he wrote, in which I have usually noticed them to assume anything like the shape of willow

leaves, is in the immediate vicinity of considerable spots, on their penumbræ, and frequently projecting beyond it for a small distance on to the umbra,—an appearance with respect to which, in April, 1852, I used the following expressions:-"The interior edge of the penumbra frequently appears extremely jagged; the bright ridges on its surface, which are directed nearly towards the centre of the spot, being seen projected to irregular distances on to the cloudy stratum (or umbra), and looking much like a piece of coarse thatching with straw, the edge of which has been left untrimmed." After nearly twelve years of careful