This tabular

summary shows:

1st. That the quarters arrange themselves in an entirely different order under the different heads, with the single exception of the second being the quarter of greatest average rainfall and also of greatest average wet-day rate of rain.

2nd. That the least average rainfall was in the quarter immediately preceding the new moon, instead of being, as Sir J. Herschel supposes, about the full moon.

3rd. That the maximum number of wet days was in the third quarter, and the minimum in the first; thus differing in every particular from the results stated by M. Arago to have been obtained in Germany and Paris, on the one hand, and in the south of France on the other, which, as we have seen, differed from one another.

This discussion may be appropriately closed, perhaps, by echoing Arago's remark, that "the question requires to be examined afresh."

V. THE APPROACHING TOTAL SOLAR ECLIPSE.

By R. A PROCTOR, F.R.A.S., &c.

THE eclipse of next December is less remarkable in many important respects than the two total solar eclipses now commonly known as the Indian and American eclipses of 1868 and 1869. The former of these was distinguished among all the eclipses of recent times by the exceptional extent to which the lunar disc overlapped, during central totality, the concealed disc of the sun. For more than six minutes at some stations no direct solar light was visible. The eclipse of last year was not distinguished in this particular way, though the duration of totality-at some stations exceeding four minutes-was far from inconsiderable. What rendered the American eclipse so extremely important, even more important than the Indian one, was the fact that a large proportion of the track of the moon's shadow lay across a region dotted over with well-armed observatories. It is probable that on no previous occasion has so large an array of practised observers been employed in scrutinizing the phenomena of a total eclipse; and it is absolutely certain that so many appliances had never before been employed to render the researches of the observers effective.

In both respects the approaching eclipse is less important. The greatest duration of total obscuration will be but 2 m. 11 s.; and the track of the moon's shadow only skirts the region within which the principal European observatories are situated. In fact, the only parts of Europe traversed by the shadow are the southern provinces of Spain and Portugal, Sicily, the southern extremity of Italy, and

parts of Greece and Turkey. And of these regions only those in the Spanish peninsula and Sicily are practically available, because in the others the duration of totality will be less and the sun will have but a small elevation. In Greece, for instance, and Turkey, though the phenomena of totality may chance to be well seen, yet the chance is not such as would justify an expedition from the principal astronomical centres of Europe. The best places of all for observing the eclipse will undoubtedly be those along or near the track of totality in Algeria. These, however, will probably be left to the astronomers of France.

Fig. 1 shows the actual presentation of the earth towards the sun, and the course and shape of the moon's shadow on December 22nd next. The hour is supposed to be solar noon at Greenwich. The earth must be conceived to be rotating in the direction shown by the arrow (on the equator), and at such a rate that any meridian line in the figure will reach the place occupied by the next meridian towards the right in two hours. The black spot to the west of Spain represents the shadow of the moon at the hour named. This shadow is surrounded by the penumbra, a portion of which, however, remains throughout the eclipse beyond the northern limits of the earth's disc. The course of the shadow is indicated by the curved line taken through the black spot. If an observer on the sun could trace the apparant path of the moon's centre across the earth's disc, he would not find it curved in this way, but appreciably straight. As the earth is rotating, however, the disc turned towards the sun undergoes an appreciable change during the duration of central eclipse, and the motion of the different points of the earth along parallels curved like those shown in the figure, causes the path of the moon's centre with reference to the earth's globe (distinguished here from her disc as seen from the sun) to have the shape indicated in the figure.

Central eclipse begins on the earth generally at twenty-six minutes before noon,-in other words the black shadow shown in the figure as already well advanced is supposed to have entered on the disc twenty-six minutes before the epoch corresponding to the figure. Central eclipse concludes for the earth generally at twentyone minutes past one, or eighty-one minutes after the epoch corresponding to the figure. The total interval during which the moon's shadow (as distinguished from her penumbra) falls upon the earth is thus 1 h. 47 m.; and the amount of motion due (during this interval) to the earth's rotation can be conceived by remembering that the southern extremity of Spain moves during totality from a place below the dark spot in the figure (and on the proper parallel, of course) to about the place occupied in the figure by Sicily.

It will be evident from a further consideration of the relations

presented in Fig. 1, that there are two respects in which this eclipse is unfavourable. First of all, the track of the shadow lies far from the centre of the disc. It is clear that, pro tanto, the shadow is rendered smaller by falling near the outer parts of the disc; because these parts lie farther than the centre from the sun. Secondly, the elevation of the sun at the time of eclipse is not considerable. Since the sun is vertical at the place which occupies the centre of the earth's illuminated disc, and on the horizon for any place which lies on the circumference of that disc, it is obvious that when the track of the moon's shadow lies as in Fig. 1, the sun's elevation is relatively small during total obscuration. In the present eclipse, at the stations where the chief observing parties will be placed, the sun's elevation will be about 30 degrees, amply sufficient for ordinary observing purposes, but not altogether so great as might be desired for spectroscopic and polariscopic researches, and still less satisfactory for photography.

Fig. 2 presents on a larger scale the track of the moon's shadow, and the actual oval shape of the black spot which seen foreshortened in Fig. 1 appears as a circle. It will be seen that Odemira in Portugal, Cadiz and Xeres in Spain, Oran and Ratna in Africa, and Syracuse in Sicily, are the principal towns which lie very close to the central line. But it is probable that the stations will be selected without special reference to the proximity of towns; indeed for many purposes the less inhabited regions of a country are best suited for such observations as have to be made during eclipse.

Although Mount Etna is not very close to the central line, there are reasons for believing that a party stationed on the summit of this mountain would be enabled to make important observations. They would be more than twice as far raised above the sea-level as those observers were, who during the American eclipse obtained such favourable views of the solar corona from the summit of White Top Mountain. It will be remembered by our readers that General Myer reports the extension of the corona as seen from this station to have exceeded more than twofold the extension observed by those at lower levels. As there will probably be an English observing party near Syracuse, it would be a matter of the utmost interest and importance to compare their observations of the corona with those made at the summit of Etna.

At present, it may be mentioned in passing, there seems every reason to believe that two important expeditions will be sent from England to observe the eclipse. As we write, the arrangements are not complete, and there still remains a possibility that the whole undertaking may fall through. But it is hoped that this may not be the case, and that the large array of volunteers whose names appear in the list of the two proposed expeditions may be enabled to devote their energies to the work they have severally undertaken.

The main object of the astronomers of this and other countries will be to determine the nature of the corona. For this purpose, each of the English expeditions is to be divided into four parties. First, there will be the spectroscopists; secondly, the polariscopists; thirdly, the photographers; lastly, there are the general observers, who in our opinion are very far from forming the least important portion of the expedition.

The spectroscopic evidence obtained during the Indian and American eclipses is contradictory and unsatisfactory. Let it be remarked in passing, however, that it is not altogether so contradictory as has been asserted. The American observers appear to have been misled into the supposition that Major Tennant saw the ordinary solar spectrum-that is, that the Fraunhofer lines could be seen in the spectrum of the corona. And indeed in Prof. Roscoe's treatise On Spectrum Analysis,' it is stated that Major Tennant saw the ordinary solar spectrum, whereas "Professor Pickering, on the other hand, saw only a continuous spectrum." But Major Tennant's account expressly asserts that the spectrum he saw was continuous. He says, "What I saw" (the italics are his) "was undoubtedly a continuous spectrum, and I saw no lines. There may have been dark lines, of course, but with so faint a spectrum and the jaws of the slit wide apart, they might escape notice." Thus the continuous spectrum seen by some of the American observers is in perfect accordance with Major Tennant's observation. Indeed the mistake is rather fortunate than otherwise, because it led the American observers to search specially for dark lines such as they supposed Tennant to have seen; and, therefore, their failure to recognize any may be regarded as all but decisive of the matter.

Where Major Tennant's observations are not accordant with those made by the American observers, these latter observations are themselves wanting in accordance. For Professor Young saw three bright lines in the coronal spectrum, and Professor Harkness saw one bright line; whereas Professor Pickering, like Major Tennant in 1868, saw only a continuous spectrum. This discrepancy will, we may fairly trust, be cleared up during the approaching eclipse. It may perhaps be found that different parts of the corona give different spectra. It may be noticed, however, that the bright line seen by Harkness and the bright lines seen by Young were delicate objects, and would almost certainly have escaped notice had these observers used a much narrower or a much wider slit than they actually employed. Professor Harkness failed to see the line till he had slightly opened the slit; but he would probably have lost it equally had he widened the slit too much. May not Major

*The total quantity of light from the bright lines would be increased by widening the slit; but the intrinsic brilliancy of the broadened bands would be no greater than before. On the other hand, the intrinsic brilliancy of the continuous background would be increased by the change.

Tennant have failed through such a course? He says, "thinking that want of light prevented my seeing the bright lines which I had fully expected to see on the lower strata of the corona, I opened the jaws of the slit." It is worth noticing that failure may arise from this very adjustment. Too narrow a slit is clearly unfavourable, because a certain quantity of light is required for distinct vision; but on the other hand too wide a slit is equally unfavourable, because a certain relative superiority in the brightness of the lines (or in this case bands) over the background of the continuous spectrum is equally requisite. The obvious conclusion is, that a telescope of large aperture and therefore of high light-gathering power should be employed, and the light of the continuous background reduced as much as possible by increasing the dispersion.

As respects the polariscopic operations, there is a similar contradiction to be explained during the approaching observations. The observers of the Indian eclipse assert positively that the light of the corona is polarized in a plane through the sun's centre; the American observers, on the other hand, as positively deny this. The Astronomer Royal (than whom no higher authority-on this particular subject-exists) solves the difficulty summarily by expressing his belief that the observers in India were not sufficiently familiar with the principles of polariscopic research to interpret what they saw. In this case, and assuming a similar state of things in the case of the American observers, we must look forward to the approaching eclipse as likely to supply the first really reliable information yet obtained respecting the polarization of the corona. We cannot doubt that the observers next December will not fail from want of knowledge, since not only has the Astronomer Royal called special attention to the necessity of their carefully preparing themselves beforehand, but the government of the party has been assigned to Professor Pritchard, who is nothing if not a master of the science of theoretical optics. Our great fear is, however, lest the methods of testing light at present in vogue may not be sufficiently effective for the resolution of the somewhat difficult problems depending on the polarization of the corona. Whatever advantage there may ordinarily be in the use of well-tried methods, it may be questioned whether in this particular case more powerful instruments than the polariscopes at present in use might not be devised and employed with advantage.

We pass over the photographic department of the expedition; in the first place because there is every reason to feel confidence that under the able supervision of Messrs. Browning and Brothers (at Gibraltar and Syracuse respectively) the photographic arrangements will be exceptionally successful, and in the second because as regards the inquiry, which is the main purpose of the expedition, photography can teach us comparatively little. Unless the whole