spheric agency. It is true that rain-water, especially when assisted by humus derived from the vegetable soil above, is capable of enlarging crevices. But the erosion resulting from its chemical action is limited to spaces where stalactitic and stalagmitic deposition is not going on. This deposition evidently rather tends to preserve caverns than facilitate their destruction. The detachment of chips from rocks by frost in favourable situations is a process likewise limited to spaces where the chips have not accumulated to too great an extent. All atmospheric agencies which can only remove matter a short distance must tend to choke up or glut their sphere of action. Without the assistance of a powerful transporting agent they can never, in such situations as the Cheddar ravine and its caves, make permanent progress in the great work of denudation. But supposing atmospheric action during millions and millions of years to be capable of producing a vacuity as large as the Cheddar ravine, its form would still remain to be explained. Sufficient time allowed, a colony of ants might be considered capable of rearing a mountain mass equal to the Alps, but an examination of the form of the Alps would at once forbid the idea of insects having been the architects. Atmospheric agents, whether operating above or under ground, are now producing nothing similar in form to the main features presented by the Cheddar ravine. That the cliffs have been modified in exposed situations by frost is evident, but the modification has been in the direction of destroying and not developing the characteristic forms of the cliffs. The same remark applies more or less to the action of fresh-water in caves.

The common notion that the Cheddar ravine is a crack or rent may, in a very limited sense, be correct. It is possible, if not probable, that at first there may have been a narrow winding fracture similar to that behind the High Tor at Matlock. It is, however, certain that no fracture ever occurred sufficient to disturb the angle at which the strata dip in a south-easterly direction, which on both sides of the ravine exactly corresponds. A very little observation will be sufficient to convince any one that the ravine has been mainly, if not entirely produced by the abstraction of an immense mass of limestone rock. The following diagram will show the stratigraphical structure of the locality, the dotted lines representing the strata which have been removed.

From the foregoing observations I think it must appear obvious that the clean removal of a stupendous quantity of rock must form the burden of any satisfactory explanation of the origin of the Cheddar Cliffs. It is true that in some places the rocks are now crumbling, but there is no agent (with the

exception of man) to carry the detritus away. In most places the rocks have preserved their original smoothness and regularity of outline. Here and there concave undercuts run hori

Section of Cheddar Ravine.-A, rocks removed; BB, remaining rocks; co, terraces; dd, undercuts; e, road.

zontally along the the faces of cliffs, while at the base of others there are cavernous recesses with water-worn roofs. But the most instructive forms of rock surface are the planes caused by jointing and bedding. They show that the mode in which the adjacent blocks or masses were carried away was not a process of granular dissolution, nor even fragmentary dilapidation, but a bodily displacement. The cause must have been equivalent to the translation of large blocks of limestone. To borrow an illustration from the well-known cheese of the neighbouring plain of Cheddar, if a farmer were to find one morning that a part of a cheese was missing, and that the surface left was smooth and regular, he would conclude that some person had cut a slice with a knife, not that a mouse had been nibbling at the cheese during the night; or, suppose the farmer were to find that a whole cheese had been removed from his storeroom, he would at once conclude that a power capable of carrying it away in a lump had been concerned in the theft. A puny agency, which carries on its work grain by grain, or bit by bit, cannot leave a smooth, plain, and regular surface of any extent; but a violent and powerful agency, while it is not incapable of leaving a rough surface (circumstances being favourable), mainly tends to produce breadth and uniformity of

contour.

The most philosophical way of trying to explain natural phenomena is to seek for similar phenomena now in course of being produced; and many modern sea-coasts exhibit fac-similes of the Cheddar cliffs. The forms of these cliffs are precisely those which would result from waves driven by storms into a narrow inlet of the sea. At a greater elevation, near the summit of the Mendip hills, smoothed (sometimes polished), rounded, hollowed, perforated, and grooved surfaces of rocks may here and there be traced. The way in which they have been shaped

may be seen on the neighbouring sea-coast at the present day. At a little lower level than the Cheddar ravine there are generally-acknowledged indications (such as sea-shells*) of the sea having once covered the plain between the Mendip range and Polden Hill. During the great glacial submergence the sea may have washed through the Cheddar ravine, and completed, if not entirely effected, its denudation.

DOUBTS AND FACTS CONCERNING LINNÉ.

THE progress of investigation into the evidence of recent changes in the lunar crater Linné has cast considerable doubt upon the opinions so positively expressed by Schmidt, and accepted by most astronomers. In order to place the question as fairly as possible before our readers, we now publish a short paper by Mr. Birt, F.R.A.S., and some extracts from an important communication just made by Mr. Huggins to the Astronomical Society, and published in the "Monthly Notices ;" and also a letter from the astronomer Wolf, recently read before the French Academy.

THE NEW CRATER ON LINNÉ.

BY W. R. BIRT, F.R.A.S.

The question of change on the moon's surface, supposed to have been manifested in the case of the crater Linné, with which our readers are acquainted, remains undecided. Respighi, on the Continent, as well as several eminent astronomers in our own country, having come to the conclusion that no change whatever has taken place in the condition of Linné, and that if any appearances have been presented indicating change, such appearances are to be explained either by defective observations, by unfavourable conditions of our own atmosphere, by variations in the angles under which we see lunar objects, or by different incidences of the solar light falling upon them. There can be no doubt that each of these circumstances materially affects the appearances of lunar objects, and it is the more important in the instance which is now exciting considerable attention, to know more fully the facts rather than to rest on the conclusions that may have been drawn from a partial examination of facts presented, it may be, by a single series of observations.

* At Burtle, in the marshes of the river Brue, there are sand-banks full of marine shells, which are believed to indicate a comparatively recent and partial submergence of the land.

The results that have as yet been arrived at, and which are supported both by English and Continental observations, are as follows:

First. The existence of a shallow crater, usually presenting the appearance of a whitish cloud, which, by the way, is of variable size; the crater itself has been very rarely seen. Respighi saw it on the 10th of May, 1867, during a perfectly tranquil state of the air. Knott caught a sight of the ring on January 12th, 1867, and, on the same evening, in moments of quiet air and good definition, Buckingham noticed the shallow depression. Webb saw the ring on April 11th, 1867.

Second. In this shallow crater or depression, a little west of the centre, a small crater with a well-marked interior shadow has been seen more or less distinctly, both in England and on the Continent, since November, 1866; in some cases as a perfect crater, in others portions only have been detected. The evidence tending to establish the existence of this small crater is certainly beyond dispute.

Third. Herr Schmidt, of Athens, carefully observed Linné from October 16th, 1866, and during November, 1866, without having detected either the large shallow crater or the small one within it. The rim of the small crater appears to have first arrested his attention on December 13th, 1866, as a delicate white hill; Buckingham seems to have first seen the shadow as a black spot on the following evening, December 14th.

In all former records of Linné nothing is said of two craters, one within the other. Linné is simply described as a crater.

In the older records the diameter of Linné is given by one authority (Schmidt) as 15 German miles, and by another (Beer and Mädler) as 1.4 German miles.

Since December 14th, 1866, the diameter of the white cloudy mass has been measured nine times. Schmidt has given two estimates of its extent, October 18th, 1866, at 2 German miles, and December 27th at 2000 toises only.

Three estimations of the size of the small crater have been given; the first, 1867, February 11th, by Secchi, at most of a second. The second, some time in April or May, 1867, by Respighi, viz., 4 seconds. The third, by Wolf, 1867, June 12, at 1 second. These estimations differ very considerably the one from the other.

In cases of measurement the values were obtained in seconds of arc. The estimations were in German miles, or

toises.

As the value in miles, or English feet, of a second of arc at the moon's apparent centre increases as the object is removed from the moon's centre in the proportion of the secant of the angular distance from the centre, it is easy to

find the value in seconds of arc of the estimations on the one hand, and in English feet of the measures on the other. From the data given, the following table has been constructed:

This table furnishes three sets of numbers: first, the diameter of the crater in 1831, and about that epoch; second, the estimations and measures of the whitish cloud, in which there are considerable variations; and third, the estimations of the small crater. Under this head the most serious difference is that between Secchi and Respighi.

It would be highly improper to question for a moment the estimation of Respighi. The difference, however, between his estimation and that of Secchi, of nearly 26,000 English feet, calls for some remark. Respighi's observations were made with great care, and it is probable that he might have seen an opening which he estimated at that diameter, especially as the small crater was seen with greater distinctness in April. The diameter of this opening may, from some cause or other, not have been permanent in its extent. I am quite satisfied, that with the Royal Society's refractor of 4 in. aperture (Respighi's was 4 French inches aperture) I could have seen and measured a crater of 4"-0 in diameter; in fact, I have many smaller on the British Association outline map, which I have not only seen, but discovered with the 44 inch aperture, power