Imágenes de páginas
PDF
EPUB

may be seen on the neighbouring sea-coast at the present day.

At a little lower level than the Cheddar ravine there are gene-rally-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.

depressir and good and, on the caught a sigbring a perfecen,

epressiona. In this scrater with ictly, bot

,. 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 1:5 German miles, and by another (Beer and Mädler) as .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

toisess the value in ment centre in in the proportios easy to

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 :

[blocks in formation]

Schmidt

36,449

5:17 | Crater B. and M.... 1831

33,482 4.83 Crater Schmidt .... 1866, Oct. 18 48,688 6.90 Whitish cloud Birt

1866, Dec. 15 | 81,920 11:61 | Whitish cloud Measured 1866, 18 | 49,886 7.07 | Whitish cloud | Measured 1866, , 19 51,650 7:32 | Whitish cloud | Measured

1866, , 21 47,627 6:75 Whitish cloud Measured Schmidt ..... 1866, 27 12.790 1.81 | Whitish cloud Birt

1867, Jan. 14 56,100 7.95 | Whitish cloud Measured Buckingham 1867, Mar. 14 | 42,336 6:00 Whitish cloud Measured Wolf .. 1867, June 12 31,752 4:50 | Whitish cloud Birt

1867, July 8 37,623 5:33 | Whitish cloud Measured 1867, , 9 49.420 7.00 Whitish cloud 1 Measured

1867, ,5 10 37,845 5:36 | Whitish cloud Measured Schmidt 1866, Dec. 13 1918:11 0.27 Delicate bill

1866, , 26 1695 0.24 Fine black point
1867, Jan. 25 1279 0:18 | Fine black point

1867, „ 25 1918:11 0.27 | Fine white peak
Secchi , 1867, Feb. 11 2352 0:33 Small crater
Respighi ...., 1867, Apr May 28,224 | 4:00 | Small crater
Wolf ......... 1867, June 12 70561.00 Small crater

- This table furnishes three sets of numbers : first, the chiameter 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 41 inch aperture, power

230. I have records of observations made on April 11 and May 11, 1867, in which I expressly state that I could not detect any object on the surface of Linné. With Mr. Barnes' silvered glass reflector (Wit) I very carefully examined Linné, on the 10th of June, 1867; but although both Mr. Barnes and Mr. Browning saw a white nucleus, I was unable to detect it, and saw nothing of a crater of the magnitude of 4":0, which I think I must have seen, had it been there.

With regard to the variations in the extent of the whitish cloud, I have observed phenomena of the same kind on the Mare Crisium, and about seventy sets of measures of Dionysius, yield the same result, though not to so great an extent as in the case of Linné.

OBSERVATIONS ON LINNÉ.

BY WM. HUGGINS, ESQ., F. R. S. In “ Monthly Notices” Mr. Huggins publishes a view of Linné, as seen by him on May 11, at 8h. 45m. It represents Linné as an oval white spot, and to the west of its centre shows a white ring surrounding a black spot. Mr. Huggins remarks, “At the time when the diagram was made, the shallow, saucer-like form of Linné was not seen; but I have detected it on other occasions. June 8, at 7h., when a great part of the light reflected from our atmosphere was removed by means of a Nicol's prism, I observed a shadow within the eastern margin of this shallow crater. When the diagram was taken, on May 11, the “interior of the small crater was in shadow, with the exception of a small part of it towards the east. The margin of the small crater was much brighter on the western side, and at this part appears to be more elevated above the surface of Linné. Under a very oblique illumination this high eastern wall appears as a small bright eminence, and casts a somewhat pointed shadow.”

On the 9th July, at 9, Mr. Huggins measured Linné, and found the

Length of the bright spot ..... 7":85
Breadth . . . . . . . . . . 6":14

Diameter of small crater . . . . . 1":71
The power employed was 500.

On the 14th Feb., 1866, Mr. Huggins examined the spectrum of the light from Linné, but could detect no lines not belonging to solar light.

Mr. Huggins quotes Schröter's description of Linné, and remarks that in Plate IX. of his “ Selenotopographische Fragmente," “ the place occupied by Linné is marked by a round white spot, and not by the figure of a crater." Schröter's words are, “Als ein weisses sehr kleines rundes Fleckchen erscheinende, etwas ungewisse einsenkung in sich hat;" which may be translated, “ Appearing as a white, very small round spot, with a somewhat uncertain depression in it.” Mr. Huggins observes that such a description coincides very well with existing aspects, and he does not think the absence of any notice by Schröter of a small interior crater is of much weight in indicating its subsequent formation. He adds, that “Lohrmann's description and that of Mädler do not appear to be in accordance with Schröter's observations, or with the present condition of the object.”

WOLF ON LINNÉ.

(From " Comptes Rendus,June 17, 1857). Since the 10th May I have noticed that the crater Linné continues to exist, but with a much smaller diameter than that of the crater indicated in the maps of Lohrmann or Beer and Mädler. In the centre of the white spot a circular black hole may be seen, bordered on the west by a portion of ground which seems prominent above the remainder of the spot. This slight extra elevation has already been described by Schmidt. Atmospheric circumstances did not allow me to obtain an irreproachable image of the moon before the 10th June. On that day, at 8 o'clock, Linné had already been in full light nearly 48 hours, and the central hole could be seen with perfect sharpness. It is a deep crater-deeper than most of the little craters surrounding it, if one may judge from the comparative intensity of the shadows; but its diameter is not equal to that of craters A and B of Beer and Mädler. The white spot which spreads radiatingly (s'étend en ragourant) round it, had, on the 12th June, a diameter of 4":5, that of Bessel being 7".7. The crater itself subtending a little less than one second. The perfect purity of the atmosphere, and the optical power of the telescope (Om. 40) which I employed, allowed a number of small craters to be seen very distinctly round Linné, or rather a number of small round holes without elevated margins, and which are not shown in Beer and Mädler's map. Six of these little craters form a very remarkable double range to the north and north-east of Linné. They are smaller than the craters in a line situate to the north-west of Linné, and noticed by Schmidt. I employed magnifications of 235, 380, and 620 times.

The brightness of Linné has not changed since Beer and Mädler's observations, for it is always equal to tbat of the white spot situated near Littrow, on the western margin or

B of Bees tend en meng that of

« AnteriorContinuar »