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middle of the obscure area, stretching straight across it from one extremity to the other, with much the same appearance as in winter in a closed chamber the sun's rays admitted through a window are wont to present, or as they are seen in the distance when cast through openings in the clouds, or like comets' tails at night in a clear sky stretched out at length in space, as we remember to have seen in the one which in the years 1680 and 1681 was so conspicuous to all Europe. This appearance, never before seen by me in this or any other lunar spot, is represented in the figure which I give below.

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❝1, 2. The lunar spot named Plato, and the ruddy ray of the sun thrown across its dark floor from the margin of the spot 1, white and turned towards the sun. It was thus observed at Rome on the Palatine Mount, Aug. 16, 1725, at 1 hour after sunset, with the 150-palm telescope of J. Campini.

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It is proposed to astronomers and physicists, for their consideration and judgment, whether this is to be taken as an indication of an aperture piercing the border of the spot which is turned towards the sun, through which opening the rays are cast and appear as through a window; or whether it is rather to be thought that they are refracted rays, which are bent from the top of the border towards the bottom, and appear of a ruddy tint as they are wont to do in our own atmosphere at sunrise and sunset, and so give reason for admitting the existence of some denser fluid like an atmosphere surrounding the lunar globe." I have the following remarks on the above, dated June 4, 1867 :"Bianchini appears to have been one of the earliest observers who noticed 'detail' more particularly. Hevel, Riccioli, Cassini, and others aimed more at delineating the entire surface, which of course included much detail that. is becoming more and more valuable every day; still such observations as Bianchini's, recorded in his Hesperi et Phosphori,' are of great value, especially as the appearances described and delineated could not find place in a more general work."

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Schröter, in his 'Selenotopographische Fragmente,' vol. i. p. 334, §§ 256, 257, refers to the observation of Bianchini, and also to one of Short's in 1751, April 22. It would appear that Bianchini's suggestion of an aperture or hole in the W. rim of Plato was not verified by Short, who seems to have observed

the shadows of the three peaks y, d, and e of B. & M., which are represented by Schröter in t. xxi. The shadows of these and other peaks on the W. wall have been very frequently observed of late years.

I am not aware that Bianchini's observation has been verified. The peculiar appearance which he has delineated depends not only on libration, but also on the angle which the terminator makes with the meridian; for it is clear that the direction of the terminator must form a tangent to a line passing equally through the depression in the wall to produce the appearance seen by Bianchini; and it is highly probable that it is of very rare occurrence, as seen from the earth, the variation in the angle of terminator with meridian being as much as 3°.

While transcribing the above (April 22, 1871) I have considered the Bianchini phenomenon more closely. During the year 1870 the opportunities for observing sunrise on Plato were comparatively numerous, and certainly not the slightest appearance of Bianchini's streak was detected; on the other hand, the positions of the earliest rays of sunlight on the floor have been determined, with some degree of precision, for the portion of the luni-solar year during the period of the observations. If the configuration of the W. wall is different now from what it was in Bianchini's time, the phenomenon may be explained by the supposition that the gap or pass N. of the peak d was lower than at present, and has been raised by "landslips" on one or both sides, which are of extensive occurrence on the moon as recognized by Nasmyth; the absence of further observations by Bianchini on the same evening, however, leaves the matter in doubt.

Short records, in the Phil. Trans. for 1751, p. 175, that on April 22, 1751, he saw a streak projected along the flat bottom of Plato. Soon after he saw another streak parallel to the first, but somewhat lower [or northerly], which in a very short time divided into two. He found a gap in the wall opposite the first streak, and also one in the direction of the lower one.

Not only is Bianchini's observation at variance with modern observations, but Short's also. The order of appearance of the streaks of sunlight on the floor on Jan. 10, 1870, is, first, the broad streak through the wide gap; second, the southern streak north of the peak y. The appearances of Short's streaks were in the reverse order.

The following record of observations by Schröter on July 30, 1789, at 9h 48m, kindly translated by Mr. Gledhill, will illustrate Mr. Elger's observation on January 10, 1870:

"Selenotopographische Fragmente,' § 250, vol. i. p. 329. "A different, more beautiful, and more magnificent view of Plato is obtained when, with the rising sun, the first traces of an extremely faint twilight are seen on the grey floor of the crater, and when the first beams of light are thrown over the mountains into the plain below. This view of Plato, which lasts only for a few minutes during the slow monthly rotation, and for which one may wait for a year and yet not see it, I saw on the 30th of July, 1789, 9h 48TM. As in the 8th figure of t. xxi., the terminator had advanced from W. to E. as far as a, B. To the W. of this the greatest part of the border lay in the light of day [or on the day side], and only the small portion to the E. of a, B was illuminated on the night side. The whole inner grey surface, on the contrary, was still hidden by the shadows of the lofty mountains on the border, and on the S. border there was also a low spot filled with shadow. While I was observing the shadows of the inner surface with power 161, I became aware of something to the E. of the middle of the floor, as if the dark surface were in a kind of fermentation. A few seconds later I saw here in

two places an extremely distinct unveiling or brightening which closely resembled a very faint twilight. Both places appeared dark, blackish, and contrasted so slightly with the other night-shadows, that at first I was uncertain whether or not I perceived a real difference in the obscurity. Meanwhile, after a few seconds both the light-spots became somewhat brighter, changed their form continually, until they soon became larger and notably brighter, and assumed the appearance given in fig. 8; and as no very marked change occurred while the observation was being made, I was by this time able to sketch them in their present clearer colour and increased size; but even yet they appeared a dark grey, so that, according to my arbitrary scale and a very approximate estimation, they were placed at only 10, or at most 10.

"Doubtless these present but always very dark colours were half-shadows, and were found there because in these two places only a part of the rising sun was visible over the irregular elevations on the western border; and these half-shadows I have often seen in the course of my observations when the terminator passes across grey surfaces. Soon after, the surface threw off the mask of night, and in a few minutes I could distinguish the line-like shadows lying across the whole floor thrown by the peaks on the western wall. If one, however, compares the shape of these two somewhat bright spots on the map with the position and shadows of the west border, and reflects that these bright spots, as I saw them, were surrounded by the shadows of night on the east, there can no longer be any doubt (if a different reflection of the light has no share in the matter) that the floor is not perfectly flat, but that these two places are somewhat more elevated; and with this supposition the observations given before quite agree."

The following notes have been kindly furnished by Mr. Pratt, relative to the foregoing description of sunrise :

"Jan. 10, 3. On 1870, March 10, I have notes of the same phenomenon, which I believe I forwarded at the time, recording the inability I experienced to rid myself of the idea that I was witnessing a true twilight. My observation of it extended over twenty-five minutes, at the end of which time I perceived the faintest trace of the formation of the spires."

"Jan. 10, 4" 18", spot No. 3. Query. Is the brightest spot of the streak, here mentioned as seen inclined to the north of No. 3, and I presume in close proximity to it, my spot No. 30? As far as I can understand the localities are identical."

"Jan. 10, 4" 50m, shadow of peak y. On a similar occasion I have observed the thin thread of the shadow lying across II E42, and have watched it slowly shortening and travelling down the interior slope of the rim, and had a good view of it lying on the floor just in contact with the foot of the slope." "Jan. 10, 8h, shadow of peak d. I do not remember to have ever seen the shadow of d otherwise than with the northern fork the longest."

On Bianchini's light-streak Mr. Pratt remarks:-" Bianchini's ruddy spire of light, which he observed at Rome, 1725, Aug. 16, and thought to be sunlight shining through an aperture in the west wall, would the want of achromaticity in his 150-palm telescope account for the colour? Still his unique view may prove valuable some day; and it is stimulating to perseverance on our part to multiply observations with our comparatively luxurious instruments to find such unwieldy telescopes capable of so much in the hands of a careful observer. I wonder if the crater G on the west exterior slope was recorded so long since, as its clean-cut form, as I have sometimes seen it, is suggestive of recent formation, and its locality such as to easily account for the filling-up of the aperture Bianchini supposed."

[The crater G is not seen in Bianchini's drawing of 1725, August 16, nor in that illustrating his observations of 1727, August 23 and September 22.W. R. B.]

Mr. Pratt remarks, that in Short's observation of 1751, April 22, the first streak of sunlight was on the upper part of the floor, followed soon after by a parallel streak somewhat lower. "It is important," says Mr. Pratt, "to learn what kind of telescope Short used during the observation; for as he was chiefly a maker of the Gregorian form, and as that construction does not invert the image, it may be possible his term lower may mean southerly instead of northerly, thus being in accord with modern observations."

"The very interesting translation of Schröter's notes of 1789, July 30, and his discovery of something on the eastern half of floor, as if a kind of fermentation was going on, and his discovery a few seconds later of an unveiling or brightening, closely resembling twilight, remind me," says Mr. Pratt, "very forcibly of my own observations before mentioned. The half-shadows of Schröter also remind me of what I have very often seen, as he describes ; but I cannot understand his explanation of them. As far as I can see, halfshadows presuppose an atmosphere; and a well-authenticated course of observations of them would be good proof of the latter's presence."

[If by the term "half-shadow" be meant the penumbral fringe of every true shadow, the rays of light emerging from opposite limbs of the sun, crossing beyond the object casting the shadow and then diverging, will fully explain such a fringe. In the case of the sun rising above the mountains, the reverse phenomenon occurs, viz. a gradual darkening fringe skirting the illuminated surface arising from less and less light arriving from the sun's disk; a true twilight is occasioned by the particles of an atmospheric medium being illuminated by the sun's rays while the luminary is below the horizon, and such I believe I have on several occasions witnessed.-W. R. B.]

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Interval 24 to 36 hours.

1870, May 9. Mr. Gledhill describes spot No. 1 as easy; a fine sharp crater, with raised walls, much black shadow within, the east inner slope bright he also records 3 and 17 as presenting the same appearance as No. 1. On October 3, at about 12h earlier illumination, Mr. Gledhill did not observe the crater character of these objects, but describes them as elevated objects. This is remarkable, as on Oct. 3 the moon's latitude was 1° to 2° S., while on May 9 it was 3° N., libration carrying Plato further from the eye, yet the crater character was more distinct. Mr. Elger records No. 17 as seen by glimpses.

As regards spots 13 and 19, the following remarks of Mr. Elger are interesting:"The northern portion of the floor, including streak a, was noted as equally light; the streak could not be traced." Mr. Gledhill writes, a not to be distinguished from the bright floor all along the north border. Mr. Elger found the same locality "all light on the 10th."

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1870, February 9. Mr. Gledhill first saw spot No. 4, its bright W. wall only. He says, This object seems to have lower walls than 1, 17, or 3." Mr. Gledhill writes: "For a few minutes I saw what appeared to be a very low ridge running from N. to S. across the floor of Plato. It runs from the N. border to spot 3, then curves to No. 1, and again bends back to the E. and reaches No. 17, and thence goes on to the S. border." [The low ridge mentioned by Mr. Gledhill is, so far as I know, new. It is not coincident with the great fault from N.W. to S.E. From a drawing subsequently sent to me by Mr. Gledhill, it would indicate a fracture, having its origin at spot

No. 1, diverging N.E. and S.E. to spots Nos. 3 and 17, and extending from them in opposite directions to the N. and S. borders.] At 5.30 Mr. Gledhill recorded that spot No. 4 is already indistinct; there is a dull yellow patch about it. No. 3 at this early stage of illumination Mr. Gledhill found to be single; he looked in vain for the other two adjacent spots, Nos. 30 and 31. 1870, Oct. 3. Mr. Gledhill records Nos. 1, 3, 17, and 30 as elevated objects. Mr. Elger found no trace of 3.

1870, March 11. Mr. Gledhill describes spots Nos. 1 and 3 as bright, circular.

Interval 36 to 48 hours.

1870, April 10. Mr. Gledhill records spot No. 1 as a large, sharp, circular crater, with internal shadow on W. side; also Nos. 3 and 17 as circular craters. Mr. Elger records Nos. 16 and 25 as frequently glimpsed.

1870, July 7. Mr. Whitley observed Nos. 1, 3, and 17 as craterlets, 4 a white spot, and glimpsed No. 11 very faint. On the same evening Mr. Neison recorded the floor as very dark, the spots indistinct, not visible continuously; and Mr. Elger could just trace the "sector."

1870, Jan. 11, 7.20. Mr. Gledhill describes spot No. 1 as a large round crater, larger than Linné, quite bright and circular, a very fine easy object. At 7.30 the same evening, he says "Linné also is now seen as a crater, with some shadow within on the west." At 7.45 Mr. Gledhill writes: "Now the N.E. inner slopes of craters Nos. 1 and 3 glow in the bright sun, while the S.W. inner slopes are in shadow. It is the N.E. inner slope which so often, in bad definition, comes out as a bright disk or semidisk."

1869, August 16. Mr. Pratt thus writes:-" Of these difficult objects [the spots], seven were seen many times during the hour; No. 1 often well defined as a crater, Nos. 3 and 4 as well-defined craters as No. 1, but accompanied with a nebulous light, perhaps caused by the companion spots to each, which, however, were never clearly defined owing to the minuteness of the objects and the short periods of definition clear enough. They both had a similar appearance."

1870, September 4. Mr. Neison records No. 4 as just observable, and 14 very faint.

Interval 48 to 60 hours.

1870, May 10. Mr. Gledhill records spots Nos. 1, 3, and 17 as elevated craters with little internal shadows. Mr. Elger records No. 5 as seen only by glimpses much fainter than 17; 16 and 14 easy.

1871, March 1. Mr. Gledhill records spot No. 1 as a crater brightest on the inner E. wall.

1870, August 6. Mr. Elger noticed the west portion of the floor of an even light colour. It is on this portion that the spots Nos. 13, 19, and 22, which have decreased of late in visibility, are situated. On the 24th of March, 1870, Mr. Gledhill observed the reverse, viz. the west part of the floor exhibited the darkest tint. It was, however, less in extent than the light portion given by Mr. Elger, and was seen under the opposite illumination. See intervals 108h to 96h, and 12h to Oh *.

1870, October 4. Mr. Gledhill records No. 1 as an elevated object. Mr. Elger found No. 14 more easy than 5 and 17; it was not seen by Gledhill. Nos. 3, 30, and 17 were seen as bright disks by Gledhill.

* These reversed tints are quite in accordance with the surface of the floor dipping on each side from the line of "fault" crossing Plato from N.W. to S.E.

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