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Third. That spots Nos. 1, 3, 17, the object halfway between No. 4 and the E. border (7), the object halfway between No. 3 and the E. border (32), the object (if any) just to the E. of No. 3 (31), and the object S.W. of No. 1 at a considerable distance away are all elevated objects.

[Some time subsequently to these observations I received from Mr. Gledhill a drawing of nine crater cones seen on Jan. 10, 1870. They were Nos. 1, 3, 30, 4, 7, 9, 11, 17, and 32. I have not received any confirmation of the object a considerable distance S.W. of No. 1.-W. R. B.]

Fourth. The order in time of the appearance of the shadows.
Fifth. The time to a minute when light first falls on the floor.

[The discussion of the observations by intervals shows that the sun's light first falls upon the floor of Plato from ten to thirteen hours after the sun has risen at 4° 6'1 of E. long. on the equator according to season: a simple computation of the epoch of sunrise at this longitude and O-8 will be a guide to ascertain the illumination of Plato within twenty-four hours of the epoch.-W. R. B.]

Sixth. The interval between the appearance of light on the floor and the distinct perception of the shadows from the W. border is about twenty-five or thirty minutes.

Seventh. The great northern streak of sunlight is seen some fifteen minutes before the southern streaks are detected. This may be caused either by difference in elevation of the gaps in the W. border, or difference in level of the floor, or both may unite to produce the effect.

What can cause the duskiness of the eastern floor except depression of the floor? 1870, Jan. 10, 9h 0m. Mr. Elger saw spot No. 1 close to the shadow of the peak situated on the S. of the great gorge or opening in the W. wall. At 9h 10m the N. peak of this shadow was about clearing it; at the same time spot No. 4 could just be seen. Mr. Elger remarked that the shading round spot No. 1 was much darker than the central portion of the floor, and that this dark shading could be traced in an easterly direction to about one fourth of the distance between the spots 1 and 4: " this," says Mr. Elger, "would appear to indicate a fall in the surface of the floor from No. 1 towards the E. in section" (fig. 5). Schröter, if I remember rightly, alludes to some observations indicating similar irregularities in the floor. From Mr. Elger's observation, combined with one of Mr. Gledhill's to be noticed under Feb. 9, 1870, it would appear that spot No. 1 is situated on the ridge marking the great fault. (See interval 24h to 36h.)

W

Fig. 5.

E

1870, May 8, 8h to 10". Close of first interval of twelve hours. Epoch 7a 21h 20m. Mr. Elger writes, " On the evening of the 8th, between 8h and 10h, I had a fine view of sunrise; the air was remarkably steady; shadows and minute details seen to perfection."

1870, May 18. Mr. Elger writes:-" Re your statement as to the dip of the floor. Is there reliable evidence that the N.E. and S.E. areas of the floor are lower near their respective borders than towards the spotless central area? In January last I saw spot No. 1 in contiguity with the shadow of No. 2 peak (western wall); the surface of the floor east of No. 1 was then, of course, seen under very oblique light. Judging from the shading and general aspect of the surface in the neighbourhood of No. 1, there appeared to be a very rapid fall from spot No. 1 to spot No. 4; if this be so, the stem of the trident' would be a depression in the surface."

1870, April 9. Twenty-three hours after epoch of sunrise at 4° 4'7 on

equator, E. long., Mr. Elger records spots Nos. 1 and 17 in contiguity with shadows of high peaks on west wall [y and d]: Nos. 1, 3, 4 very plain [seen also by Mr. Pratt], 17 faint, 25 only glimpsed, 7 suspected; no markings seen. Mr. Pratt records on same day shadows of y, d, and e on floor nearly similar to 1869, Nov. 12, excepting that & showed a second point south of chief one, and that of e did not exhibit a cleft.

The importance of such careful observations as those which have furnished the data for this interval cannot admit of question. The determination of the epoch at which the floor first becomes illuminated, as compared with the epoch of an easily computed phenomenon (sunrise at a given longitude on the equator), places at once within our reach the means of ascertaining when the appearances witnessed during the interval 10 to 24 hours after sunrise, at 4° E. long. on the equator, will be repeated*. This is, however, a small result compared with the forms and progressions of the shadows; for by their aid, especially if well sketched, and their lengths carefully measured, or even estimated in parts of those of the three measured peaks y, d, and €, the distance of the west wall from the terminator being at the same time ascertained, the irregularities of the west wall at sunrise, and by a similar process those of the east wall at sunset, may be obtained with tolerable precision by B. & M.'s method described in Der Mond,' § 65, p. 98, and in the Report of the Lunar Committee of the British Association, Report,' 1867, p. 15. We have thus the power, by multiplying such observations, of becoming intimately acquainted with the breaks and gaps, the elevations and towering pinnacles of the wall, and are in a position for handing down to our successors details that may enable them to detect changes, if such should occur, of sufficient magnitude to become perceptible. The shadows which I enumerated on Jan. 10, 1870, were six,-the longest y, one between and ♪, d with its two peaks or saddle form, one south of €, and €. Mr. Joynson, of Liverpool, gives in his drawing of the same date two peaks to d. The irregularities both of the floor and border have come out by these observations with marked distinctness, and tend greatly to settle for the present epoch the main features. If, however, changes are in progress, they may be, as on the earth, extremely slow.

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The appearances recorded on January 10, 1870, being so different to that witnessed by Bianchini, August 16, 1725, the following translation, by my friend Mr. Knott, from Bianchini's work Hesperi et Phosphori Nova Phenomena' (Romæ, 1728), will doubtless be read with interest :

"Under the auspices of the Cardinal de Polignac, two large telescopes, 94 and 150 Roman palms long, by Campini, were prepared and erected, and on the 16th of August, 1725, the following observations of Plato were made.

"Although on that night we were only able to turn the telescope 150 palms long, on the moon we detected, in the lunar spot named Plato, a phenomenon not previously observed. The moon was at the time a little past its first quadrature with the sun, which it had attained on the previous day, and the spot Plato fell on the periphery of solar illumination, where is the boundary of light and darkness in the lunar hemisphere exposed to the The whole of the very elevated margin, which on all sides surrounds the spot like a deep pit, appeared bathed in the white rays of the sun. The bottom of the spot, on the other hand, was still in darkness, the solar light not yet reaching it; but a track of ruddy light, like a beam, crossed the

sun.

*The longitudes of the terminator at 60° N. latitude on the equator, and at 60° S. latitude, Greenwich, midnight, during the lunation, are given monthly in the Astronomical Register.'

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. "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:—

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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 48. 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 40, or at most 0.

"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, 4h 18m, 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."

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"Jan. 10, 4h 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 casily account for the filling-up of the aperture Bianchini supposed."

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