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breaks in the S. and N. borders passed through the western ends of the earliest streaks of light thrown on the floor. This line appears to be coincident with the great fault crossing Plato. With reference to this I have the following note:-" This phenomenon, the western extremities of the streaks falling in a line with the breaks in the N. and S. borders, was well observed in January 1870. An elevation of the ground in the direction of this fault has been seen. It would, however, appear that differences in the lengths of the streaks would depend not on any unevenness of the ground, but on the relative depths of the gaps in the W. border."

1870, January 10, 2h to 8.-From ten to sixteen hours after epoch of sunrise at the equator, E. long. 4° 6'·∙1, © — 8, Jan. 9, 16h, equal to 170° 27' 8. This was by far the finest observation of sunrise on Plato by no less than seven observers, viz. Messrs. Gledhill, Pratt, Elger, Neison, Birmingham, Joynson, and Birt. Mr. Gledhill's record is so full and so interesting that a reproduction of it will convey a vivid impression of the progress of illumination of a lunar formation as the sun rises upon it.

Jan. 10, 2. Cloudless. Terminator just on the E. border of Plato; can just see the outline of the crater, which now lies in deep shadow. On the E. side the lofty steep wall just N. of a triangular formation marked II E 2 glowed intensely in the solar rays.

3. The E. wall from the great breaks in the S. and N. borders appeared as a bright narrow band. The curved outline of the N.E. border was bright, sharp, and narrow, but the lower slope within could not be seen. I could fancy that the W. part of the floor is, if possible, deeper in shadow than the E. half. [This phenomenon has often been witnessed, and has been attributed to the reflection of the strong light of the eastern interior from the dark floor. Upon attentively contemplating this degradation of shadow near its eastern boundary, it will often be seen that it is not simply a reflection from the floor, but apparently the illumination of a something above the floor.-W. R. B.]

3h 45m. A bright narrow broken line was seen between the two breaks on the E. and N.E. The outline of II E42 is not yet visible.

4h 18m. At this moment (12 hours 18 minutes after epoch) the first streak of light fell upon the floor. Within it and near its western extremity was seen No. 3 as two elevated objects, very near each other, but quite distinct. I could not detect shadow between them after hard gazing, although it was easily seen to the N.E. of the lower object. The streak was three times the breadth of the two objects together where it enclosed them, and it became broader near the N.E. border of Plato; it was brightest about and to the west of No. 3, and inclined a little downwards at the E. end. * The two components of No. 3 are of the same size apparently, are equally but not very bright; they lie nearly E. and W. of each other, but the E. component is a very little to the N. of the other.

4h 30m. The streak widens. I could not detect motion in it. I now carefully placed the wire on the great gap in the west border; the line passed along the axis of the streak. The west angle of the streak is not sharp, but rounded, and lies a little beyond No. 3. The lower of the cones of No. 3 touches the lower edge of the streak. It now assumed a fan shape, being broadest at the E. end, which is now more than halfway to the E. border. 4h 40m. The streak is now much wider. I think I see a minute elevation a little to the E. of No. 3 and in the streak. The two components of No. 3 are now bright and sharp, with shadow on the east. Another streak has been barely visible or suspected for a few minutes; it lies to the S. of the

former and near the S. border. It runs parallel with the northern streak, SIT is about half its length, and has its western extremity over a point a little E. of No. 3. It is narrow, and extremely faint and difficult." A minute of two later it was seen better, also a still fainter and narrower line to the north of it, which is parallel with it and the northern streak. The most southern streak produced to the E. would graze the southern edge of II E42. 4 50m. Now the shadows from the W. wall take shape. The south shadow, which extends up to the S. border, goes directly into the gap at the S. edge of II Ev2. The next pointed shadow to the N. of this goes direct to the middle of II E42; it is extremely pointed at its E. end for more than half its length, and is suddenly wider at the W. end. [This appears to indicate that the peak which throws the shadow is very needle-like.] I cannot be quite sure that this shadow for the next 10m or 15m really extended up to the E. border. It became so faint and narrow and line-like that it could not be well seen near the border. Then, again, the floor for some distance (say a distance equal to the width of II E42) lay in rather dark shadow. The floor between the shadows was not bright up to the E. border of Plato; all along the foot of the E. slope a dark shadow lay, and this interfered with an exact determination of extremities of shorter shadows from the W. wall. The next shadow to the north was a broad parallel-sided belt, which proceeded to the E. border as such. Its upper or S. edge extended to the N. end of II E42, and its lower or N. edge cut the border of Plato just below, or to the north of II E42. A line through No. 3 to the gap in the S.E. border cuts the W. angles of the two southern bright spaces between the shadows.

5. No. 3 lies on the lower edge of the lowest bright space or upper edge of the lowest shadow. The shadow still clings to or is in contact with No. 3, and either extends to the E. of it, or No. 3 throws a shadow to the E. The floor along the E. border is still dusky; it is brightest at that part in line with No. 3,

5h 5m. A very fine narrow shadow is now seen to stand off from the shadow below and in contact with No. 3; it is this which touches No. 3.

5h 15m. The upper shadow is now clearly pointed, and falls short of the border. [This is probably the shadow of the peak between B. & M.'s y and d.] I still see a minute elevation just to the N.E. of No. 3. It is now just on the tip of the lowest pointed shadow, and about halfway from 3 to the N.E. border. [This spot is No. 32; it was discovered in streak ẞ by Mr. Elger on December 15, 1869.-W. R. B.]

5h 45m. Floor at the foot of the E. border is still dark, except at the extreme N. The long broad shadow is now retiring from the E. border, and is seen faintly bifurcated; the lowest or northern fork is the longer, but this broad shadow still seems to have its N. and S. edges parallel.

6h. Now the dark shadow on the S. border breaks up, and a fine pointed shadow separates from its northern side, which if produced goes quite into the gap at the southern edge of II E2. The bright W. angle above this shadow goes back towards the W. until under the great gap in the S. border. The great central shadow is now easily seen bifurcated; the lower peak is the longest, and reaches nearly up to the east border. The tip of the shorter shadow to the N. reaches just to No. 3; the next to the N. is rather longer. 6h 20m. The object to the N.E. of 3 (32) is easy, elevated, and bright. Now 4 is seen, also a large elevated object (7) about halfway from it to the N. extremity of II E42, and on this line.

6h 30m. The great S. band of shadow goes straight into the gap at the S. end of II E42. The E. portion of the floor for some distance from the foot

of the slope is still dusky. The shadow of the N.E. component of No. 3 is easy, and lies to the N.E. A line from the lower edge of the shadow in the great gap of the west border along the lower edge of the central shadow goes into the gap at the N. end of II E42. This shadow is now finely bifurcated; the lower or northern peak is the longer.

8h. Spot No. 1 is now seen as a large striking object. It seems to be in the path of the upper fork of the central shadow, and looks like the shadow of one of Jupiter's satellites on the disk. [In Mr. Birmingham's sketch of May 19, 1869, -8=286° 37′·3, the upper or southern fork of the central shadow is longest, while in the present series of observations the northern is the longest. This is not a solitary instance of variation in the shadow of this peak. Mr. Birmingham is in agreement with Mr. Gledhill in referring spot No. 1 to the upper or southern fork. In my paper on the spots and shadows of Plato (Transactions of Sections, p. 17, Report of British Association for the Advancement of Science,' 1869), I remark that Rosse and Birmingham have drawn No. 1 with the shadow of d just receding from it. Challis's shadow of terminates by a straight line; neither fork was visible, for he carefully measured the two angular points. Rosse drew the termination of the shadow as from two pinnacles upon the summit, with No. 1 between them. These variations are doubtless azimuthal; nevertheless they are of great importance, as we hope presently to show.]

8h 5m. Spot No. 1 is a large, lofty, very prominent cone. Close to the N.E. component of No. 3, and to the N.E. of it, is seen a black shadow curved to the N.E., with a bright elevated object close to the curve. I see the two components of No. 3 as bright distinct objects; then, close to the N.E. foot of the N.E. component, comes a large circular shadow quite black, embracing a bright object to the N.E.

8h 15m. Spot No 4 is already getting rather difficult and hazy, although it lies far away in the bright eastern floor. Spot No. 17 is now seen just on the lower edge of the uppermost pointed shadow. No. 1 is bright and large, free from the long shadow. Shadow still lies on the eastern floor at the foot of the slope. Mr. Pratt, the same evening, Jan. 10, noticed a peculiar feature of the eastern part of the floor corroborative of Mr. Gledhill's observation of the dip to the foot of the east border. He says, "A peculiar feature of the eastern part of the floor in sunlight observed. Between what was probably the eastern margin of the sector b and the foot of the interior slope of the E. rim was a decidedly darker tint, as if that part of the floor was lower than the rest, and perhaps falling towards the border; the western margin followed very closely the form it would have if the whole space between the sector b and the border were depressed." In my own record, Jan. 10, 4h 48m, the Crossley equatorial 7-3-in. aperture, eye-piece No. 4, power 122, with slot, I say: The S. spire of sunlight apparent; it is directed towards the middle of II E42. Neither of the spires of light reach the border, indicating the floor to dip near the border."

Mr. Gledhill summarizes his observations, under the head of "points determined," as follows:

First. The position, size, alignment, and order of development of the streaks [of sunlight, as distinguished from those that make their appearance afterwards] which first fall on the floor. They are evidently the solar rays passing through the gaps on the border.

Second. The floor on the E. at the foot of the inner slope lies in shadow more or less deep until the giant shadows from the W. border have retreated westward beyond the centre of the crater.

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. w Gledhill's to be noticed under Feb. 9, 1870, it would

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Fig. 5.

appear that spot No. 1 is situated on the ridge marking the great fault. (See interval 24h to 36".)

1870, May 8, 8h to 10h. 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, è, 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 e, 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 E. 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.

γ

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 :

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"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.

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'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.'

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