[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.] : 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." 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. cular. Mr. Gledhill describes spots Nos. 1 and 3 as bright, cir 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. the inner E. wall. Mr. Gledhill records spot No. 1 as a crater brightest on 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 illuminaSee intervals 108h to 96h, and 12h to Oh *. tion. Mr. 1870, October 4. Mr. Gledhill records No. 1 as an elevated object. 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. Interval 60 to 72 hours. 1870, July 8. Mr. Gledhill records Nos. 1 and 17 as bright spots badly seen. Mr. Elger records No. 5 as seen only by glimpses, but brighter than No. 1. 1869, August 17. Mr. Pratt inserted the positions of the spots observed by him "by independent estimation," also "their relative positions with respect to light streaks" were very carefully determined as follows: No. 1. On the dark surface near the junction of two streaks. 3. In the middle of a light streak. 4. In the middle of a light streak (sector)*. 17. On the dark surface close to a light streak (W. edge of sector). 14. Near the margin of a light streak. Interval 72 to 84 hours. 1870, April 11. Mr. Elger records No. 5 nearly as bright as 17, which he regarded as fainter than at last lunation; 14 and 16 were easy, 24 and 25 seen by glimpses. Mr. Gledhill records Nos. 1, 3, 30, and 17 as bright circular disks. Mr. Pratt detected the six spots which he observed with difficulty. 1870, March 13. Mr. Gledhill writes: "Unless I am very much mistaken indeed, 34 was an easy object, i. e. No. 1 came out easily double;' also, as the E end of the floor slopes to the east, spots Nos. 6 and 7 may be seldom seen on this account (?)." To this I add: "This may be the case while the moon is passing from perigee to apogee." Mr. Gledhill says further: “No 3 (and 30) very easy, wide, double; 3 is the larger, both equally bright: 30 is not seen nearly so often as 3; when only one is seen it is 3." 1870, June 9. Mr. Elger recorded 5 as brighter than 17. 1870, February 11, 6.30. Mr. Gledhill found spots Nos. 1 and 17 as very sharp bright disks, but could not detect interior shadows; he describes Nos. 1, 17, and 3 as sparkling. Of No. 1, he says, it often comes out double; last year I often saw it thus. I am now almost sure I see a minute object close to the west of it (34). Interval 84 to 96 hours. 1870, December 4. Mr. Elger writes:-"The marking connecting the middle and east arm of trident, which was, I believe, first seen by Mr. Pratt last spring, I found a very easy object, fully as bright as the brightest portions of the trident;' it follows the curvature of the south border, and crossing the last arm of the trident, terminates about halfway between the latter and the west limit of the sector.' During the May and June lunations, I had faint glimpses of it; but it was then a very much more difficult object than it is now." The apparition of this streak appears in some way to be connected with spot No. 5, the variations in visibility of which are considerable. As, from the discussion of visibility, the connexion of these variations with illuminating, visual or atmospheric (terrestrial), changes appears to be untenable, it may be suggested that, if the first maximum, Aug.-Sept. 1869, resulted from increased activity, ejecta may have been thrown out and produced the faint streak which was seen on the west of No. 5 by two observers. At or about the second epoch of increased activity, a larger quantity of ejecta *Mr. Gledhill has frequently observed spot No. 4 at the angle formed by the converging sides of the "sector." may have been thrown out, producing a brighter streak, extending eastward as well as westward. The most interesting circumstance connected with this streak is its conformity in direction to that of the south border, as if some peculiarity of the surface existed in the neighbourhood of No. 5, of a depressed character, which received the outflow or outthrow of the ejecta. Another noteworthy circumstance is, that this streak was not recorded earlier than May 13, 1870. 1870, September 6. Mr. Gledhill records Nos. 1, 3, 17, and 30 as bright disks, also that definition was good, and that the streaks and spots seemed to stand out in relief. 1869, November 15. Mr. Gledhill writes::- "The spots Nos. 1, 17, and 3 do not appear as a mere white spot on the floor of Plato would do. There is a sharpness and clearness of contour and a brightness (uniform) of surface which could only belong to a crater or peak. I have often been struck with this. This remark applies to them whenever they are well seen. I can only liken them to the small round disks of bright stars seen in the transitinstrument. Spot No. 4 never looks like Nos. 1, 17, or 3." To this I append the following query :-Do the clearness and sharpness of the contour of spots Nos. 1, 17, and 3 result from seeing the shadowless interiors of the craterlets? If so, on what agency does the appearance of the mere white spots depend? Do Nos. 1, 17, and 3 vary in this respect with good states of our atmosphere? Mr. Pratt records a spot new to him on the N.W. of 3, about half as far from 3 as is 4 on the opposite side, and aligning with 3 and 4; he speaks of it as exceedingly small. I have numbered it 29. He also observed spot No. 8, which he describes as fainter than 29, and situated about one third the distance from 3 towards 4. On this evening Mr. Pratt very carefully scrutinized No. 3 and its immediate neighbourhood; the following are his notes transmitted to me:"First. The second spot, which I have always observed with 3 (and which I learn from Mr. Birt I have always placed in the same relative position as has Mr. Dawes, who discovered it, and of whose alignment I was before quite unaware), is exceedingly close to 3 on the N.E. I estimate the distance at 2", and its position with respect to 1 was very carefully judged to be 145° to 150°, reckoning from S. round by E., which I afterwards found by comparison to be about the angle represented in my former sketches. Second. A third spot, S.E. of 3, and twice as far from it as Mr. Dawes's, was observed. Its relative size was judged to be one fourth, while that of the second spot was one third of 3. The direction was from 3 towards 4." [This spot I take to be 8.-W. R. B.]. "Another peculiarity in 3 was, that it was just included by the light streak, but still quite on its edge, as was also its smallest companion. I now determined very carefully the colour of the immediate localities of all spots visible. After independently noting it for each spot, I found on summing up that the whole were upon the light streaks, with the exception of No. 1, around and towards which the light streak was softly shaded off." 1870, July 9. Mr. Whitley glimpsed spot No. 17 with difficulty. Interval 96 to 108 hours. 1870, April 12. Mr. Gledhill records Nos. 1, 3, and 30 as bright circular disks, 17 as a bright disk, also 6, but seen only, once or twice. Mr. Pratt records No. 1 as very dense and bright, 3 and 4 as hazy, and 16 and 22 difficult. 1870, May 12. Mr. Gledhill records Nos. 1, 3, and 17 as fine bright disks, No. 4 a spot, but seldom seen. Marking a, Mr. Gledhill records as the brightest, and Mr. Elger mentions the part east of No. 16 as very bright and well defined; this, as well as the remarks of Mr. Elger on May 9, may tend to throw some light on the decreased visibility of Nos. 13 and 19 (see Interval 24 to 36 hours). On this evening Mr. Whitley observed and described the markings, giving a sketch of the same. Mr. Elger's sketch of the north part of Plato and Mr. Whitley's are not in accordance. The time at which Mr. Whitley made his observations is not mentioned; Mr. Elger's 8.45 to 11. 1870, March 14. Mr. Elger writes: "The markings were not well seen; the eastern arm of the trident' was the brightest, and could be traced from the south rim to No. 1, passing to the west of No. 5: the marking y was very plain, the rest of the markings were faint and difficult to make out." In contrast with this indistinctness on Plato, Mr. Elger says, "[In spite of the haziness of the sky, the markings and minute details of the Mare Imbrium were seen with unusual distinctness]." In the English Mechanic,' No. 312, March 17, 1871, p. 602, article " Mars," by F.R.A.S., the author speaks of the indistinctness and partial dimming on the surface of the planet, accompanied by the presence of dark lines in its spectrum, coincident with those referable by Father Secchi to the vapour of water. The indistinctness and dimming of detail are alike distinguishable on Mars and the Moon; and in addition we have on the Moon a number of spots becoming vividly bright with a high sun. From Dr. Huggins's observations, the spectral lines of the vapour of water are absent in the lunar spectrum. 1870, June 10. Mr. Elger recorded No. 17 decidedly brighter than No. 5 and equal to No. 3; 14 only glimpsed once or twice; 16 and 25 frequently seen. 1869, December 15. This evening Mr. Elger discovered spot No. 32. He described it as N.E. of spot No. 3, nearly aligning with 17 and 4, and situated on a brush of light (Gledhill's streak 6), extending from No. 3 to the N.E. rim of Plato. 1871, March 3. Mr. Pratt observed 16 spots, viz. 1, 3, 4, 5, 14, 17, 21, 20, 23, 29, 0, 18, 13, 19, 7, 6, arranged according to relative brightness. Of these Mr. Pratt speaks of Nos. 20 and 21 as being far above their usual brightness. Situated as they are near the north border, the Moon going north in latitude, they were not in the most favourable position for observation; their great brightness is therefore remarkable, and connected with this is an increase of brightness in the streak a. The new streak between Nos. 5 and 17 Mr. Pratt saw with ease, joining the east arm of the "trident" with the "sector" from closely south of 17 to opposite 5. 1870, October 6. Mr. Gledhill records Nos. 1, 17, and 30 as fine bright disks; Nos. 5 and 6 equal. Mr. Elger observed Nos. 14 and 16, not seen by Mr. Gledhill; 14 was equal to 5. Interval 108 to 120 hours. 1870, September 7. Mr. Gledhill records Nos. 1 and 3 as fine sparkling disks, and 4 as a hazy spot. Mr. Neison records Nos. 1, 3, 4, and 5 pretty distinctly visible; 17 brilliant but not well defined; 14 and 16 faint and very faint respectively. 1869, November 16. Mr. Gledhill says, "I never saw the floor so bright. The spots 1, 17, 9, 3, and 30 appeared just like small stars in the transitinstrument on a windy night." At 10, 11, and 12 hours Mr. Gledhill remarked that spots Nos. 3, 1, 9, and 17 formed a sparkling curve, and were fine easy objects, seen at a glance at any moment; he says they were very striking. On the contrary, he speaks of spots 23, 16, 19, 13, and 14 as very difficult objects; none were ever easy objects. Of 9 and 11 he says, "I never saw them so easily and well as to-night." The following notes are |