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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).
13 and 19. In the middle of a light streak.

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.

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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.

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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 From Dr. Huggins's observations, the spectral lines of the vapour of water are absent in the lunar spectrum.

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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

important:-"Nos. 1, 3, and especially 1 (which surpasses all in sharpness, and perhaps in brightness sometimes) are fine easy objects, with moderate altitudes. Now Linné never appears like these except when near the evening terminator. As to y Posidonius I never see it sharp and crater-like (white and bright) when the sun is up. I could not see it at all the other day when the morning terminator was a degree or two from it." Of white spots Mr. Gledhill remarks: "I called some spots mere white spots, because I have never seen them otherwise; by-and-by I may catch them near the terminator, and have reason to change the term. I fancy that when the terminator is a morning one the effect on objects differs from that given by the evening terminator."

Interval 120 to 132 hours.

1871, March 4. Mr. Neison saw spot No. 14 very indistinct, and barely brighter than a longitudinal steak running in a direction from No. 13 to past No. 14, which was then situated upon it. It appeared to have its origin at the point of convergence of Gledhill's and 8. On the same evening, Mr. Gledhill recorded but not d. On March 4, Mr. Neison saw No. 16 (once only) as a peculiar light-marked spot on a patch of broken light trending westward. Mr. Neison also recorded parts of the N.W. and S.E. portions of the floor indistinct from broken light and light streaks.

1870, June 11. Mr. Elger recorded spots Nos. 5 and 16 as seen only by glimpses.

Interval 132 to 144 hours.

Fig. 7.

1870, April 14. Mr. Gledhill records Nos. 1, 3, 4, 17, 9, 11, and 30 as bright round disks. Mr. Elger writes, under date of April 26, 1870, relative to his observations of April 14, as follows:-"That the visibility of the spots is connected with the position and brightness of the markings (as you suggest) is, I think, most probable: it is clear that the spots at present known are mainly confined to the districts occupied by the markings, and that the floor of Plato is divided by the latter into three nearly equal areas, A, B, C, as on sketch. Areas A and C are covered with markings, but area B is devoid of them. If we compare the number of spots in area B with the number of spots in areas A and C, we shall find that there are only two spots (23 and 11) in area B, while in area A there are ten, and in area C no less than twenty-three. It is true that small portions of the areas A and C are without

markings; but the spots within those areas are, without an exception, situated either upon the light streaks or close to their borders. These facts seem to me very suggestive, and point to an intimate relation between the spots and markings. As observations accumulate, your present belief in a connexion between the phenomena will, I think, be placed beyond doubt." In connexion with the above, the following quotation from a letter by Mr. Pratt, dated 1870, April 22, is interesting:"Very curious the difficulty there is in observing such delicate detail; possibly instruments and eyes will show differently, independently of the mental bias and accumulation of pre1871.

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vious impressions; and I rather fear that telescopes much larger than my own cannot help us out of the difficulty."

The difficulty to which Mr. Pratt alludes is particularly felt with regard to that indispensable method of determining positions "measurement." Mr. Gledhill has executed some measures of the positions of the principal spots and the extremities of the light markings, and Mr. Pratt has aligned several of the spots with objects on the border; but so exceedingly delicate are the details, and so seldom is the state of the atmosphere sufficiently translucent and free from agitation, that to obtain an approximate plan of the spots and markings from measurement is necessarily a work of time. Pending this, in the above sketch both spots and markings have been inserted, partly on alignment and partly by estimation. The two light regions are well sprinkled with spots, as pointed out by Mr. Elger; and it is not a little interesting to notice that the nearly spotless area coincides with the region between the "trident" and the "sector," with its prolongation to "Webb's Elbow" near the N.W. border. In the absence of more accurate detail, which is likely to be obtained from Mr. Gledhill's measurements, the sketch (fig. 7) will serve as a guide for ascertaining if the spots and markings preserve their relative positions; and in this connexion the remarkable change of locality, if it be so, of spot No. 5 may be mentioned, Mr. Elger having seen and recorded on three occasions (1870, March 14, May 13, and October 10) its position on the eastern edge of the eastern arm of the "trident." It is possible there may be two neighbouring spots in this locality which have not yet been seen together. The importance of recording with every observation of spot No. 5 its position with regard to the eastern arm of the "trident" is obvious. The light streak supposed to be connected with No. 5 is too far south, or the spot is too far north, on the sketch.

1870, May 13. Vide "Indications of intermittent visibility" (p. 88).

1870, January 15. Mr. Gledhill observed as many as 22 spots, the second greater number seen on any one occasion. Vide "Indications of intermittent visibility." Spots Nos. 1, 3, and 17 are described as very easy, large, bright, sharp objects; No. 4 as jumping into view and not steadily seen. No. 34 was discovered this evening; it has not been observed since March 13, 1870, when it was recorded as an easy object.

1869, August 20, 21, and 23. Mr. Gledhill gives three spots close to the N.W. border, which he has marked 13, 19, and 16. No. 16 being too far east for that spot, I have regarded it as 20; if, however, Mr. Gledhill really saw 16, its degree of visibility would be slightly increased. On August 23 Mr. Pratt gives 16 in its proper position, and he observed the same number of spots as Mr. Gledhill; but Mr. Gledhill saw No. 12 and 31, which Mr. Pratt did not see, Mr. Pratt recording Nos. 7 and 30, not seen by Mr. Gledhill.

1870, September 8. Mr. Neison records spot No. 4 as a flat indistinct spot; 17 sharp but bright, darkening on one side, and showing traces of a crater-formation.

Interval 144 to 156 hours.

1870, August 10. Mr. Neison records spot No. 3 as apparently oval; the longer axis of the ellipse is in the direction of No. 31.

1870, October 8. Mr. Elger mentions No. 14 as very casy, 16 easy, and 17 seen only occasionally.

Interval 156 to 168 hours.

1870, May 14. Mr. Elger recorded No. 16 easy; 5, 14, and 17 faint; 25 and 32 seen by glimpses. Mr. Gledhill records 1, 17, 3, and 6 as bright disks, 4 not well seen, and 5 as a bright spot.

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