important: Nos. 1, 3, and especially 17 (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 et the point of convergence of Gledhill's and d. 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.

G

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 easy, 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.

1870, September 9. Mr. Elger recorded No. 5 faint, 17 especially faint, 14 and 22 glimpsed, and 14 difficult.

Interval 168 hours to Meridian Passage.

1870, June 13. Mr. Gledhill has this remark: "For some time I have thought that when power 115 was used, spot No. 4 was almost at any time to be seen, or at any rate a condensation of the sector' at its apex was seen. On applying 240, however, the appearance vanishes, and no condensation or spot is seen, or perhaps only sometimes and at intervals."

Interval Meridian Passage to 168 hours.

1870, July 13. Mr. Gledhill records No. 1 as very bright.

1870, September 10. Mr. Elger records Nos. 25 and 16 as easy, No. 14 as seen by glimpses.

Interval 168 to 156 hours.

1870, August 12. Mr. Neison records "a spot seen on the border of No. 3, very small and hardly visible except at intervals, but pretty bright on edge only of the light marking." Mr. Neison suspected it to be No. 31, which it undoubtedly is according to the position which he has accorded to it on the diagram. Mr. Neison was the only observer who detected No. 31 during this lunation, on the 10th and 11th of August, as an elongation of No. 3. Mr. Elger, Mr. Gledhill, and Mr. Pratt appear to have missed it. Query, was the group Nos. 3, 30, and 31 in greater activity about this time? Mr. Neison has this note, "3. Faint indications of its being a crater very distinct." Mr. Pratt records: "During the long period since I last saw the light streaks I have had little opportunity to study former sketches, and so was free in a measure of the bias of them. Yet on sketching those seen, the forms, positions, and directions coincide with former drawings, notably the trident a, 6, n, l." Mr. Pratt also notices a remarkable increase in brightness of spot No. 22, so as to attract especial attention. Neither Messrs. Elger, Neison, Ormesher, nor Gledhill noticed this spot, although they were observing on the same evening as Mr. Pratt, who further remarks "that in moments of best definition the area comprised between Nos. 19, 1, and 4 was not nearly so well displayed as the rest of the floor, giving a strong impression of an obscuring medium located there." [This observation of the streak 7, the existence of which has been questioned, is perfectly independent of any suspicion of its non-existence, as it occurred some months before the question was raised.]

1870, October 10. Mr. Elger found spot No. 5 on the E. edge of the E. arm of the " trident;" its position, as given by Mr. Pratt, is on the W. edge of the E. arm. He also found that Nos. 5 and 14 were far inferior to 17. Spot No. 25 was easy. Mr. Elger did not see spots Nos. 9, 11, 18, 23, nor 30 recorded by Mr. Gledhill, nor did Mr. Gledhill see No. 14. For a special note on the position of spot No. 5, which Mr. Elger also saw on the E. edge of the "trident" on May 13, 1870, see Interval 132 to 144 hours. 1870. On the 12th of August, and on September 7, 11, and 12, Mr. Neison made a series of observations with apertures varying from 4 to 5 inches, with differences of of an inch. 4

The spots seen were Nos. 1, 3, 4, and 17 with 4 and 44 inch apertures, the same and No. 5 with 42 and 43; with 5 inches aperture spot No. 14 was detected and marked as faint, and with 54 inches No. 16 was discerned

the last two, Nos. 14 and 16, were in all cases marked as faint," sometimes extremely so.

These seven spots are precisely those which have the highest degrees of visibility for 18 lunations, as under :

Spots

1

Visibility.. 1.000

3 4 17 5 14 16 897 887 ·830 510 433 .294

From these observations, it appears that spots Nos. 1, 3, 4, and 17 may be detected with instruments between 4 and 44 inches of aperture, that spot No. 5 requires an extra half inch, or 4 to 5, and that 5 and 5 will bring out spots No. 14 (5 inches) and 16 (54 inches).

Aperture, of course, is an important element of visibility; and as these spots are seen with apertures under six inches, as the observations increase, and the normal degrees of visibility become well determined, variations in the visibility of these spots may be detected with instruments of 6 inches aperture, provided the observations extend over a sufficiently long period.

Elements of Visibility.

Lunar.-Brightness and size of spots.

Terrestrial. Clearness and steadiness of atmosphere.

Instrumental.-Goodness of figure of object-glass or mirror, and extent

of aperture.

Physiological.-Keenness of eyesight.

Interval 156 to 144 hours.

1870, July 14. Mr. Gledhill records No. 1 as a fine, large, bright spot, No. 17 as a small bright spot, Nos. 3 and 30 as bright spots, and No. 5 a bright spot, seen now and then. Mr. Ingall records No. 1 as very plain and sharp, No. 4 as steadily seen, and Nos. 3, 31, 30 a misty spot, probably consisting of these three.

1869, August 23. Mr. Pratt records that "spots Nos. 1, 3, 4, 17, 6, and 14 were very bright compared with their usual appearance, and all easily seen. No. 4 was not well defined; there was a persistent oval light round it (N.W. and S.E), and I several times believed it to be double, but could not be positive it was so. So remarkably clear was the vision that several times as many as four or five spots were held in view at once, without looking directly for them, and two or three times as many as six were so seen, viz. Nos. 1, 3, 4, 17, 5, and 14; again, Nos. 1, 3, 4, 17, 6, and 5. Nos. 4, 7, 6, 17 were a group seen together, and Nos. 5, 14, 22, and 1 were a similar one; yet still so exceedingly delicate are the fainter spots and the fainter traces of light on the floor that it needs a most concentrated attention to see either. In looking for the faint spots the faint traces of light will escape notice ; again, when looking for the latter, the former are most likely not to be seen. This exceeding delicacy too interposes a serious difficulty in aligning them with objects on or near the border: the eye cannot hold so wide a view and at the same time retain a sufficiently correct impression of objects at once so faint and small. These remarks do not apply to the easier spots and light streaks. Once, for a few minutes, a narrow, dark, straight line, like a pencil-mark, was visible from m towards Rambleta [i. e. from N.W. to S.E.], probably the crack Mr. Birt has discovered. It was not seen again this evening."

1870, September 11. Mr. Neison records No. 1 as very distinct, No. 3 as distinct and brilliant, Nos. 5 and 14 as faint, 5 as rather so.

Interval 144 to 132 hours.

1871, March 8. Mr. Ormesher records a spot near the S.W. border, which he queries "14, a long way off" from its position. Is it a spot not before recorded?

1870, August 13. Mr. Gledhill records spots Nos. 3 and 17 as fine bright disks, No. 1 as a fine, large, bright disk, and No. 4 as a nebulous object. Mr. Pratt remarks that "on this evening, as well as in 1870, August 12, the tint of the dark portions of the floor was much intensified close to the rim. It was the case all round, but especially so between b and , between e and , and between ß and ŋ.'

1869, December 20. Mr. Pratt places a spot nearly due north of No. 1 on the diagram of this evening, which he queries as 23. I query it as uncertain. Spots Nos. 1, 0, 23, and 16 very nearly align. The line passing through Nos. 1, 0, and 23 passes slightly west of No. 16. Mr. Pratt's spot is very decidedly east of this line. [1871, March 31. The spot registered by Mr. Pratt on Dec. 20, 1869, not having been reobserved, it is probable that it may have been, as Mr. Pratt queried, No. 23. I have now entered it as such.-W. R. B.]

1870, September 12.

Interval 132 to 120 hours.

Mr. Neison records of No. 22, "a spot very faint, and difficult to make out in the midst of a patch of light."

Interval 108 to 96 hours.

1870, July 16. Mr. Gledhill records spot No. 1 as "a fine, large, bright disk; looks like an elevation;" also Nos. 3 and 17 as bright disks. I have made the following note on the Form :-" "9 and 0. These do not appear in their precise localities, especially 0. It may be that the spot thus marked by Mr. Gledhill is a new one.'

1870, December 12. Mr. Pratt writes: "A faint crepuscular kind of shade has crept over the western part of the floor, and is deepest near the western border; but the gradation is very delicate, 12 hours to 12 hours 40 minutes." [1870, March 24. Mr. Gledhill noticed a darker tint at the west part of the floor, and furnished a tinted sketch: see remarks under this date (p. 87); also Mr. Elger's observations of the same portion of the floor being light, under date 1870, August 6, interval 48 to 60 hours.]

66

1870, November 11. Mr. Gledhill records spots Nos. 1, 3, 30, and 17 as bright spots. On the 13 of September (same interval) he recorded them as bright or fine craters;" with the exception of Mr. Neison's record on August 12 of No. 3 as a suspected crater (interval 168 to 156 hours), this interval (108 to 96 hours) is the earliest in the declining day that the four have been seen as craters. The terminator is recorded as west of Fracastorius.

1870, September 13. Mr. Gledhill records spots Nos. 1, 17, and 30 as bright or fine craters, and says of 17, "fine crater as 1 and 3;" but of 3 he says, "fine disk." I have marked 3 as a crater.

Interval 96 to 84 hours.

1870, August 15. Mr. Pratt records that the darker margins of the shaded parts of the floor are still visible as on the 12th and 13th August, but not in such striking contrast.

1870, October 13. Mr. Pratt records spot No. 1 as brilliant, the others dimmer than usual.

Interval 84 to 72 hours.

1869, August 26. Mr. Pratt remarked a decided difference in definition