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in different parts of the floor, even in so contracted an area, the whole northern half being less well defined, the south-east part the best so by far. Traces of the line from m to Rambleta were caught, and the floor appeared unlevel, the central and south parts appearing highest, and the south-west part next so. This, Mr. Pratt says, requires confirmation.

1870, September 14. Mr. Gledhill records No. 3 as a fine wide double spot (i. e. 3 and 30). Mr. Neison (same day) remarks as follows of Nos. 1, 3, and 17, seen by Mr. Gledhill as craters: No. 1 not very distinct; No. 3 sharp and shaded, not very bright; No. 17 very distinct.

Interval 72 to 60 hours.

1870, August 16. Mr. Pratt observed 3 spots only this evening. On October 14 (same interval) 16 were observed, 9 by Mr. Gledhill and 7 by Mr. Pratt, in addition. They both record the definition of the border as "good;" Mr. Pratt says, "with interruptions." On August 16, Mr. Pratt records the definition of the border as "bad." The following remark of Mr. Pratt is interesting in connexion with this paucity of spots :-"The darker parts or shaded portions of the floor were just perceptible with attention. Tint of floor' medium, much paler than on the 13th inst."

Interval 48 to 36 hours.

1870, August 17. Mr. Gledhill records No. 1 as a fine, large, open crater, 3 and 30 as craters, 17 as a small crater, and 4 as a bright but not definite spot.

Interval 36 to 24 hours.

1870, March 23. Mr. Gledhill writes: "The shadow of the elevated object on the east border (the rock ), close to the N. of W. II E42, was on the floor, and the adjacent floor to the N.W. was very bright, much brighter than a or the 'sector,' and it extended one third of the distance from the border to spot No. 4, as in sketch." Mr. Gledhill could not determine its form, but considered that it was the streak ŋ intensified.

1870, July 19. Mr. Gledhill observed the four craters 1, 17, 3, 30 only; he described No. 1 as a large circular crater with raised walls, but not much brighter than the floor.

1869, August 28. Mr. Pratt writes: "The LEVEL of the floor was conspicuously divided by the line from m to c, the ground sloping east and west of this line, the eastern part being brighter than the part on its west, while the locality of spot No. 4 was judged to be the highest of the whole floor." In connexion with this remark of Mr. Pratt it may be well to notice that, combined with Mr. Elger's observations on 1870, Jan. 10, of a depression in the floor east of No. 1 (see Interval 12 to 24 hours), the two suggest that this depression does not extend so far as No. 4. Again, comparing this observation of the western part of the floor being darker than the eastern, which is in accordance with Mr. Gledhill's on March 24, 1870 (see Interval 12 to 0 hours), it would appear that Mr. Elger's observation of the bright western area on 1870, May 9 and 10 and August 6, was an intensified brightness of the ordinary brilliancy of the floor, sloping to the west. The Intervals 24 to 36 and 48 to 60 hours, the season spring, with the sun's altitude about 14°, seem to indicate that the increased brightness was quite independent of illuminating angle.

Speaking of the apparent changes observed, not only on Plato, but over a wider range, between August 16 and 28, 1869, Mr. Pratt says: "Thus, among apparent changes of a particular character, and restricted to certain

small localities, there does appear to have been a wider and more general disturbance in the brightness and definition of objects, all which disturbance appears to be confined to the low-lying lands of that part of the moon observed. Not that changes were not visible in high regions; but these are more easily referred to changes of illuminating and visual angle, while the disturbances above mentioned are not so easily accounted for, especially those changes in the visibility of the light-streaks on the floor and the striking differences of brightness of the spots."

1869, October 26. In connexion with Mr. Gledhill's return of this date I remark, "Crater Row' being so well seen, and the border of Plato so sharp and distinct, it is remarkable that spots Nos. 5, 6, 7, 13, 14, and 16 should not have been well and easily seen, although it appears they were seen, also that spot No. 3 should have been seen single, and that only sometimes, when it was seen double the previous night."

1870, November 14. Mr. Gledhill observed Nos. 1, 3, 30, and 17 as craters, and says, "they look like bright elevated rings."

Interval 24 to 12 hours.

1870, March 23. See ante, Interval 36 to 24 hours.

1869, September 27. Mr. Gledhill recorded a broad band of brightness parallel to the north border, enclosing spots Nos. 13, 19, and 16; he does not say they were seen as well as the bright band. I have, however, recorded them as having been seen.

Interval 12 to 0 hours or sunset.

1870, November 15. The four craterlets Nos. 1, 3, 30, and 17 are described by Mr. Gledhill as elevated crater-cones.

1870, March 24. Mr. Gledhill writes:-" Terminator on N.E. end of Apennines; the eastern shadows lie on the floor. A line drawn along

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the west edge of the sector,' and produced to the north border, separates the bright east part of the floor from the darker west part; the inner slope of the west wall glows in sunlight, while the floor near it is the darkest portion of the crater [Plato]." See p. 95, line 9.

ADDITIONAL NOTES.

Differences of Visibility of neighbouring Objects.

1869, August 26, 11 hours 30 minutes. Definition frequently exceedingly good but disturbed, with much boiling at times. Mr. Pratt has furnished the following record :

"There was a marked difference between the M. Imbrium, the M. Serenitatis, and the M. Frigoris, in respect of the visibility of minute objects on their surfaces. The Mare Imbrium was literally covered with small white spots and streaks. The three streaks from Aristillus to the south border of Plato were again traced. Archimedes had roughly four light streaks E. and W., and about nine or ten easily discerned white spots. Beer and Mädler and neighbourhood looked invitingly for a close study.

"The Mare Serenitatis was of a dull grey, with few white spots and comparatively few features visible. Of those visible all were very indistinct, EXCEPT THE MORE ELEVATED ONES; thus, of the small objects round Linné, most were invisible, a few indistinct, even I E01, I E02, I Ey3 [the three small eraters N.W. of Linné] were almost obscured. Linné itself a cloudy white spot, with knot of light in centre, but not nearly so bright as when seen on the 23rd inst. Posidonius y was brighter and half the size of Linné. Bessel was tolerably clear. About half the number of white spots S.E. of

Bessel were very indistinctly seen, the remainder invisible.

Posidonius, just within the terminator, was fairly defined. Sulpicius Gallus and one or two near it on the pleateau were clear; so that the MORE AN OBJECT WAS RAISED above the general level of the Mare the clearer was its definition, while those on the level of it were more or less obscured.

"The Mare Frigoris was very hazy indeed; even close to the foot of the north slope of Plato objects could not be defined, while those raised a little above the Mare were remarkably well defined indeed. The whole northern

slope of Plato appeared everywhere rugged and uneven."

Indications of intermittent Visibility and of possible volcanic Activity. On the evening of the 13th of May, 1870, no less than twenty-seven spots were seen on the floor of Plato, 26 by Mr. Pratt, and an extra one by Mr. Elger. This extraordinary display occurred between 132 and 144 hours after the terminator had passed 4° E. long. It is, however, not a little remarkable that, on the same evening, Mr. Gledhill, at Halifax, observed four spots only. The great number seen by Mr. Pratt, as compared with the small number seen by Mr. Gledhill, is doubtless due to a fine state of the earth's atmosphere at Brighton.

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With regard to the streaks seen by Mr. Pratt on the same evening he remarks-"I could not see the small streaks on the western part of the floor, and sometimes even my old 'trident' and the streak < were so indistinct as to be difficult. What was the cause? Surely not the earth's atmosphere; for at the same time spots could be seen. Perhaps we shall discover that spots are raised at a higher level than light streaks, and thus visible when streaks are obscured."

This remark of Mr. Pratt's is important: certainly the state of the earth's atmosphere could not have affected the two classes of objects in different ways. If the intensity of the spots depended upon the purity of our atmosphere, one would think that the brightness of the streaks would also have been increased; but in Mr. Pratt's experience it was not so. Mr. Elger speaks of some as bright and others faint. Mr. Gledhill, with a bad atmosphere, speaks of them as bright; but he saw only four spots. Are the spots really brighter than the streaks? But, then, why do both vary in brightness? Mr. Pratt having perused [carefully] the MS. has furnished me with the following remarks:

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May it not be well to mention that, on the occasion referred to, 1870, May 13, I observed fifteen streaks, one of which was a new one. [This was the streak from spot No. 5 towards No. 14.] This number was much above the average, the curious fact being that although so many were perceptible with attention, yet the increase in their brightness was in a lower ratio than that of the spots. There are two possibilities which may affect the discrepancy [difference]between the notes of Mr. Gledhill and myself in relation to the streaks:-First, the times at which we observed may have been different. As for myself, I tested the chance of working with any thing like satisfaction once at least every half hour during the whole of the evening, and before I tried for the last time, at 11 hours, had been unable to perceive either one spot or streak. Secondly, priority of observation bestowed on objects of one class may detract from the estimated brilliancy of the other class. In my own case, immediately I went to the telescope, at 11 hours, I saw several spots conspicuously, and in consequence searched for spots alone for nearly an hour. A search for so long a time for one class possibly may, in a slight measure, reduce the sensibility of the eye for objects of the other class, whether spots or streaks."

The following extracts from Mr. Pratt's letter, dated 1870, May 19, are

interesting:-"Some spots having at different times been observed as cra→ terlets, their character as volcanic is settled in my own mind. Whether all spots are analogous I should be glad to know; but on the supposition of such similarity existing, the suggestion naturally arises whether the light streaks be not scoriæ or lava, or a mixture of both, resulting from the action of the craterlets with which they seem to be connected."

A comparison of the curves for the 20 lunations, April 1869 to November 1870, is suggestive of the craterlets being a distinct class of objects. The phenomena characterizing the cratelets, as indicated by the curves, differ very materially from the phenomena manifested by the spots; for example, in the correspondence of the maxima at the time of the supposed outbreak of Aug.-Sept. 1869, we have an increase of visibility in spots, the behaviour of the craterlets being altogether different. Certain neighbouring spots, to which allusion has been made, declined greatly in visibility, and were very seldom seen during a period in which the craterlets were almost always visible; and in connexion with this it may be remembered that craterlets are characterized by high degrees of visibility, while of many spots which have large ranges the normal degrees of visibility are low.

That a connexion exists between the streaks and spots is, as Mr. Pratt remarks, "self-evident;" and Mr. Elger has shown that most of the spots occur on the streaks. Now as both spots and streaks vary in brilliancy and visibility, may not the steaks consist, as Mr. Pratt suggests, of ejecta from the volcanic orifices of the craterlets? The increased brightness of the streaks in the neighbourhood of the border has been frequently noticed, as well as the unevenness of the floor. It may be possible that newly ejected matter (especially if it be of the character of "broken glass," suggested, I believe, by Dr. Huggins as explanatory of the appearance of Linné) may reflect light more strongly, and thus contribute to the brighter appearance of the streaks about the time at which the craterlets manifest increased activity, and this may become so great as even to conceal the craterlets themselves. On the other hand, although we are perfectly ignorant of any meteorological or chemical action occurring at the surface of the moon, it may be permissible to suggest that, if such action be possible, the reflective power of the ejecta may become impaired, and the streaks in consequence rendered less bright.

It is exceedingly difficult to conceive that volcanic action can be in existence on the moon's surface without "vapour" of some kind escaping from the orifices. If this be the case, condensation must follow, and the orifice may be covered by the condensed vapour, the upper surface of which may strongly reflect the light and produce the appearance of a spot when not in a state of actual eruption; and this spot may be seen on a surface covered with ejecta, the reflective power of which has been impaired since it left the orifice.

One of the brightest portions of the floor of Plato is the S.E., which is characterized by the "sector" or "fan." On the 10th of January, 1870, Mr. Gledhill observed as many as nine crater-cones on the eastern part of the floor, viz. Nos. 1, 9, 11, 17, 4, 3, 30, 7, and 32. It is easily conceivable that ejecta from some of these may be the perennial source of the reflective power of the "sector."

“It is, as far as I can see," says Mr. Pratt, "not at all proven that it is impossible that they, the spots, may not be small acting volcanos at this present moment; and you will please credit me with having noted that, on the 13th of May, although the spots were very greatly in excess of their usual brightness, the relative brilliancy of the light streaks was not nearly

in the same proportion, indeed not so high as on some nights when fewer spots have been visible. The supposition of Schröter of an exceedingly low atmosphere, confined to the lower regions, seems to me especially consonant with the above observations, for the following among other reasons:

"A thin atmosphere, the only possible detection of which is confined to the lower parts of the floor [that is within the mountainous enclosure of Plato], may obscure the streaks partially [to effect this there must be condensed material of some kind] without affecting the spots, which, if craterlets, are raised more or less above the level of the streaks [the low fogs, the upper surfaces of which are at a less elevation than ordinary buildings are high, may be cited as examples]; for such an atmosphere would probably be rendered more dense by and during the supposed activity in the spots, which on that night were unusually bright and, according to the hypothesis, in action. [It must not be forgotten that on comparing the observations of Mr. Pratt with those of Mr. Gledhill, the presumption is that the unusual number and brilliancy of the spots was simply the effect of a finer atmosphere at Brighton as compared with that at Halifax. The phenomenon which is at variance with this is the less brilliancy of the streaks as recorded by Mr. Pratt; still we have the bright streaks of Mr. Gledhill supporting the hypothesis of the effects of the earth's atmosphere.] Hence after a subsidence of the brightness of the spots and the restoration of the normal state of the atmosphere, we might expect to see the streaks come out more distinctly."

It will be remarked that, in my suggestions above, the increased brightness of the streaks is supposed to depend upon the craterlets actually ejecting material, while the increased brightness of the spots depends upon the escape of vapour. I have not quoted Mr. Pratt's remarks for the purpose of controverting them; they appear to me to be exceedingly valuable, and in the present state of selenological inquiry it is important to canvass every view that may be put forward. It is quite consonant with both our views that increased activity in a spot may, and doubtless does, manifest itself by increased brilliancy; and it is not unlikely that the forma tion of a spot in the way suggested over a volcanic orifice otherwise invisible may precede an actual eruption, contributing to an increased brilliancy of the streaks if they really result from volcanic ejecta.

On the agencies capable of affecting the visibility of objects on the moon Mr. Pratt remarks:-"To my own mind the only likely agencies that can exist in the moon capable of affecting the visibility of objects are the everywhere-denied lunar atmosphere and real volcanic activity; as far as I can learn, the observations of some favour the one agency, while other observations do the same for the other, at the same time that different observers alternately deny the possible existence of either. Surely they are very closely related. If volcanic activity be established, can it exist without an atmosphere? While if a low atmosphere be established, would not the stronger objection to present volcanic activity be removed? The hope that persistent and minute observation of a suitable region might produce a result sufficient either to weaken or strengthen the supposition has been at once the impetus and bond which has induced me to give a large share of attention to Plato. We may not have attained such a result even yet; but possibly continued application may be rewarded. I hope so. The close study of typical species is generally the best method of acquiring a good knowledge of genera."

Mr. Pratt further adds: :-"The reverse of what I have here stated I have several times observed, viz. that the light streaks on those occasions were

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