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much brighter relatively to their best state than were the spots, of which F. generally at those times few have been discernible."

1870, May 13. Mr. Pratt has not only specified the order of brightness as follows:

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Spots No.:
Visibility:

20.
⚫046 ⚫046
25.

23.

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⚫072

•150

⚫036

⚫046

⚫057 026 -222 *062

30.

31. 12.

7.

Spots No.: 2.
Visibility: 046 ·144 •139 031 ·031 •113

which we can compare with the degree of visibility for the 18 lunations as given immediately under the number of each spot (from this comparison we see that the brightness on May 13 was not strictly accordant with the visibility), but he has described the character of visibility by the words easy, conspicuous, &c., thus forming with the spots not seen eight classes of objects, an analysis of which may be interesting.

Class I. contains one spot only, No. 1, deg. of vis. = 1.000.
Pratt. Exceedingly bright and dense.

Elger. Unusually bright.

Gledhill. Bright spot.

Class II. contains one spot only, No. 4, deg. of vis. = .892.
Pratt. Bright but hazy.

Elger. No remark.

Gledhill. Spot.

Class III. contains one spot only, No. 3, deg. of vis.

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Pratt. Distinct; he inserts 5 between 3 and 17.
Elger. 3 and 17 nearly equal.
Gledhill. Bright spot.

Class IV. contains four spots, viz. Nos. 17, 5, 14, 22,—

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Mr. Pratt observed the three components of the group 3, 30, 31: he described 30 and 31 as steadily seen; they occur in Class VI. Mr. Pratt accorded to spot No. 22 a high degree of brightness on this evening, and described it as 66 conspicuous: "neither Mr. Elger nor Mr. Gledhill detected it; this doubtless depended upon the state of our own atmosphere. It may, however, be remarked that the spot was less visible on May 13, 1870, as compared with its visibility in August 1869, when it was seen by every observer.

The position of spot No. 5, as observed by Mr. Pratt on August 26, 1869, was on the west border of the eastern arm of the "trident." The spot No 5, discovered by Challis, and possessing a normal visibility of 510, has been so frequently observed as almost to warrant its stability of position; and should its relative position, as regards the eastern arm of the trident, be found to vary, it will afford evidence of a probable variation in the position of the Schröter's drawings of the Mare Crisium indicate similar movements of the streaks from Proclus over the Mare.

arm.

Class V. contains eight spots, viz. Nos. 16, 6, 13, 19, 18, 20, 23, 29.

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Of the spots in this class, and which Mr. Pratt describes as easy, one only, No. 16, was seen by Mr. Elger. This spot has a higher degree of visibility than 22 in Class IV., "conspicuous;" and this is perhaps another indication that the visibility of No. 22 on May 13 did not wholly depend upon the state of the earth's atmosphere.

The normal degrees of visibility in this class range from 294 to 036, furnishing a strong indication that they were seen in consequence of a fine state of the earth's atmosphere.

Class VI. contains five spots, viz. Nos. 9, 30, 24, 31, 21.

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The same remark may be applied to this class as to Class V., viz. that the spots were seen in consequence of a fine state of the earth's atmosphere. The two spots Nos. 9 and 30, with comparative high degrees of visibility, are very frequently seen by Mr. Gledhill, and doubtless were not seen by him in consequence of the bad state of the atmosphere at Halifax.

Class VII. contains six spots, viz. Nos.

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25, 7, 10, 2, 0, 12.

Gledhill.
Not seen.

Vis.

•144

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Spot No. 25, vis. 144, is frequently seen by Mr. Elger.

In addition to the above, Mr. Elger frequently glimpsed No. 32. The WHOLE of the above spots, as well as the streaks recorded by Mr. Pratt, were observed three separate times at intervals of about twenty minutes. The majority was seen much oftener.

The following spots were not seen on the evening of May 13:

Spot: 11. 34. 8. 15. 33. 27. 26. 28. 35. Vis. 144 026 015 015 010 010 005 005 005 With the exception of spot No. 11, which is frequently seen by Mr. Gledhill, these spots were doubtless concealed by or, rather, required a still finer state of the atmosphere to bring them out. It is difficult to say why Mr. Pratt did not detect spot No. 11 when he saw thirteen spots with lower degrees of visibility. It is one of those spots to which special attention

should be directed. Of the remainder, three have been observed once only by Mr. Gledhill, viz. Nos. 26, 28, and 35; two have been observed twice, viz. Nos. 27 and 33; two thrice, both old spots, viz. 8 (Gruithuisen) and 15 (Dawes); and one, No. 34, six times between January 15 and March 13, 1870*. In his letter dated 1870, May 19, Mr. Pratt says that "spot No. 8 could not be recovered even with the most minute attention." Of spot No. 1 he says, "it was brighter than I have seen it before, quite round and dense, much like the image of a star on a good night surrounded by the very least trace of a ring of light. [Neither] internal nor external shadows could be seen, although I constantly expected a slight glimpse."

Spot No. 22.

In reference to this spot Mr. Pratt writes, under date 1870 August 26, as follows:

"Spot No. 22, according to my observations, has manifested a remarkable increase of brightness, and those parts of the shaded portions of the floor of Plato which are nearest to the rim have come out more conspicuously darker than the rest than I remember to have previously noted. The tint of the floor, toe, has progressively paled. These three phenomena [the increased brightness of spot 22, the intensification of the darker parts of the floor near the rim, and the progressive paling of the floor] may possibly be connected by a common cause; for certainly in this lunation there is somewhat of a coincidence amongst them; for instance, spot 22 is intensely bright at the time the marginal portions of the shaded parts are most conspicuously dark, and these two, again, coincide with the time when the general tint of the floor is at its darkest. Again, after August 12 and 13, spot 22 decreased in relative intensity, although I am not ready to hazard the assertion that it had on August 16 positively declined to its usual intensity, as it was not seen. [It was on this evening that Mr. Pratt observed three spots only.] Two similar instances, I believe, I have noted before, when 22 manifested a singular brightness at sunrise. But the connexion between the visibility of the deeper-tinted margin and the general deepening of colour is perhaps more close still, as both certainly paled after August 13. The perplexity seems to be that the variation in intensity of the margin is relative in respect of the general colour; and if differences of angles of illumination and vision do affect the general tint, it might be supposed that they would in the same manner affect the margin and so produce no relative variation of intensity."

In connexion with the relative intensity of which Mr. Pratt speaks, the state of the border is somewhat important. August 12 and 13, when the marginal portions of the floor were intensified in colour, Mr. Pratt recorded of the border :-"Definition fair at times, with much tremor, wind N.E." This was on the 12th. On the 13th the record is: " Border, definition bad,

The history of spot No. 34 is curious; the following are the only records which exist of it. The observations were all made by Mr. Gledhill with the Halifax 93-inch equatorial in the Observatory of Edward Crossley, Esq.

1870, January 15, 10 to 13 hours. "I am continually thinking I see an object close to

No. 1 and to the west of it."

February 11, 6.45. "No. 1 often comes out double; last year I often saw it thus. I am now almost quite sure I see a minute object close to the west of it." February 12, 6.0. 'Saw 9, 11, 30, and object close west of No. 1."

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March 12, 6 to 8 hours. No. 34 mentioned as having been seen.

March 13, 6 to 12 hours. "Unless I am very much mistaken indeed 34 is an easy object, i. e. No. 1 comes out easily double."

There are no records after this date. Instruments less than 9-inches aperture are not likely to redetect it.

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much boiling, wind N.E." On the 12th, definition fair, the floor was recorded as very dark." On the 13th it was dark, but not so much so as on the 12th. On the 16th, as well as on the 15th, the definition of the border was "bad." These records clearly throw a doubt upon the supposition of the "paling" having resulted from some lunar action, inasmuch as when the deeper tint was observed the definition was "good," the "tremor" and boiling" having a tendency to confuse the portions of the floor. On the other hand, spots have been much more numerous with bad definition than 3 as observed by Mr. Pratt on the 16th; and this would lead to the supposition that the apparent extinction of the spots with a pale floor was in some way differently connected than by a deteriorated state of the earth's atmosphere. I have often observed that the passage of a thin cloud over the moon has greatly contributed to intensify the tints of the darker portions of the surface; but in this case the intensification has been general and not partial, as it would be if dependent upon local lunar action.

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Mr. Pratt records a case of partial obscuration which was well seen on August 13. "It appeared," says Mr. Pratt, on this wise. A general view of the floor showed it much speckled and streaked in other parts; but over the area specified [Mr. Pratt has not mentioned the particular part of the floor; but from what follows I apprehend it must be in the neighbourhood of No. 3] there seemed an absence of markings; close attention, however, enabled some to be seen, but not nearly so richly as the remainder of the floor, and we know well enough that that particular area is not wanting in markings. The evening's view has just occurred to memory when I first discovered that spot 3 was a triple one, and had a remarkable view of its neighbourhood [Qy. Was this on May 13 ?], therefore exactly the reverse being the case. August 13 seems as conclusive a proof as one observer is likely to obtain in a year's work."

Of four observers on the same evening, two record No. 3, and the other two appear not to have seen it. Taking them in chronological order, Neison, 9.5 to 9.15, records it as distinct; Pratt, 10.30 to 12.30, did not observe it; Ormesher, 11.0 to 11.30, does not show it in his drawing; Gledhill, 14", records it as a bright disk: he also records 30. As these observations are not contemporaneous, with the exception of Ormesher's, having been made while Pratt was observing, it appears, from its absence in both their records, that from 10.30 to 12.30 it was really not visible; and this tends to support Mr. Pratt's idea that for the time it was hidden by something like an obscuring medium. What this could have been it is difficult to surmise. The remark, however, of Neison that 30 was not to be seen between 9.5 and 9.15 is interesting in connexion with Gledhill recording both spots at a later epoch, 14h, and also detecting five not seen by Pratt, viz. 3, 30, 9, 11, 18. Neison suspected he saw 14, not recorded by Gledhill nor Pratt, but seen by Ormesher. Pratt saw 22, not seen by either of the others. The case of 14 is a little perplexing; it might, however, have been missed by Pratt on account of the bad definition. With regard to the greater number of spots seen by Gledhill, two circumstances may have contributed to this result, the larger aperture of Mr. Crossley's instrument and the epoch at which Mr. Gledhill observed. It may possibly be found that the greater number of spots recorded after the sun's meridian passage at Plato depend upon the steadiness and purity of the air mostly experienced after midnight.

Sunset and Sunrise on Plato.

Extracts from Mr. Pratt's notebook, 1870, Oct. 17, 11h to 12h. Defini

*

tion fair, with boiling. Plato is a grand and striking sight. Tint of floor medium. More than half the floor in shadow. Terminator just including the W. rim. The rim of the crater on the N. exterior slope finely seen. In three parts the rim appeared broken down to level of floor-close to m, opposite to c, and nearly so at W. II E42 [the breaks at m and opposite c are in the line of the well-known fault crossing Plato from N.W. to S.E.]. was throwing a long spire of shadow the full length of the floor at 11 40m. That part of the floor contiguous to the W. and S.W. rim was deeply shaded, with streaks of shade running towards the centre of the floor. Between the break near c and the shadow of a straight shading as of a narrow valley was well seen. [These shadings appear to be roughly coincident with the dark spaces on the floor as seen under high illumination, the straight shading being, as Mr. Pratt suggests, between the "sector" and the E. arm of the "trident." Is there really a valley here running into the central depression between 1 and 4, seen by Mr. Elger in January, 1870, and observed much earlier by Schröter?] Between these shadings and the shadow of the E. rim were three roundish lighter regions, the higher parts of the floor giving the appearance of a strongly marked convexity."

"A strong suspicion arises that the apparently higher portions of the floor are the light streaks usually seen, and the highest parts are spots 1, 17 and 5." Mr. Pratt further suggests that the light streaks are coincident with formations analogous to "spurs" from the chief centres of the residual activity on the floor.

It is not a little remarkable that on the occasion of such a very favourable oblique illumination the craterlets 1 and 17 should not have been detected by Mr. Pratt; both have raised rims of the nature of true volcanic cones, and 1 has been seen, and I believe 17 also, with interior shadows and bright interiors facing the sun. Mr. Pratt does not appear to have seen even the remotest semblance of a shadow. The spots properly so called do not appear generally until the sun has attained an altitude of 20°. If craterlets are recorded as spots earlier, it is probably in consequence of bad definition confusing the crater-form appearance. Is it possible that on the two occasions mentioned by Mr. Pratt, Oct. 17 and Nov. 1, the craterlets 1, 17, 3, and 4 were by some means concealed? As regards Nov. 1, the observation of the crater-cones as the shadows gradually recede from E. to W. is very frequent; indeed the surface of Plato as it just emerges out of night appears to be in a very different state to what it is about mid-day; objects are much sharper, and it is difficult to conceive of any agency so affecting such visible objects as to render them invisible at a time when they are generally most conspicuous. So far as contemporaneous observations are capable of throwing light on this phenomenon, three spots only were recorded on the same evening; No. 1 by Mr. Elger, who noticed it from 9h to 9h 5m, near the shadow of the summit of the middle peak of the W. wall, three hours later than Mr. Pratt's observation. Mr. Gledhill at 6", same as Mr. Pratt, says, "Moon so low and air so thick that very little light from moon can reach us;" he says also, "I see 3 as double elevated cones [i.e. 3 and 30]. No other objects can be seen." Mr. Neison, 5.10 to 8.15 [probably 8.10 to 8.15] succeeded in seeing 3 only, which he records as very faint. He does not give the state of the atmosphere as to definition; but from his remarking that " a deep cleft in west edge of wall was very distinctly seen," I should suppose that it was pretty good. Taking the four sets of observations it would appear that at sunrise on Plato Nov. 1, 1870, some agency was in operation capable of concealing the craterlets; and combining these observations with those of

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