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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.
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:
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 craterlets, 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" “ 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 formation 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
much brighter relatively to their best state than were the spots, of which generally at those times few have been discernible.”
1870, May 13. Mr. Pratt has not only specified the order of brightness as follows:
Spots No. : 1. 4. 3. 5. 17. 14. 22. 6. 13. 16. Visibility : 1.000 .892 .897 •510
.830 .433 •175 .222 -156 .294 Spots No. : 20. 23. 18. 19. 29. 0. 24. 21. 9. 10, Visibility: .046 046 •072 •150
-036 046 ·057 ·026 .222 062 Spots No. :
2. 25. 30. 31, 12. 7. Visibility : 016 144 .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.
Gledhill. Bright spot.
Pratt. Bright but hazy.
Pratt. Distinct; he inserts 5 between 3 and 17.
Gledhill. Bright spot.
Vis. 17. Conspicuous. Nearly equal to 3. Bright spot.
.830 Very faint on east 5. border of eastern
-510 arm of “ trident.” 14. Seen by glimpses.
•433 22. Not seen.
.175 Mr. Pratt observed the three components of the group 3, 30, 31: be 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 conspicuous :" neither Mr. Elger nor Mr. Gledhill detected it; this doubtless depended upon the state of our own atmosphere. 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 arm. Schröter's drawings of the Mare Crisium indicate similar movements of the streaks from Proclus over the Mare.
Class V. contains eight spots, viz. Nos. 16, 6, 13, 19, 18, 20, 23, 29.
Vis. 16. Easy. Easy.
036 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 conse
ence of a fine state of the earth's atmosphere.
Class VI. contains five spots, viz. Nos. 9, 30, 24, 31, 21.
·026 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 rery 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. 25, 7, 10, 2, 0, 12.
.062 2. Hazy.
•031 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