change takes place is as yet undetermined; but there can be no question that it is of the same nature as that of the appearance of the spots on Plato greatly intensified.

The result of the discussion may be briefly stated as being very strongly suggestive of the existence of present lunar activity, the exact nature of which requires further and more extensive observations to determine. Intimately connected with the spot-changes are the variations of appearance and intensity of reflective power of the streaks and markings on the floor of Plato. In the observers' and other notes which form the Appendix to this Report will be found allusions to the connexion between the spots and streaks; but it manifestly requires a similar discussion of the streaks and markings to arrive at a definite conclusion on the subject. Most of the observers have furnished observations of these interesting phenomena, so that a discussion of them could at once be proceeded with if it should be the pleasure of the Association to carry on the inquiry. The principal results of the discussion of the spot-observations relative to visibility, irrespective of solar altitudes, and treated in pairs of lunations from April 1869 to November 1870, based on 1594 observations during 20 lunations, are contained in Lunar Map Circular VIII.; and some further remarks occur in a paper on the subject, published in the Philosophical Magazine, March 1871. This discussion, on an entirely different principle to that employed in the preparation of the present Report, and leading to a similar result, tends to confer on both a character in which confidence may be placed, for either without the other is incomplete; together they point to present lunar action as the originating agency producing the phenomena.

Although measurements for position of such delicate objects as the spots on Plato are difficult to execute, Mr. Gledhill has succeeded in obtaining three sets of micrometrical measures, on September 13 and December 9, 1870, and on May 1, 1871, a combination of which has enabled me to draw the outline of the crater, and to insert from these measurements four streaks and the sector as seen generally by Mr. Gledhill. The streaks are , e, a, and B. The streaks and e are rather westward of their places as given on

the tinted plate in the 'Student' of April 1870, p. 161. The spots whose positions have been determined by measures are Nos. 1, 4, 3, and 17. The effect of the measures is to bring them closer together and more towards the centre of the crater than in the printed plans. On each occasion that the measures were made, a diameter of the crater passing through spots Nos. 1 and 4, from A to B, was measured, also one at right angles to this from C to D, passing through No. 1. All the remaining measures of spots and streaks were referred to these diameters, spot No. 1 being the origin of the coordinates, and the longest diameter being considered as unity. The ratios of the means of the measures were determined to be as follows:

In order to plot the spots that have been laid down by alignment and estimation, it is necessary to align with the measured spots, and particularly with objects on the border, a process that will be adopted in the preparation or a monogram of Plato.

APPENDIX.

OBSERVERS' NOTES.

These are arranged in each interval of 12 hours according to season, so as to give increasing altitudes of the sun from O 8=270°. Winter in the

northern hemisphere.

Interval 0 to 12 hours.

1869, Oct. 13, 71 (O— 8 =76° 24'-8, Oct. 12d 21").—Ten hours after the epoch of sunrise at the equator in E. long. 4° 0'6, the first streak of sunlight was seen by Mr. Gledhill to fall on the floor of Plato through the gap in the west wall between B. & M.'s peaks and e, the W. extremity lying on or near the fault from N.W. to S.E., and bringing into visibility the craterlet No. 3, which is seen earliest of all the spots. Mr. Gledhill gives the sun's azimuth equal to 87° 31', the altitude being equal to the angle formed by the height of the depression in the wall between the peaks above the point of the floor on which the sun's rays first impinge.

Interval 12 to 24 hours.

1870, July 6, 8h.-Twelve hours and a half after epoch of sunrise at the equator, E. long. 4° 11'-5, -8, July 5, 19, 30-354° 54'4. Mr. Gledhill again witnessed the first streak of sunlight fall on the floor of Plato, and observed spot No. 3 just within it, and remarked that the streak lay parallel with the longest diameter, and did not incline from No. 3 as it did in January. [On the 13th of October, 1869, at 7h, Mr. Gledhill remarked that the streak was a little inclined to the N., and not quite parallel with the rim.] At 9h of July 6, 1870, Mr. Gledhill remarked that a line through the two gaps or

breaks in the S. and N. borders passed through the western ends of the earliest streaks of light thrown on the floor. This line appears to be coincident with the great fault crossing Plato. With reference to this I have the following note:-"This phenomenon, the western extremities of the streaks falling in a line with the breaks in the N. and S. borders, was well observed in January 1870. An elevation of the ground in the direction of this fault has been seen. It would, however, appear that differences in the lengths of the streaks would depend not on any unevenness of the ground, but on the relative depths of the gaps in the W. border."

1870, January 10, 2 to 8.-From ten to sixteen hours after epoch of sunrise at the equator, E. long. 4° 6'∙1, © — 8, Jan. 9, 16h, equal to 170° 27'-8. This was by far the finest observation of sunrise on Plato by no less than seven observers, viz. Messrs. Gledhill, Pratt, Elger, Neison, Birmingham, Joynson, and Birt. Mr. Gledhill's record is so full and so interesting that a reproduction of it will convey a vivid impression of the progress of illumination of a lunar formation as the sun rises upon it.

Jan. 10, 2h. Cloudless. Terminator just on the E. border of Plato; can just see the outline of the crater, which now lies in deep shadow. On the E. side the lofty steep wall just N. of a triangular formation marked II E✨2 glowed intensely in the solar rays.

3. The E. wall from the great breaks in the S. and N. borders appeared as a bright narrow band. The curved outline of the N.E. border was bright, sharp, and narrow, but the lower slope within could not be seen. I could fancy that the W. part of the floor is, if possible, deeper in shadow than the E. half. [This phenomenon has often been witnessed, and has been attributed to the reflection of the strong light of the eastern interior from the dark floor. Upon attentively contemplating this degradation of shadow near its eastern boundary, it will often be seen that it is not simply a reflection from the floor, but apparently the illumination of a something above the floor.-W. R. B.]

3h 45m. A bright narrow broken line was seen between the two breaks on the E. and N.E. The outline of II E2 is not yet visible.

4h 18. At this moment (12 hours 18 minutes after epoch) the first streak of light fell upon the floor. Within it and near its western extremity was seen No. 3 as two elevated objects, very near each other, but quite distinct. I could not detect shadow between them after hard gazing, although it was easily seen to the N.E. of the lower object. The streak was three times the breadth of the two objects together where it enclosed them, and it became broader near the N.E. border of Plato; it was brightest about and to the west of No. 3, and inclined a little downwards at the E. end. * The two components of No. 3 are of the same size apparently, are equally but not very bright; they lie nearly E. and W. of each other, but the E. component is a very little to the N. of the other.

4h 30m. The streak widens. I could not detect motion in it. I now carefully placed the wire on the great gap in the west border; the line passed along the axis of the streak. The west angle of the streak is not sharp, but rounded, and lies a little beyond No. 3. The lower of the cones of No. 3 touches the lower edge of the streak. It now assumed a fan shape, being broadest at the E. end, which is now more than halfway to the E. border. 4h 40m. The streak is now much wider. I think I see a minute elevation a little to the E. of No. 3 and in the streak. The two components of No. 3 are now bright and sharp, with shadow on the east. Another streak has been barely visible or suspected for a few minutes; it lies to the S. of the

former and near the S. border. It runs parallel with the northern streak, is about half its length, and has its western extremity over a point a little E. of No. 3. It is narrow, and extremely faint and difficult. A minute or two later it was seen better, also a still fainter and narrower line to the north of it, which is parallel with it and the northern streak. The most southern streak produced to the E. would graze the southern edge of II E42. 4h 50m. Now the shadows from the W. wall take shape. The south shadow, which extends up to the S. border, goes directly into the gap at the S. edge of II E42. The next pointed shadow to the N. of this goes direct to the middle of II E42; it is extremely pointed at its E. end for more than half its length, and is suddenly wider at the W. end. [This appears to indicate that the peak which throws the shadow is very needle-like.] I cannot be quite sure that this shadow for the next 10m or 15m really extended up to the E. border. It became so faint and narrow and line-like that it could not be well seen near the border. Then, again, the floor for some distance (say a distance equal to the width of II E42) lay in rather dark shadow. The floor between the shadows was not bright up to the E. border of Plato ; all along the foot of the E. slope a dark shadow lay, and this interfered with an exact determination of extremities of shorter shadows from the W. wall. The next shadow to the north was a broad parallel-sided belt, which proIceeded to the E. border as such. Its upper or S. edge extended to the N. end of II E42, and its lower or N. edge cut the border of Plato just below, or to the north of II E42. A line through No. 3 to the gap in the S.E. border cuts the W. angles of the two southern bright spaces between the shadows.

5. No. 3 lies on the lower edge of the lowest bright space or upper edge of the lowest shadow. The shadow still clings to or is in contact with No. 3, and either extends to the E. of it, or No. 3 throws a shadow to the E. The floor along the E. border is still dusky; it is brightest at that part in line with No. 3,

5h 5m. A very fine narrow shadow is now seen to stand off from the shadow below and in contact with No. 3; it is this which touches No. 3.

5h 15m. The upper shadow is now clearly pointed, and falls short of the border. [This is probably the shadow of the peak between B. & M.'s y and 8.] I still see a minute elevation just to the N.E. of No. 3. It is now just on the tip of the lowest pointed shadow, and about halfway from 3 to the N.E. border. [This spot is No. 32; it was discovered in streak ẞ by Mr. Elger on December 15, 1869.-W. R. B.]

5h 45m. Floor at the foot of the E. border is still dark, except at the extreme N. The long broad shadow is now retiring from the E. border, and is seen faintly bifurcated; the lowest or northern fork is the longer, but this broad shadow still seems to have its N. and S. edges parallel.

6h. Now the dark shadow on the S. border breaks up, and a fine pointed shadow separates from its northern side, which if produced goes quite into the gap at the southern edge of II E2. The bright W. angle above this shadow goes back towards the W. until under the great gap in the S. border. The great central shadow is now easily seen bifurcated; the lower peak is the longest, and reaches nearly up to the east border. The tip of the shorter shadow to the N. reaches just to No. 3; the next to the N. is rather longer. 6h 20m. The object to the N.E. of 3 (32) is easy, elevated, and bright. Now 4 is seen, also a large elevated object (7) about halfway from it to the N. extremity of II E42, and on this line.

6h 30m. The great S. band of shadow goes straight into the gap at the S. end of II E42. The E. portion of the floor for some distance from the foot

of the slope is still dusky. The shadow of the N.E. component of No. 3 is easy, and lies to the N.E. A line from the lower edge of the shadow in the great gap of the west border along the lower edge of the central shadow goes into the gap at the N. end of II E42. This shadow is now finely bifurcated; the lower or northern peak is the longer.

8h. Spot No. 1 is now seen as a large striking object. It seems to be in the path of the upper fork of the central shadow, and looks like the shadow of one of Jupiter's satellites on the disk. [In Mr. Birmingham's sketch of May 19, 1869, O-8=286° 37'-3, the upper or southern fork of the central shadow is longest, while in the present series of observations the northern is the longest. This is not a solitary instance of variation in the shadow of this peak. Mr. Birmingham is in agreement with Mr. Gledhill in referring spot No. 1 to the upper or southern fork. In my paper on the spots and shadows of Plato (Transactions of Sections, p. 17, Report of British Association for the Advancement of Science,' 1869), I remark that Rosse and Birmingham have drawn No. 1 with the shadow of d just receding from it. Challis's shadow of terminates by a straight line; neither fork was visible, for he carefully measured the two angular points. Rosse drew the termination of the shadow as from two pinnacles upon the summit, with No. 1 between them. These variations are doubtless azimuthal; nevertheless they are of great importance, as we hope presently to show.]

8h 5m. Spot No. 1 is a large, lofty, very prominent cone. Close to the N.E. component of No. 3, and to the N.E. of it, is seen a black shadow curved to the N.E., with a bright elevated object close to the curve. I see the two components of No. 3 as bright distinct objects; then, close to the N.E. foot of the N.E. component, comes a large circular shadow quite black, embracing a bright object to the N.E.

8h 15m. Spot No 4 is already getting rather difficult and hazy, although it lies far away in the bright eastern floor. Spot No. 17 is now seen just on the lower edge of the uppermost pointed shadow. No. 1 is bright and large, free from the long shadow. Shadow still lies on the eastern floor at the foot of the slope. Mr. Pratt, the same evening, Jan. 10, noticed a peculiar feature of the eastern part of the floor corroborative of Mr. Gledhill's observation of the dip to the foot of the east border. He says, "A peculiar feature of the eastern part of the floor in sunlight observed. Between what was probably the eastern margin of the sector b and the foot of the interior slope of the E. rim was a decidedly darker tint, as if that part of the floor was lower than the rest, and perhaps falling towards the border; the western margin followed very closely the form it would have if the whole space between the sector b and the border were depressed." In my own record, Jan. 10, 4h 48m, the Crossley equatorial 7-3-in. aperture, eye-piece No. 4, power 122, with slot, I say: The S. spire of sunlight apparent; it is directed towards the middle of II E42. Neither of the spires of light reach the border, indicating the floor to dip near the border.”

Mr. Gledhill summarizes his observations, under the head of "points determined," as follows:

First. The position, size, alignment, and order of development of the streaks [of sunlight, as distinguished from those that make their appearance afterwards] which first fall on the floor. They are evidently the solar rays passing through the gaps on the border.

Second. The floor on the E. at the foot of the inner slope lies in shadow more or less deep until the giant shadows from the W. border have retreated westward beyond the centre of the crater.