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Sixth maximum. Interval 24. 1870, Sept. 14, 16 spots, 23 for the whole interval, from 3 observations.
When we take the mean numbers of spots seen at each interval, the curve is comparatively flat, rising but little above the mean line of 7.9 spots visible at any interval, and this is about the mean number visible on any evening. The flatness of the curve is not accordant with an increase of spots dependent on an increase of solar altitude or greater angle of illumination, otherwise the apex would be much more decided. We may, however, trace from the number of spots actually seen and contributing to the maxima of the spot-curve, as well as from the observations adduced above, that the change of illuminating angle does exercise an influence on the appearance of spots, inasmuch as on a few occasions the largest number of spots have been seen with higher illuminations. The actual curve, however, derived from two years' observations is not sufficiently decided to refer the appearances of the spots to this agency.
By dividing the whole of the twelve hourly intervals into six series or groups, and taking spot No. 1 as the standard of comparison, we have the data for computing Tables III. and IV. containing the visibilities of each spot for each group of intervals: sunrise, or 0 to 60 hours, altitudes 0° to 17°; 60 to 120 hours, altitudes 15° to 34°; 120 hours to meridian passage, altitudes 31° to 42°; meridian passage to 120 hours, altitudes 42° to 31°; 120 to 60 hours, altitudes 34° to 15°; and 60 to 0 hours, or sunset, altitudes 17° to 0°. From the results in these Tables, Table V. has been formed, in which we have a bird's-eye view of the visibilities during the luni-solar day. Generally the visibilities are low during the first 60 hours, i. e. compared with spot No. 1, the smaller spots are but seldom seen; and this is so far indicative of solar light in some way developing or bringing the spots into visibility. During the next 60 hours some spots have risen considerably in visibility, while others have been seen more frequently during the afternoon hours of the luni-solar day. The numbers are, however, too irregular to allow us to conclude that the smaller and less frequently seen spots are influenced in their visibility by further changes of illuminating angle beyond their first development; and this is very strikingly manifested by the curves which these numbers furnish; for example, the diurnal curves of spots Nos. 5, 14, and 16 in fig. 2 generally agree in exhibiting greater visibilities from 60 hours to meridian passage, while spots Nos. 9 and 11 are more frequently seen from 120 to 60 hours before sunset. These, as well as the peculiarities of the other curves, strongly suggest that the variations of visibility of certain spots are not to any great extent dependent upon an increase of intensity of solar light, but rather upon some agency more particularly connected with the spots themselves. It is important to remark that another series of observations may furnish totally different diurnal curves, should the variations in visibility depend upon local lunar action.
In nearly every case the spots seen during the first 60 hours of the lunisolar day have increased during the day in visibility, i. e. they were seen less
frequently during this group of intervals than during the succeeding sixty hours. This increase, however, has not been regular, which it would have been from changes of illuminating angle alone, some spots having been seen, as before stated, more frequently during the second group of intervals, while others have declined in visibility and not attained their maxima until the period 120 to 60 hours before sunset. The diurnal curves of spots Nos. 14, 5, and 16 in the first category, and those of Nos. 9 and 11 in the second, have already been referred to; that of spot No. 22 (fig. 3) differs from the others by its showing an increase of visibility from sunrise to 120 hours before sunset. The visibilities of many spots are lower during the last 60 hours of the luni-solar day.
The curves of visibility during the luni-solar day are essentially different from the curves of visibility as deduced from the observations of twenty-four lunations, although both lead to the same result; and from both a very important conclusion may be drawn, viz. that upon assuming other agencies to be in operation than changes of illuminating angle, such as present activity, the epochs at which such activity was manifested varied to such an extent, and were so far separated from each other in time, as to coincide, in the case of spots Nos. 14, 5, and 16, with the period in the luni-solar day of 60 to 120 hours after sunrise, while the activity manifested by spots Nos. 9, 11, and 22 occurred at a later period of the luni-solar day, 120 to 60 hours before sunset. So far as the variations of visibility of spots Nos. 14, 5, 16, 9, 11, and 22 are concerned, they do not appear to depend exclusively on changes of illuminating angle, even if a certain intensity of solar light contributes generally to render the spots visible.
While the four craterlets Nos. 1, 3, 30, and 17 are visible during the whole of the luni-solar day, the spots on their sites are seldom seen until the sun attains an altitude of about 30°, and then they appear as "bright round disks;" and this characteristic attaches as well to the craterlets as to other spots when the sun attains this altitude. With altitudes between 30° and 40° a different class of phenomena is manifested; the sharp and distinct cha
2: 2282: 2: 3: 32: 29: 34: 82: 228: ±±882:
8: 46: : : 8288228638828: ❀±88;
2EE8: 2: 28288: 28282: 2~3228: 2898
racter of the craterlets is no longer observed. Some put on a hazy appearance, and they all assume the same aspect as those spots which have not been observed as craterlets. This state of things continues until the declining latitudes approach the limit at which the crater form was lost in the advancing day, then it once more appears accompanied by a disappearance of most of those spots which came into visibility as the sun rose higher. We have an analogous phenomenon to this in the well-known crater Aristarchus. Shortly after sunrise its outline is sharp and distinct, while its interior is partly covered with a well-marked shadow and partly glowing in strong sunlight. As the sun rises above its horizon these characteristics are lost; the ridge extending from it to Herodotus becomes brighter, and to some eyes, and with some instruments, it is confounded with the interior, the whole appearing as a very vivid brush of light. The exact solar altitude at which the 1871.
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 5, e, a, and B. The streaks and e are rather westward of their places as given on 5
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.
These are arranged in each interval of 12 hours according to season, so as to give increasing altitudes of the sun from -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