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£200 have been received towards the cost of publication for the first year, up to April 1872.
There is good reason to believe that the expectations entertained of the usefulness of these Reports will be fully realized by their continuance on the present system, and that they will be found largely to conduce to the progress of the science wherever the English language is spoken.
Report of the Committee for discussing Observations of Lunar Objects suspected of Change. The Committee consists of the Rev. T. W. WEBB and EDWARD CROSSLEY, Secretary.
THE Committee have much pleasure in presenting their first Report on the above subject. Though much attention has been given of late years to a large number of lunar objects, your Committee felt that they could not accomplish their purpose better than by confining their Report to the discussion of a limited and well-observed portion of the lunar surface. No person seeking to discover evidence of geologic change would be constantly travelling over the whole surface of our globe, but would of necessity confine his attention to a small area for a considerable period of time. This has been the course adopted on the moon. Plato, a vast crater, containing 2700 square miles, in 51° N. lat. and 10° E. long., has presented a most interesting and important variety of features, which we have endeavoured to photograph, so to speak, with pen and pencil, with a view, if not at once to obtain our ultimate object, at least to lay out the groundwork for future observers. The Report has been carefully drawn up by Mr. W. R. Birt on behalf of the Committee. Time has only permitted the discussion of the observations of the bright spots and craterlets seen on the floor of Plato; whereas your Committee consider that it is equally important that the observations of the numerous streaks, with the faults and other peculiar features noticed on the floor and walls of this fine formation, should be likewise discussed, in order that something like a complete description of this object as observed at the present time may be presented to the Association for the use of future selenographers.
Your Committee would therefore request that a further grant of £20 may be placed at their disposal for this purpose during the ensuing year.
Report on the Discussion of Observations of Spots on the Surface of the
In executing the task confided to me of discussing certain observations of the spots on the lunar crater Plato, one of the first points which I deemed it important to ascertain was the effect which the intensity of the sun's light as a function of his altitude might produce on the visibility of the spots. The number of spots actually observed between April 1869 and April 1871 inclusive, amounted to 37, the greater portion (21) having been discovered in this interval. In order to become acquainted with phenomena possibly connected with an increase of light on the floor of the crater, the observations have been arranged under intervals of twelve hours, from sunrise to sunset on Plato, and a ledger formed for each interval, the number of which is 31. From these ledgers the results in Table II. have been deduced, viz. the mean number of spots visible during each interval, and the actual number of spots observed during each interval. For illustrating the results
the curves in fig. 1 have been projected. The first curve is that of solar altitudes at the moon, epoch the equinoxes, locality 50° north or south latitude. The second curve is that of the mean number of spots visible during each interval.
We may regard the various maxima of the spot-curve as indicative:- First, of a greater number of observations during the intervals which furnish the maxima. It is true the column of observations may countenance this view; but it does not hold in all cases, neither are the greater number of observations so pronounced as the maxima of the curve. Second, of a clearer state of the earth's atmosphere than usual, enabling us to see more spots than when it is ordinarily translucent. This may to some extent explain the occurrence of maxima separated by several intervals, and probably those instances where we have a larger number of spots with a smaller number of observations. Third, of an actual increase of visibility of the spots themselves at different and widely separated epochs, the observations of such increased visibility falling at those intervals at which the maxima were recorded. The following are the epochs at which the greatest number of spots were observed corresponding with the maxima of the curve:
First maximum. Interval 2. 1870, Jan. 10, 12 spots, 15 for the whole interval, from 7 observations.