246 REPORT— 1872. 



The results of the present work may be briefly characterized as confirming, 

 by a direct reference to the sun's altitude above the horizon of Plato, the sup- 

 position that variations of tint in some measure depend on increasing and 

 decreasing altitudes. The ascending and descending branches of the curve 

 obtained from independent estimations of tint by the several observers are 

 sufficiently near those of the sun's altitude to enable me to delineate a nor-^ 

 mal curve representative of the sun's influence in darkening the floor of Plato, 

 or else in overspreading it vrith something of the nature of a dark covering, as 

 his rays strike the surface at the increased angle of about 40 degrees. While 

 this darkening influence comes out most unmistakably, there are variations 

 in the lighter and darker portions of the floor which seem quite irreconcHe- 

 able with solar influence of a gradual character. The treatment of the obser- 

 vations under intervals of the luni-solar day fails to bring out any regularity 

 in these variations, and it is only by treating the observations clironologicaUy 

 that the true sequence of the changes can be detected. To do this for every 

 separate streak woiild not only swell this Report to an unseemly length, but 

 woiild consume more time than can be devoted to the inquiry. I have, 

 nevertheless, considered separatelj'' the changes which were observed in 

 August 1869; and in order to assist in showing more distinctly these changes 

 and their connexion inter se, I have introduced the hypothesis of a dark 

 obscuring medium. Not that I lay any stress upon a mere hypothesis of this 

 kind ; it serves to connect the observations, and that is all ; it may or it 

 may not be true, and should therefore be held very lightly. In addition to 

 this examination of the distribiition of the light and dark spaces on the floor, 

 I have traced from day to day the appearances of a single streak, that desig- 

 nated a, from its flrst detection in September 1869 to the close of the obser- 

 vations; and to show more conclusively that the variations manifested by this 

 and neighbouring streaks were not dependent upon the same solar influences 

 which contributed to the darkening of the floor, I have arranged all the ob- 

 servations bearing upon them in the order of intervals of the luni-solar day. 

 The principal divisions of the present Report are: — 1, the influence of the sun 

 on the floor of Plato ; 2, an examination of changes recorded in August 

 1869 ; 3, the history of streak a ; and 4, observers' notes arranged in 

 intervals of the luni-solar day, and embodying generally the results of the 

 two years' observations. 



It may contribute to a better understanding of the nature of the streaks, 

 their connexion with, the spots, and their variability, if the physical charac- 

 teristics of Plato be described. We have : — First, a mountain-cinctured plain, 

 of about sixty miles in diameter, the wall rising to the average height of 

 nearly 4000 Enghsh feet. This waU is surmounted at four points by needle- 

 like pinnacles of rock, which rise to a further elevation of 3000 feet, so that 

 their summits attain the height of about 7000 feet above the plain, which is 

 not strictly level, the border having suffered from dislocation, which has 

 raised the floor in a direction from S.E. to N.W. Second, two systems of 

 streaks, as seen between April 1869 and April 1871, They are related to the 

 " fault " produced by dislocation. The S.W. system consists of the " trident," 

 the N.E. of the streaks 0, ij, and y (see fig. 1). These two systems, which 

 are opposite in direction, are intimately connected with certain spots in their 

 respective neighbourhoods, the S.W. radiating from spot No. 1. Of the N.E. 

 system, streak /3 emanates from spot No. 3, r] from spot No. 4, and y from 

 spot No. 6. The most prominent streak on the floor is the sector which takes 

 its rise from spot No. 4, but has occasionally been seen in the opposite direc- 

 tion, extending as far as spot No. 3. The S.E. portion, that extending to the 



