250 



REPORT 1872. 



Nos. 5 and 14. Some months afterwards a continuation of this streak east- 

 ward of No. 5 was observed, and very lately it has been seen between Wos. 

 14 and 22 {p, fig. 1, on p. 247). A very remarkable characteristic of this 

 streak is its parallelism with the south border. Taking all the circumstances 

 of the observations into account, it can scarcely be doubted that this is a neiv 

 streak, the eastern and western portions being connected with spot No. 5, 

 and the further continuation westward with spot No. 22. 



If it should be well established that new streaks make their appearance 

 from time to time, we may be able to understand that many recorded differ- 

 ences from the older delineations are to be referred not so much to errors of 

 the earlier selonographers, as to real changes in the 

 objects themselves ; for example, this very area of 

 Plato is figured by Beer and Madler, in the first 

 edition of the large map, as being crossed from N. to S. 

 hj four light streaks, as in the annexed sketch, fig. 2. 

 That Madler actually saw these four streaks there can 

 be little doubt, as they are distinctly mentioned in 

 ' Der Mond.' That they are not in existence at present 

 is quite certain; for the disposition of the streaks is noWp, 

 very different from that figured by Beer and Madler. 



I. 



Inflttence or the Sun on the Floor op Plato. 



Previous to an examination of certain non-periodic changes of brightness, 

 colour, and the forms of streaks, it is essential to ascertain the normal varia- 

 tions of tint as dependent upon the gradual increase and decrease of the sun's 

 altitude before and after the sun's meridian passage. This has been ascer- 



-Beer and Madler. 



13 6 7 9 U 13 15 17 19 2]. 



•S6 2b 27 29 31 



Curve of variation of tint on the floor of Plato. 



tained by noting the tint of the floor in accordance with the directions 

 specified on the form for receiving the records of the observations. A 

 medium tint has been regarded as the mean line, and its value fixed at 0-50 ; 

 and as the curve of the sun's altitude consists of an ascending and a descending 



