57 2 THE POPULAR SCIENCE MONTHLY. 



powerful telescopes are indispensable. Unfortunately, it is a difficult 

 matter to study, for on only one or two days in the year is Messier 

 likely to be found in a proper condition to be observed. 



Other instances of change in the lunar surface have been detected 

 by selenographers, but the evidence on which they rest is not so over- 

 powering as is needed to induce their acceptance. 



Periodical variations in the color and brightness of lunar regions, 

 such as would result from processes of vegetation, were first noticed 

 by Beer and Madler, but they regarded the absence of masses of water 

 upon the moon as a fatal objection to this explanation. The variation 

 in the floor of Plato is one of the most interesting of these changes. 

 Plato is a circular plain 60 miles in diameter, surrounded by a belt of 

 highlands from 3,000 to 3,500 feet in height. These highlands at sun- 

 rise are a pale yellowish gray, gradually changing to grayish white. 

 At sunrise the interior of Plato appears of a cold gray, but, as the 

 sun rises higher above the horizon of Plato, and the solar rays fall 

 more perpendicularly on this region, the whole surface grows rapidly 

 brighter, until, about two days after sunrise, the interior of the for- 

 mation attains its brightest tint. It is then a cold, light yellow gray, 

 often approaching a pale yellow, in fact, and brighter than the sur- 

 face of the Mare Imbrium on the north, while the surrounding high- 

 lands are a bright grayish white, tinted here and there with gray. 

 Judging from what occurs in any of the numerous other formations 

 resembling Plato, this may be considered the normal tint, inasmuch 

 as those other formations which present exactly the same phenomena 

 up to this time, and which, under similar conditions, present exactly 

 the same appearance, retain this tint unaltered until near sunset. Af- 

 ter, however, the second day, the floor of Plato commences to under- 

 go a most extraordinary and anomalous change, which renders it unique 

 on the moon, for, instead of growing lighter, the interior commences 

 to become darker. Four days after sunrise it is materially darker 

 than the northern Mare, and a cold gray in tint, while the surround- 

 ing highlands are a bright white in color, tinted with gray ; the ap- 

 pearance they retain until the thirteenth day after sunrise, growing a 

 little, though not very much, brighter toward full moon. Two days 

 later the floor of Plato has become a dark gray; at full moon it is 

 deep steel-gray ; and, about two days after full, reaches its darkest 

 tint, a very deep steel-gray, almost approaching a black color. Under 

 these conditions, it is one of the very darkest portions of the entire 

 lunar surface, though, seven days prior, it was one of the lightest por- 

 tions of the surface of its kind. After this, it gradually lightens in 

 tint, but much slower, and never reaches so light a tint. 



This extraordinary periodical change in the tint of the floor of 

 Plato has hitherto received no explanation, but its existence has been 

 put beyond the pale of doubt, and Mr. Birt has, at the instance of a 

 British Association Committee, carefully discussed a numerous series 



