PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 81.7 



Some of tliese ring raountiiin formations have a diameter of over 

 150 miles, while upon the other hand they decrease in magnitude 

 until the}' cease to be visible in the telescope ; many of them, also, 

 though perfect types of the ring formation have no system of streaks 

 radiating from them, and this peculiarity does not depend upon their 

 size, as some basins that are much larger than Tycho have ray 

 systems, while all sizes, including many of the very smallest, project 

 rays varying in quantity, magnitude and intensity. As they decrease 

 in magnitude, however, the aspect of the rims becomes less broken 

 and rugged, until in those of the lesser diameters the rampart 

 becomes smooth, and the interior regularly concave, but as their 

 genei'al characteristics are the same, it follows that they are all alike 

 the result of a common cause. 



As the ring mountains decrease in size, their numbers increase in 

 a corresponding ratio, and the smaller a basin the more recent seems 

 to have been the period of its formation, if we may judge by the 

 sharpness of the outline as compared with that of the larger rings, 

 from whicli we may infer the progressive modification of the active 

 forces which have constructed the topography of the moon's surface,, 

 and it would also indicate that anciently, that whole globe must have- 

 been in a condition in which the material directly producing the force 

 acted in large masses, and, therefore, with corresponding power ; 

 hence the early results present the most marked evidence of violence. 



As geological epochs rolled into the past, the active agent became 

 more generally disseminated as a necessity of the very modification* 

 it was engaged in producing, and the more general dissemination 

 increasing the number of points to which it was directed, decreased in 

 the same proportion the indi\ndual volume of the active agent, and, 

 consequently, the violence of the force with which it acted ; hence 

 the gradual diminution in the size of the ring mountains, until the 

 miniature proportions of the younger members of the family remain 

 yet to be discovered by an increase in the power of the telescope. 



The ray systems which proceed from the ring formations are always 

 higher in the immediate neighborhood of the center, and decrease 

 their altitude in proportion to the increase of distance from that 

 point; this is shown by all the examples which are favorably situated 

 for observation upon the moon's surface. 



In the stereoscopic pictures made by Mr. Whipple, of Boston, with 

 the Cambridge refractor, a small sized ring formation named Thales, 

 comes in such a position upon the terminator, as to show' very dis- 



[Inst.] .52 



