132 OUTLINES OF NATURAL PHILOSOPHY. 



plains them, but has led us to distinguish many 

 more, of which the accumulated effect was percei- 

 ved, by the disagreement of the calculated with the 

 observed places of the moon, though their laws 

 were unknown. These will be considered under 

 the head of Physical Astronomy. 



Of the Moon's Revolution on her Axis. 



132. The moon, as she revolves in her orbit 

 about the earth, preserves always, at least near- 

 ly, the same face turned toward the earth, and 

 therefore must revolve on her axis, in the same 

 direction, and in the same time, that she re- 

 volves in her orbit. 



a. The moon's surface, when viewed with a telescope, 

 presents an object so strongly characterised, as to 

 leave no doubt of its being always the same. 



The accurate examination of the spots, which the 

 tracing of the progress of the illumination has na- 

 turally induced, has discovered some apparent ine- 

 qualities in the moon's revolution on her axis, to 

 which the name ofLibration has been given. They 

 are optical appearances, and argue no real ine- 

 quality in the moon's rotation. 



133. If 



