70 ROTATION OF JUPITER'S SATELLITES. SECT. IX. 



nent meridian, to which the different spots on her surface have 

 been referred, and their positions are determined with as much 

 accuracy as those of many of the most remarkable places on the 

 surface of our globe. According to the observations of Professor 

 Secchi at Rome, the mountains of the moon are mostly volcanic 

 and of three kinds. The first and oldest have their borders 

 obliterated, so that they look like deep wells ; the second, which 

 are of an intermediate class, have elevated, and, for the most 

 part, regular unbroken edges, with the ground around them 

 raised to a prodigious extent in proportion to the size of the 

 volcano, with generally an insulated rock in the centre of the 

 crater. The third, and most recent class, are very small, and 

 seem to be the last effort of the expiring volcanic force, which is 

 probably now extinct. 



The distance and minuteness of Jupiter's satellites render it 

 extremely difficult to ascertain their rotation. It was, however, 

 accomplished by Sir William Herschel from their relative bright- 

 ness. He observed that they alternately exceed each other in 

 brilliancy, and, by comparing the maxima and minima of their 

 illumination with their positions relatively to the sun and to their 

 primary, he found that, like the moon, the time of their rotation 

 is equal to the period of their revolution about Jupiter. Miraldi 

 was led to the same conclusion with regard to the fourth satellite, 

 from the motion of a spot on its surface. 



