124 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



Saturn. Dr. H. thinks himself sufficiently authorized to make this conclusion, 

 though it may have hap[)ened sometimes that the light of the satellite has suf- 

 fered an occasional change, of short duration, from other causes; for the same 

 reason that we should certainly allow those who first saw the spots in the sun to 

 be in the right to assign the period of its rotation nearly, when they perceived 

 that the same spot made several revolutions, though that spot might afterwards 

 vanish. But Dr. H. thinks he may go further, and ascertain on sufficient 

 grounds, that this satellite turnsonceon its axis, exactly in the time it performs one 

 revolution round its primary planet. This degree of accuracy is obtained by 

 taking in the observations of M. Cassini, in the Memoires de I'Academie des 

 Sciences, 1705, page 121 ; where we find it mentioned, that " the 5th satellite 

 of Saturn disappears regularly for about one half of its revolution, when it is to 

 the east of Saturn." The same memoir contains also a conjecture of this satel- 

 lite's rotation on its axis; but this surmise is contradicted as premature, in 1707, 

 page 96; where we find the following paragraph. " M. Cassini gives an ex- 

 ample of the danger there is in these sort of determinations, that are made too 

 hastily. The 5th satellite of Saturn, of which we have said, in the History of 

 1705, page 121, that it became invisible, in the eastern half of the circle it 

 describes about Saturn, began, in the month of Sept. 1705, to be there visible, 

 as well as in the western half, where it always was so. Hence the conjectures 

 we have related cease to be well founded." 



Now, without determining whether the satellite, from some cause or other, 

 ceased to change its brightness, or whether its phenomena were not sufficiently 

 followed to come to a proper conclusion. Dr. H. thinks that with the assistance 

 of observations at so great a distance of time as those of M. Cassini, he may 

 sufficiently establish the period of this satellite's rotation. For since he had 

 traced the regular and periodical change of light, tlirough more than 10 revolu- 

 tions, and found them, in all appearance, to be contemporary with its return 

 about Saturn, it leads us to a strong presumption that its rotation on its axis, 

 like that of our moon, strictly coincides with its revolution round its primary 

 planet; and the observations of M. Cassini confirm, this conclusion. For had 

 he seen the satellite brightest in any other part of its orbit, their observations 

 would not have agreed together; but since the year 1705 the satellite has made 

 about 397 revolutions; and yet the phenomena described by Cassini answer now 

 as exactly to Dr. H.'s observations, as the spots in our moon, viewed in Cassini's 

 time, answer to those we now observe. 



If it should be objected, that the 5th satellite of Saturn has not been con- 

 tinually observed, and that consequently these appearances might either not 

 happen at all, or fall on difl^erent places in its orbit; Dr. H. answers, that a 

 period of more than 10 revolutions is a strong argument that no such change 



