VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 125 



has taken place; for if the satellite had but made a single rotation on its axis 

 more or less than it has made revolutions round Saturn, the change must amount 

 to nearly 1° per revolution; that is, to about 10° during the time of taking 

 notice of it; which is a quantity he might have perceived. However, to remove 

 all doubt, we have some valuable observations of M. Bernard, who in the year 

 1787 also found the 5th satellite of Saturn subject to the same change of light 

 that M. Cassini had observed.* Now, by joining those to Dr. H.'s, we have a 

 short period of nearly 20 revolutions that agree together, so as to preclude all 

 doubt of any intermediate change; and therefore we cannot be liable to err, 

 when we extend this period to all the 397 revolutions since Cassini's time, 

 and by that means ascertain that the 5th satellite of Saturn turns on its axis, 

 once in 79 days, 7 hours, and 47 minutes. 



I cannot help reflecting, with some pleasure, on the discovery of an analogy, 

 which shows that a certain uniform plan is carried on among the secondaries of 

 our solar system ; and we may conjecture, that probably most of the moons of 

 all the planets are governed by the same law; especially if it be founded on such 

 a construction of the figure of the secondaries, as makes them more ponderous 

 towards their primary planets. For if even the 5th satellite of Saturn, which is 

 at so great a distance from its planet, is affected by such a law, of course the 

 other satellites are not very likely to have escaped its influence. 



From the considerable change in the brightness of the 5th satellite of Saturn, 

 we may be certain that some part of its surface, and this by far the largest, 

 reflects much less light than the rest; and, from the points of its orbit in which 

 it appears brightest to us, we conclude that neither the darkest nor brightest 

 side of the satellite is turned towards the planet, but partly one and partly the 

 other; though probably rather less of the bright side. 



The great regularity of this change of brightness seems to point out another 

 resemblance of this satellite with our moon. It is well known that we see the 

 spots of the moon pretty nearly of the same brightness, so as not to be overcast 

 in a very strong degree by dense clouds to disfigure them, and therefore have 

 great reason to surmise that her atmosphere is extremely rare; which indeed we 

 also know from other principles: In like manner, on account of the uninter- 

 rupted changes in the brightness of the 5th satellite of Saturn, we may suppose 

 that it also partakes of a similar fate with respect to its atmosphere, which is 

 probably as rare as that of our moon. 



On the distance of the 5th satellite. — The distance of the 5th satellite from 



Saturn is allowed to be the most proper for obtaining a true measure of the 



quantity of matter contained in the planet; for which reason Dr. H. took many 



measures of it with the 20-feet reflector. He gave them at full length, but it is 



" See Memoires de 1' Academic, 1786, page 378. 



