192 PHILOSOPHICAL TRANSACTIONS. [aNNO 1797* 



ness of the light of the satellites with the different brightness of the disc of Jupiter ; 

 but these endeavours will always fail, on account of the little assurance we can 

 have that the parts of the disc, setting aside its quick rotation, will remain for any 

 time of the same lustre. 



A very material difficulty arises from the magnifying power we use in our esti- 

 mations. If it be a low one, such as for instance 180 (for a lower should not even 

 be attempted), then we run the risk of being disappointed in bright nights by the 

 sparkling of the brilliant light of the satellites. Besides, we cannot then see the 

 bodies of them, and judge of their comparative magnitude, with the same power 

 that we view their light. If we choose a higher magnifier, we shall be often dis- 

 appointed in the state of the atmosphere, which will of course occasion an inter- 

 ruption in the series of our observation, of which the regular continuance is of the 

 greatest consequence. If we change our power according to the state of the at- 

 mosphere, we introduce a far worse cause of confusion; for it will be next to im- 

 possible to acquire, for each magnifying power, an ideal standard of comparative 

 brightness to which we can trust with confidence. 



If the magnitudes are not attended to, and carefully contra-distinguished from 

 the intenseness of light, we shall run into considerable error, by saying that a 

 satellite is large, when we mean to express that it is bright. It is so common to 

 call stars that are less bright than others, small, that we must be careful to avoid 

 such ambiguities, when the condition of the satellites is under investigation. Nor 

 is it possible to throw the size and light into one general idea, and take the first 

 coup d'ceil in looking at them, to decide about the general impression this com- 

 pound may make. When our attention is forcibly drawn by a considerable power 

 to the apparent size of the satellite we are looking at, its brightness can no longer 

 be taken in that general way, but must be abstracted from size. 



Let us now see what use may be drawn from the observations I have given. It 

 appears in the first place very obviously, that considerable changes take place in the 

 brightness of the satellites. This is no more than might be expected. A variegated 

 globe, whether terraqueous like the earth, or containing regions of soil of an un- 

 equal tint, like that side of the moon which is under our inspection, cannot, in its 

 rotation, present us with always the same quantity of light reflected from its surface. 

 In the next place, the same observations point out what we could hardly expect to 

 have met with; namely, a considerable change in the apparent magnitude of the 

 satellites. Each of them having been at different times the standard to which 

 another was referred, we cannot refuse to admit a change so well established, sin- 

 gular as it may appear. 



The first of these inferences proves that the satellites have a rotatory motion on 

 their axes, of the same duration with their periodical revolutions about the primary 

 planet. The 2d either shows that the bodies of the satellites are not spherical, but 

 of such forms as they have assumed by their quick periodical and slow contemporary 

 rotatory motions, and which forms in future may become a subject for mathematical 



