C 332 3 
XV. Observations of the changeable Brightness of the Satellites 
of Jupiter , and of the Variation in their apparent Magnitudes ; 
with a Determination of the Time of their rotatory Motions 
on their Axes. To which is added , a Measure of the Diameter 
of the Second Satellite y and an Estimate of the comparative 
Size of all the Four. By William Herschel, LL.D. F.R. S. 
Read June 1, 1797. 
It may be easily supposed when I made observations on the 
brightness of the 5th satellite of Saturn, by way of determining 
its rotation upon its axis, and found that these observations 
proved successful, that I should also turn my thoughts to the 
rest of the satellites, not only of Saturn, but likewise of Jupiter, 
and of the Georgian planet. Accordingly I have from time to 
time, when other pursuits would permit, attended to every cir- 
cumstance that could forward the discovery of the rotation of 
the secondary planets ; especially as there did not seem to lie 
much difficulty in the way. For since I have determined, by 
observation, that the 5th satellite of Saturn is in its rotation 
subject to the same law that our moon obeys, it seems to be 
natural to conclude that all the secondary planets, or satellites, 
may probably stand in the same predicament with the two I 
have mentioned; consequently a few observations that coincide 
with this proposed theory, will go a good way towards a con- 
firmation of it. 
I had another point in view when I made the observations 
