35° Dr. Herschel’s Observations of the 
The 3d is always white ; but the colour is of different inten- 
sity, in different situations. 
The 4th is dusky, dingy, inclining to orange, reddish and 
ruddy at different times ; and these tints may induce us to sur- 
mise that this satellite has a considerable atmosphere. 
I shall conclude this paper with a result of the observation of 
the diameter of the second satellite, taken by its entrance upon 
the disc of the planet, July 28, 1794, and marked in fig. 2. at 
176' degrees. 
The duration by the observation is fixed at 4 minutes ; in 
which time it passes over an arch in its orbit of 16' 52", 9. 
Now as its distance from the planet is to its distance from the 
earth, so is 1 6' 52", 9 to the diameter of the satellite ; or the 
mean distance of the 2d satellite may be rated, with M. de la 
Lande, at 2' 57", or 177". Then putting this equal to radius, 
we shall have the following analogy. Radius is to 177", as the 
tangent of 1 6' 52", 9 is to the angle, in seconds, which the dia- 
meter of the second satellite subtends when seen from the earth. 
And by calculation, this comes out o",87 ; that is less than 
nine-tenths of a second. 
I have not been scrupulously accurate in this calculation, as 
the real distance of Jupiter at the time of observation should 
have been computed, whereas I have contented myself with the 
mean distance. Nor am I very confident that the angle of the 
greatest elongation, admitted to be 2 ; 57", is quite accurate ; 
but I judged it unnecessary to be more particular, because the 
time of my observation in the beginning of the transit upon the 
disc, I find was only taken down in whole minutes of the clock. 
The end, however, is more accurately determined, by the ob- 
servation which was made 45" after the immersion ; when a 
