1875.] The Atmospheres of the Planets. 459 
would be entirely undistinguishable from the earth. And 
the entire absence of any detectable retardation of the oc- 
cultation of Jupiter’s satellites is in itself sufficient to show 
the absence of any great depth of atmosphere; for if the 
increase of density was so slow as to admit of this, the 
decrease beyond the cloud layers would be sufficiently slow 
to admit of the refraction being detected. 
There is one point in connection with the maximum 
depth of the atmosphere of Jupiter that requires further 
attention, and that is certain phenomenaseen during thetran- 
sits of the shadows of the satellites across the planet. By 
numerous observers it has been noticed that occasionally 
the form of the shadow is not a black circle, but a slightly 
elongated ellipse, which though it has been estimated to be 
as much as halfas long again as broad, probably is very rarely 
so markedly elongated, but that from the character and very 
delicate nature of the observations, the ellipticity has been 
as usual considerably over-estimated. It has been noticed 
that this elongation only occurs when the planet is near 
quadrature, so that the shadow on Jupiter is seen from a 
point inclined at a considerable angle to the axis of the cone 
of shadow. 
It has, however, been considered* that this indicates a 
peculiar character and great depth in the atmospheric enve- 
lope of this giant planet, and from a series of nine rough 
estimates of the amount of this elongation, an attempt has 
been made to deduce the depth of the atmosphere of Jupiter 
by assuming the cause of this appearance to be due to the 
passage of the shadow cone, though a multitude of very 
thin, delicate, almost translucent clouds, suspended in the 
-atmosphere of Jupiter. For assuming the atmosphere of 
this planet to be filled with delicate clouds of this nature, 
and so extraordinarily transparent as to permit shades to be 
visible through immense thicknesses of it, it can be 
shown that when Jupiter is near quadrature, the shadow 
column of the satellite will appear as an elliptical dark spot, 
with an equatorial penumbra. Supposing these conditions 
to hold, from nine observations the depth of the atmosphere 
of Jupiter is shown by a rough method to vary in minimum 
depth from 3200 to 9200 miles, the mean of nine estimates 
being 5400 miles. Even granting that the constitution of 
the atmosphere supposed in this hypothesis can be considered 
conceivable, and crediting the atmosphere of Jupiter to be 
filled with delicate clouds sufficiently transparent to enable 
graduation of shadow to be distinctly visible through an 
* Burton, M.N. Roy. Ast. Soc., vol. xxxv., p. 65. 
