Page:Popular Science Monthly Volume 11.djvu/99

Rh activity of the forces at work below them—in other words, the intense heat of Jupiter's real globe. As to the actual depth of the semitransparent atmosphere in which these cloud-layers and. cloud-masses float, it would be difficult to express an opinion. We do not know how many cloud-layers there are, how thick any cloud-layer may be, how great may be the depth of the vast rounded masses of cloud whose upper surface (that is, the surface remotest from Jupiter's true surface) we can alone see under favorable conditions. But we can indicate a minimum than which the atmosphere's depth is probably not less; and, from all the observations which I have examined as bearing on this point, I should be disposed to assign for that minimum at least 6,000 miles. I am strongly of opinion that in reality the depth of the Jovian atmosphere is still greater. I cannot doubt that Jupiter has a solid or liquid nucleus, though this nucleus—glowing, as it must be, with a most intense heat—may be greatly expanded; yet I should conceive that, with the enormous attractive power residing in it, containing as it must nearly the whole mass of the planet, its mean density cannot be less than that of the earth. Now, a globe of the mass of Jupiter, but of the same mean density as our earth, would have one-fourth of Jupiter's volume—the mean density of Jupiter, as at present judged, being equal to one-fourth that of the earth. The diameter, therefore, of such a globe would be less than the present diameter of Jupiter, in the same ratio that the cube root of unity is less than the cube-root of 4, or as 1 is less than 1.5874. Say, roughly (remembering that the atmosphere of Jupiter must have a considerable mass), the diameter of Jupiter's nucleus would, on the assumptions made, be equal to about five-eighths of his observed diameter, or to about 53,000 miles. This is less than his observed diameter by about 22,000 miles, so that the radius of his nucleus would be less than his observed radius by about 11,000 miles—which, therefore, would be the probable depth of his atmosphere.

But we have still to consider the velocities with which rounded masses of cloud travel in the very deep atmosphere of Jupiter. "There is clear evidence," I have pointed out in the article "Astronomy" of the "Encyclopædia Britannica," "that spots on Jupiter are subject to a proper motion like that which affects the spots on the sun. Schmidt, in No. 1,973 of the Astronomische Nachrichten, gives a number of cases of such proper movements of spots, ranging in velocity from about seven miles to about 200 miles an hour. It may be noted, also, that from a series of observations of one spot, made between March 13 and April 14, 1873, with the great Rosse reflector, a period of 9 h. 55 m. 4 s. was deduced, while observations of another spot in the same interval gave a rotation period of 9 h. 54 m. 55.4 s." The actual difference of velocity would depend in this case on the actual latitudes of the two spots, which were not micrometrically measured. Taking 200,000 miles as about the circumference of a