Page:Astronomy for Everybody.djvu/227

gated strata of clouds, apparently due to the same cause that produces stratified clouds on the earth, namely, currents of air. Among these clouds round white spots are frequently seen. The clouds are sometimes of a rosy tinge, especially those near the equator. They are darkest and most strongly marked in middle latitudes, both north and south of the equatorial regions. It is this that produces the appearance of dark belts in a small telescope.

The appearance of Jupiter is, in almost every point, very different from that of Mars or Venus. Comparing it with Mars, the most strongly marked difference consists in the entire absence of permanent features. Maps of Mars may be constructed and their correctness tested by observations generation after generation, but owing to the absence of permanence, no such thing as a map of Jupiter is possible.

Notwithstanding this lack of permanence, features have been known to endure through a number of years, and some of them may be permanent. The most remarkable of these was the great red spot, which appeared in middle latitudes, on the southern hemisphere of the planet, about the year 1878. For several years it was a very distinct object, readily distinguished by its colour. After ten years it began to fade away, but not at a uniform rate. Sometimes it would seem to disappear entirely, then would brighten up once more. These changes continued but, since 1892, faintness or invisibility has been the rule. If the spot finally disappeared, it was in so uncertain a way that no exact date for the last observation