Page:Astronomy for Everybody.djvu/230

dence of vapour as shown by the constantly changing aspect of the planet, we have another almost conclusive piece of evidence in the law of rotation. It is found that Jupiter resembles the sun in that its equatorial region rotates in less time than the regions north of middle latitude, although the circuit they have to make is longer. This is probably a law of rotation of gaseous bodies in general. It seems, therefore, that Jupiter has a greater or less resemblance to the sun in its physical constitution, a view which quite corresponds with its aspect in the telescope. The difference in the time of rotation at the equator and in middle latitudes is, so far as we yet know, about five minutes. That is to say, the equatorial region rotates in nine hours fifty minutes and those in middle latitudes in nine hours fifty-five minutes. This corresponds to a difference of velocity of the motion between the two amounting to about two hundred miles an hour; a seemingly impossible difference were the surface liquid.

It is a singular fact that no well-defined law of rotation in different latitudes has yet been made out, as has been done in the case of the sun. Were we to accept the recults of the meagre observations at our disposal we might be led to the conclusion that the difference of time is not a gradually varying quantity, as we go from the equator toward the poles, but that the change of five minutes occurs very near a certain latitude and almost suddenly. But we cannot assume this to be the case without more observations than are yet on record. The subject is one of which an accurate investigation is greatly to be desired.