The volume of Jupiter is about 1300 times that of the earth, _i.e._ it would take 1300 earths rolled into one to equal Jupiter in size. But its mean density is slightly less than one quarter of the earth's, so that its mass is only 316 times greater than the earth's. The force of gravity on its surface is 2.64 times the earth's. A body weighing 100 pounds on the earth would weigh 264 pounds on Jupiter. It will be observed that Jupiter's mean density is very nearly the same as that of the sun, and we conclude that it cannot be a solid, rigid globe like the earth. This conclusion is made certain by the fact that its period of rotation on its axis is variable, another resemblance to the sun. The equatorial parts go round in a shorter period than parts situated some distance north or south of the equator. It may be supposed that there is a solid nucleus within, but if so, no direct evidence of its existence has been found.