288 THE POPULAR SCIENCE MONTHLY. 



correspond to his bulk, for it exceeds the mass of the earth only three 

 hundred times. So that, if the disk our astronomers see and measure 

 actually represents the true globe of the planet, his substance must be, 

 on the average, much less dense than that of the earth. In fact, while 

 the earth's density is nearly six times as great as that of water, the 

 density of Jupiter {thus judged) would exceed that of water by barely 

 one-third. This vast globe rotates, in less than ten hours, on an axis 

 nearly upright or square to the level in which the planet travels. This 

 rapidity of rotation so great that points on the planet's equator travel 

 twentyseven times as fast as points on the terrestrial equator results 

 in a considerable flattening of the planet's globe ; insomuch that the 

 polar diameter is less than the equatorial by about a twelfth part, or 

 by fully 7,000 miles. And it may be remarked in passing, that this 

 circumstance the fact, namely, that the poles of the planet are drawn 

 in, as it were, 3,500 miles as compared with the equatorial regions, or 

 1,750 miles as compared with the mid-latitudes in either hemisphere 

 affords a striking illustration of the enormous amount of energy really 

 represented by the rotation of Jupiter. It may also be added that the 

 velocity with which points on Jupiter's equatorial zone are carried 

 round, exceeds the corresponding velocity in the case of all the planets 

 in the solar system, and is nearly six times greater than the equatorial 

 velocity of the sun himself. It amounts, in fact, to about 7^ miles 

 per second. 



We do not propose to consider here at any length the system of 

 satellites over which Jupiter bears sway ; but this preliminary sketch 

 would be incomplete without a few words on the subject. It is worthy 

 of notice that, although our earth in some sort resembles the outer 

 planets in being accompanied by a satellite, yet the relation which our 

 moon bears to the earth is altogether different from that which the 

 satellites of the outer planets bear to their respective primaries. Our 

 moon is by no means a minute body by comparison with the earth, and 

 compared with Mars or Mercury she may be regarded as having very 

 respectable dimensions. We may, indeed, look upon the moon as a 

 fifth member of the inner family of planets a member inferior to the 

 rest, doubtless, but still not so far inferior to Mercury as Mercury is 

 inferior to the earth. In the case of the outer planets, however, and 

 especially in Jupiter's case, moons hold an utterly subordinate position. 

 Taking the accepted measurements, we find the largest of Jupiter's 

 moons less than the 16,000th part of its primary as respects bulk, 

 while its mass or weight is less than the 11,000th part of Jupiter's. 1 



1 It is not uncommonly stated in our text-books of astronomy, that the density of 

 Jupiter's moons is far less than Jupiter's density ; and Lardner goes so far as to say that 

 "the density of the matter composing these satellites is much smaller than that of any 

 other body of the system whose density is known." But this is a mistake. All the sat- 

 ellites, save one, are of greater density than Jupiter, and that one the innermost ia 

 denser than Saturn, Uranus, or Neptune. 



