A GIANT PLANET. 89 



the same degree that those of the sun exceed 

 Jupiter's. But his mass, though gigantic compared 

 with the earth's, does not altogether correspond to his 

 bulk, for it exceeds the mass of the earth only three 

 hundred times. So that, if the disc 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. His 

 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 twenty- seven 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 



