GRAVITY OX JUPITER. 



ii would require the admission, thatf the organised world upon 

 it must be totally different from that which exists upon the earth. 



But Hi'-' the materials of which Jupiter is composed similar to 

 those of the earth ? If not, the conclusion at which we have 

 arrived must be modified. The whole question resolves itself 

 into tlu deierminatiou of the actual quantity of gravitating 

 matter composing this stupendous planet compared with the 

 quantity composing the earth. Now it will be apparent, that if 

 we could ascertain the attractions which Jupiter and the 

 earth would exert upon bodies placed at equal distances from 

 them, these attractions would be the exact exponents of the 

 quantities of gravitating matter composing these two globes. 



Tiiis we are happily enabled to accomplish by a very simple 

 and obvious arithmetical operation. The moon, as is well known, 

 revolves in its monthly orbit round the earth, and is retained in 

 that orbit by the attraction of the mass of gravitating matter 

 composing the earth. If that mass were greater the moon would 

 revolve faster, if less, slower. Its rate of motion is therefore an 

 index to the quantity of gravitating matter composing the earih. 



Jupiter like the earth is also attended, but by four and not by 

 one nioon. Each of these four nioons is retained in its orbit 

 s-ound Jupiter by the attraction of the gravitating mass com- 

 posing that planet. If it had happened that one of these four 

 moons were at exactly the same distance from Jupiter's centre 

 as the earth's moon is from its centre, then the motion of that 

 moon would at once prove whether the quantity of matter com- 

 posing Jupiter is greater or less than the quantity of matter 

 composing the earth. If the moon of Jupiter being thus at the 

 same distance moved faster than the earth's moon, the mass of 

 Jupiter would be greater, and if it moved slower it would be less 

 than the in:iss of the earth. 



Although all the moons of Jupiter are more distant from its 

 centre than the moon is from the earth's centre, the nearest of 

 these moons to Jupiter is not much more distant. Yet this moon 

 makes a complete revolution round Jupiter in forty-two hours, 

 while the earth's moon, though a little nearer to the attracting 

 mass, takes nearly 656 hours to make a revolution. 



It is obvious, therefore, that the gravitating mass composing 

 Jupiter must be vastly greater than that which composes the 

 earth. 



By allowing for the difference of distance of the two moons 

 from the centres of the two planets, and by taking into account 

 the exact proportion of their velocities, it has been found that the 

 jna.ss of gravitating matter composing Jupiter is 338^ times the 

 i;iass of the earth. 



29 



