296

THE PLANET JUPITER.

so small au average density seems once more to be hardlyconsistent with the immensity of its size, and the consequentintensity of pressure which, as we have already hinted, mustexist in the vast depths within it. Moreover, the fact, thatthe calculated value of the attraction of the force of gravity,upon its surface, is found to be nearly 2^ times * as great asupon the surface of the Earth , is another reason for theexistence of a much greater, rather than a smaller, averagedensity in the planet as a whole. We may, therefore, concludethat the joint action of two causes affords the most probableexplanation of the apparently small density of the planetJupiter : the one, that its real and more solid part lies farbeneath the cloudy envelope which bounds our view; the other,that even that central portion may be in a state of veryconsiderable heat (although not sufficiently hot to be decidedlyluminous), and be consequently expanded to a greater size anda lighter density than it would possess if it were cooler.

We have been led to make the preceding preliminary remarkstouching upon some deeply interesting physical speculationswhich we shall presently treat at greater length, by our use,in the first paragraph of this lecture, of the expression “ the

* This may be approximately calculated by the following method,which, with the necessary alteration of figures, will equally well apply toother planets :—

A sphere attracts any external object as if all the matter in the spherewere condensed at its centre. The weight of Jupiter being found, fromthe rotation-period of any one of its satellites, or by some other method,,to be about 300 times that of the Earth , gravity upon its surface wouldbe 300 times as great as upon the Earth 's surface, if the respective dis-tances from the centre to the surface w r ere the same in each planet.But the distance in question being about 11 times as great in the case ofJupiter , and gravity acting inversely as the square of this distance, theattraction will be diminished about 121 times. Since 121 will divide into300 rather less than 2| times, it results that gravity upon Jupiter ’s surfaceis somewhat less than 2i times as great as it is upon the Earth ’s. Aman weighing 15 stones upon the Earth would therefore weigh about37 stones upon Jupiter , and a pound avoirdupois would weigh about39 ounces. A body dropped from rest would fall about 39 feet instead of1G feet in the first second of its fall, and would at the end of that secondpossess a velocity 2£ times as great as it would upon the Earth .