The Plurality of Worlds

36

Hersch. 820.

37

The periodical character of this star was discovered by David Fabricius, a parish priest in East Friesland, the father of John Fabricius, who discovered the solar spots. (Cosmos, iii. 234.)

38

Hersch. 825. In Humboldt's Cosmos, iii. 243, Argelander, who has most carefully observed and studied these periodical stars, has given a catalogue containing 24, with the most recent determinations of their periods.

39

Hersch. 821. Humboldt (Cosmos, iii. 238 and 246,) gives the period as 68 hours 49 minutes, and says that it is 7 or 8 hours in its less bright state. If we could suppose the times of the warning, and of the greatest eclipse, given by Herschel, to be exactly determined, as 3

/

and

/

, that is, in the proportion of 14 to 1, the darkening body must have its effective breadth

/

of that of the star. But this is on the supposition that the orbit of the darkening body has the spectator's eye in its plane; if this be not so, the darkening body may be much larger.

40

Hersch. Outl. Astr. 821. Another explanation of the variable period of Algol, is that the star is moving towards us, and therefore the light occupies less and less time to reach us.

41

Humboldt, very justly, regards the force of analogy as tending in the opposite direction. "After all," he asks, (Cosmos, iii. 373,) "is the assumption of satellites to the Fixed Stars so absolutely necessary? If we were to begin from the outer planets, Jupiter, &c., analogy might seem to require that all planets have satellites. But yet this is not true for Mars, Venus, Mercury." To which we may further add the twenty-three Planetoids. In this case there is a much greater number of bodies which have not satellites, than which have them.

42

Consolations in Travel. Dial. 1.

43

What is said in Art. 15, that in consequence of the time employed in the transmission of visual impressions, our seeing a star is evidence, not that it exists now, but that it existed, it may be, many thousands of years ago; may seem, to some readers, to throw doubts upon reasonings which we have employed. It may be said that a star which was a mere chaos, when the light, by which we see it, set out from it, may, in the thousands of years which have since elapsed, have grown into an orderly world. To which bare possibility, we may oppose another supposition at least equally possible:—that the distant stars were sparks or fragments struck off in the formation of the Solar System, which are really long since extinct; and survive in appearance, only by the light which they at first emitted.

44

More recently, at the meeting of the British Association in September, 1853, Professor Phillips has declared, that astronomers can discern the shape of a spot on the Moon's surface, which is a few hundred feet in breadth.

45

A person visiting the Eifel, a region of extinct volcanoes, west of the Rhine, can hardly fail to be struck with the resemblance of the craters there, to those seen in the moon through a telescope.

46

Bessel has discussed and refuted (it was hardly necessary) the conjecture of some persons (he describes them as "the feeling hearts who would find sympathy even in the Moon") that there may be in the Moon's valleys air enough to support life, though it does not rise above the hills.—Populäre Vorlesungen, p. 78.

47

The doctrine that the interior nucleus of the Earth is fluid, whether accepted or rejected, does not materially affect this argument. It appears, that in some cases, at least, the melting of substances is prevented, by their being subjected to extreme pressure; but the density, the element from which we reason, is measured by methods quite independent of such questions.

48

Herschel, 512. Bessel, however, holds that the oblateness of Jupiter proves that his interior is somewhat denser than his exterior. Pop. Vorles. p. 91.

49

Herschel, 513.

50

A difficulty may be raised, founded on what we may suppose to be the fact, as to the extreme cold of those regions of the Solar System. It may be supposed that water under such a temperature could exist in no other form than ice. And that the cold must there be intense, according to our notion, there is strong reason to believe. Even in the outer regions of our atmosphere, the cold is probably very many degrees below freezing, and in the blank and airless void beyond, it may be colder still. It has been calculated by physical philosophers, on grounds which seem to be solid, that the cold of the space beyond our atmosphere is 100° below zero. The space near to Jupiter, if an absolute vacuum, in which there is no matter to receive and retain heat emitted from the Sun, may, perhaps, be no colder than it is nearer the Sun. And as to the effect the great cold would produce on Jupiter's watery material, we may remark, that if there be a free surface, there will be vapor produced by the Sun's heat; and if there be air, there will be clouds. We may add, that so far as we have reason to believe, below the freezing point, no accession of cold produces any material change in ice. Even in the expeditions of our Arctic navigators, a cold of 40° below zero was experienced, and ice was still but ice, and there were vapors and clouds as in our climate. It is quite an arbitrary assumption, to suppose that any cold which may exist in Jupiter would prevent the state of things which we suppose.

51

Herschel, 508.

52

It may be thought fanciful to suppose that because there is little or no solid matter (of any kind known to us) in Jupiter, his animals are not likely to have solid skeletons. The analogy is not very strong; but also, the weight assigned to it in the argument is small. Valeat quantum valere debet.

53

Herschel, 522.

54

Herschel, 510.

55

According to Bessel, Schroeter once saw one bright point on the dark ground, near the boundary of light in Venus. This was taken as proving a mountain, estimated at 60,000 feet high. Pop. Vorles. p. 86.

56

Herschel, 509.

57

Other speculators also have regarded Saturn's Ring as a ring of cloud or water. See Cosmos, iii. 527 and 553.