Part 99

Footnote A29:

The philosophy of Aristotle retained terms so very obscure, that it seems the Devil himself did not understand, or at least could not explain them; otherwise we can hardly suppose, that, when the good patriarch of Venice had summoned his attendance for this very purpose, he would have been so rude as to put him off with an answer not only unintelligible but inarticulate. See Bayle, in Art. Barbaro.

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And now the Astronomy of our planetary system seemed compleated. The telescope had discovered all the globes whereof it is composed, at least as far as we yet know. Newton with more than mortal sagacity had discovered those laws by which all their various, yet regular, motions are governed, and reduced them to the most beautiful simplicity: laws to which not only their great and obvious variety of motions are conformable, but even their minute irregularities; and not only planets but comets likewise. The busy mind of man, never satiated with knowledge, now extended its views further, and made use of every expedient that suggested itself, to find the relation that this system of worlds bears to the whole visible creation. Instruments were made with all possible accuracy, and the most skilful observers applied themselves with great diligence to discover an annual parallax, from which the distances of the fixed stars would be known. They found unexpected irregularities, and might have been long perplexed with them to little purpose, had not Dr. Bradley happily accounted for them, by shewing that light from the heavenly bodies strikes the eye with a velocity and direction, compounded of the proper velocity and direction of light, and of the eye, as carried about with the earth in its orbit; compared to which, the diurnal motion and all other accidental motions of the eye, are quite inconsiderable. Thus, instead of what he aimed at, he discovered something still more curious, the real velocity of light, in a way entirely new and unthought of.

All Astronomical knowledge being conveyed to us from the remotest distances, by that subtle, swift and universal messenger of intelligence, LIGHT; it was natural for the curious to enquire into its properties, and particularly to endeavour to know with what velocity it proceeds, in its immeasurable journeys. Experimental Philosophy, accustomed to conquer every difficulty, undertook the arduous problem; but confessed herself unequal to the task. Here, Astronomy itself revealed the secret; first in the discovery of Roemer, who found that the farther Jupiter is distant from us, the later the light of his satellites always reaches us; and afterwards in this of Dr. Bradley, informed us, that light proceeds from the sun to us in about eight minutes of time.

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Footnote A30:

Alluding to the experiments made in France, for determining the velocity of light; which, though unsuccessful, discovered a noble philosophical spirit.

Footnote A31:

This prodigious velocity of light can be no argument against its materiality, as will appear from the following considerations. The greatest velocity which we can communicate to any body, is that of a cannon-ball, impelled by gun-powder; this may be at the rate of about 20 miles in a minute of time. The planet Saturn moves about 360 miles in a minute, that is 18 times swifter than a cannon-ball; and the comet of 1680, in its perihelion, moved near 56.66 times swifter than Saturn, or 990.5 times swifter than a cannon-ball. Now these are material bodies, moving with very various, and all of them exceedingly great velocities; and no reason appears why the last mentioned velocity should be the utmost limit, beyond which nature cannot proceed; or that some other body may not move 7 or 8 hundred times swifter than a comet, as light is found to do.

That the different refrangibility of the rays of light, on which their colours depend, arises from their different velocities, seems so natural a conjecture, that it has perhaps occurred to every one who has thought on this subject. To this there are three principal objections. The first is, that, according to this hypothesis, when the satellites of Jupiter are eclipsed, their colour ought to change, first to a green and then to a blue, before their light becomes extinct; which is contrary to experience. But this objection appears to me of no weight; for we do not lose sight of the satellite because there is no light coming from thence to the eye, but because there is not light enough to render it visible. Therefore at the time a satellite disappears, there is still light of all colours arriving at the eye: and though the blue light should predominate on account of its slower progress, yet the red may predominate on another account; for along the edge of Jupiter’s shadow, as it passes over the satellite, a greater proportion of red light, than of blue, will be thrown by the refraction of Jupiter’s atmosphere. The second objection is, that since the velocity of the earth in its orbit, causes an aberration of about 20 seconds in the place of a star, if the different colours of light depended on different velocities, the aberration of blue light ought proportionably to exceed that of red light, which would give such an oblong form to a fixed star as might be discovered with a good telescope. This objection is of no more force than the former. The effect ought indeed to follow, but not in a sensible quantity; for at the altitude of 70 degrees, the apparent place of a fixed star is likewise removed 20 seconds by refraction, and the very same separation of the rays must take place; yet this I think is not discoverable with the best telescope. Perhaps by uniting these two equal causes, which may be readily done, and thereby doubling the effect, it may become sensible.

The third objection arises from that curious discovery of Dollond, by which we are enabled so greatly to improve refracting telescopes. And this objection I shall for the present leave in its full force; as well against the above hypothesis, as against every other which I have seen for the same purpose.

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As the apparent motion of the fixed stars, arising from this cause, was observed to complete the intire circle of its changes in the space of a year, it was for some time supposed to arise from an annual parallax, notwithstanding its inconsistency in other respects with such a supposition. But this obstacle being removed, there followed the discovery of another apparent motion in the heavens, arising from the nutation of the earth’s axis; the period whereof is about nineteen years. Had it not been so very different from the period of the former, the causes of both must have been almost inexplicable. This latter discovery is an instance of the superior advantages of accurate observation: For it was well known that such a nutation must take place from the principles of the Newtonian Philosophy; yet a celebrated astronomer had concluded from hypothetical reasoning, that its quantity must be perfectly insensible.

The way being cleared thus far, Dr. Bradley assures us, from his most accurate observations, that the annual parallax cannot exceed two seconds, he thinks not one; and we have the best reason to confide in his judgment and accuracy. From hence then we draw this amazing conclusion; that the diameter of the earth’s orb bears no greater proportion to the distance of the stars which Bradley observed, than one second does to the radius; which is less than as one to 200,000. Prodigiously great as the distance of the fixed stars from our sun appears to be, and probably their distances from each other are no less, the Newtonian Philosophy will furnish us with a reason for it: That the several systems may be sufficiently removed from each other’s attraction, which we are very certain must require an immense distance; especially if we consider that the cometic part, of our system at least, appears to be the most considerable though so little known to us. The dimensions of the several parts of the planetary system, had been determined near the truth by the astronomers of the last age, from the parallax of Mars. But from that rare phenomenon the transit of Venus over the sun’s disk, which has twice happened within a few years past, the sun’s parallax is now known beyond dispute to be 8 seconds and an half, nearly; and consequently, the sun’s distance almost 12,000 diameters of the earth.

If from the distances of the several planets, and their apparent diameters taken with that excellent instrument, the micrometer, we compare their several magnitudes, we shall find the Moon, Mercury, and Mars, to be much less than our Earth, Venus a little less, but Saturn many hundred times greater, and Jupiter above one thousand times. This prodigious globe, placed at such a vast distance from the other planets, that the force of its attraction might the less disturb their motions, is far more bulky and ponderous than all the other planets taken together. But even Jupiter, with all his fellows of our system, are as nothing compared to that amazing mass of matter the Sun. How much are we then indebted to Astronomy, for correcting our ideas of the visible creation! Wanting its instruction, we should infallibly have supposed the earth by far the most important body in the universe, both for magnitude and use. The sun and moon would have been thought two little bodies nearly equal in size, though different in lustre, created solely for the purpose of enlightening the earth; and the fixed stars, so many sparks of fire, placed in the concave vault of heaven, to adorn it, and afford us a glimmering light in the absence of the sun and moon.

But how does Astronomy change the scene!—Take the miser from the earth, if it be possible to disengage him; he whose nightly rest has been long broken by the loss of a single foot of it, useless perhaps to him; and remove him to the planet Mars, one of the least distant from us: Persuade the ambitious monarch to accompany him, who has sacrificed the lives of thousands of his subjects to an imaginary property in certain small portions of the earth; and now point it out to them, with all its kingdoms and wealth, a glittering star “close by the moon,” the latter scarce visible and the former less bright than our Evening Star:—Would they not turn away their disgusted sight from it, as not thinking it worth their smallest attention, and look for consolation in the gloomy regions of Mars?

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Footnote A32:

Mars appears to be surrounded by a very great and dense atmosphere.

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But dropping the company of all those, whether kings or misers, whose minds and bodies are equally affected by gravitation, let us proceed to the orb of Jupiter; the Earth and all the inferior planets will vanish, lost in the sun’s bright rays, and Saturn only remain; He too sometimes so diminished in lustre, as not to be easily discovered. But a new and beautiful system will arise. The four moons of Jupiter will become very conspicuous; some of them perhaps appearing larger, others smaller than our moon; and all of them performing their revolutions with incredible swiftness, and the most beautiful regularity:—varying their phases from full to new and from new to full, and frequently eclipsing the sun and each other, at least to the equatorial parts of Jupiter; and almost in every revolution suffering eclipses themselves by falling into Jupiter’s shadow; excepting that the outermost will seem, like a traveller fond of the sun-beams, cautiously to avoid the shadow for whole years together. Since we are advanced so far, if not tired of the journey, let us proceed a step further; it is but 400 millions of miles to the globe of Saturn. Here again all will be lost, but Jupiter itself. The Sun will put on something of a starlike appearance, but with excessive brightness. The five satellites of Saturn will exhibit appearances similar to those of Jupiter, but they will very rarely eclipse the Sun, or suffer eclipses themselves. The particular phænomena of Saturn’s ring, we cannot explain, unless we knew the time and plane of Saturn’s revolution on his axis. But this we know, that it must sometimes appear, by night, like a prodigious luminous arch, almost equal to one quarter of the heavens; and at other times, dark, so as to afford no light itself, but to intercept the light of every star beyond it, by night, and of the sun itself by day. And to conclude, if borne on the wings of a comet we should travel with it to the remotest part of its orbit; our whole planetary system would disappear, and the sun become a star, only more refulgent than Sirius perhaps, because less distant.

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Footnote A33:

Dr. Herschel discovered, in the year 1789, (fourteen years after the delivery of this Oration,) two other satellites of Saturn. These are the innermost of his (now) seven secondary planets.

W. B.

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The opinion of the earth’s rotation on its axis was once violently opposed, from a notion of its dangerous tendency with respect to the interests of religion: But, as truth is always consistent with itself, so many new proofs were furnished from time to time by new discoveries, that a mistaken interpretation of some passages in the bible was compelled to give way to the force of astronomical evidence. The doctrine of a plurality of worlds, is inseparable from the principles of Astronomy; but this doctrine is still thought, by some pious persons, and by many more I fear, who do not deserve that title, to militate against the truths asserted by the Christian religion. If I may be allowed to give my opinion on a matter of such importance, I must confess that I think upon a proper examination the apparent inconsistency will vanish. Our religion teaches us what philosophy could not have taught; and we ought to admire with reverence the great things it has pleased divine Providence to perform, beyond the ordinary course of Nature, for man, who is undoubtedly the most noble inhabitant of this globe. But neither religion nor philosophy forbids us to believe that infinite wisdom and power, prompted by infinite goodness, may throughout the vast extent of creation and duration, have frequently interposed in a manner quite incomprehensible to us, when it became necessary to the happiness of created beings of some other rank or degree.