APPENDIX.
AN ORATION,DELIVERED FEBRUARY 24, 1775,BEFORETHE AMERICAN PHILOSOPHICAL SOCIETY,HELD AT PHILADELPHIA,FOR PROMOTING USEFUL KNOWLEDGE.BY DAVID RITTENHOUSE, A. M.MEMBER OF THE SAID SOCIETY.(INSCRIBED)
AN ORATION,DELIVERED FEBRUARY 24, 1775,BEFORETHE AMERICAN PHILOSOPHICAL SOCIETY,HELD AT PHILADELPHIA,FOR PROMOTING USEFUL KNOWLEDGE.BY DAVID RITTENHOUSE, A. M.MEMBER OF THE SAID SOCIETY.(INSCRIBED)
AN ORATION,
DELIVERED FEBRUARY 24, 1775,
BEFORE
THE AMERICAN PHILOSOPHICAL SOCIETY,
HELD AT PHILADELPHIA,
FOR PROMOTING USEFUL KNOWLEDGE.
BY DAVID RITTENHOUSE, A. M.
MEMBER OF THE SAID SOCIETY.
(INSCRIBED)
To the Delegates of the thirteen United Colonies, assembled in Congress, at Philadelphia, to whom the future liberties, and consequently the virtue, improvement in science and happiness, of America, are intrusted, the following Oration is inscribed and dedicated, by their most obedient and humble servant, the Author.
Gentlemen,
It was not without being sensible how very unequal I am to the undertaking, that I first consented to comply with the request of several gentlemen for whom I have the highest esteem, and to solicit your attention on a subject which an able hand might indeed render both entertaining and instructive; I mean Astronomy. But the earnest desire I have to contribute something towards the improvement of Science in general, and particularly of Astronomy, in this my native country, joined with the fullest confidence that I shall be favoured with your most candid indulgence, however far I may fall short of doing justice to the noble subject, enables me chearfully to take my turn as a member of the society, on this annual occasion.
The order I shall observe in the following discourse, is this: In the first place I shall give a very short account of the rise and progress of astronomy, then take notice of some of the most important discoveries that have been made in this science, and conclude with pointing out a few of its defects at the present time.
As, on this occasion, it is not necessary to treat my subject in a strictly scientific way, I shall hazard some conjectures of my own; which, if they have but novelty to recommend them, may perhaps be more acceptable than retailing the conjectures of others.
The first rise of astronomy, like the beginnings of other sciences, is lost in the obscurity of ancient times. Some have attributed its origin to that strong propensity mankind have discovered, in all ages, for prying into futurity; supposing that astronomy was cultivated only as subservient to judicial astrology. Others with more reason suppose astrology to have been the spurious offspring of astronomy; a supposition that does but add one more to the many instances of human depravity, which can convert the best things to the worst purposes.
The honour of first cultivating astronomy has been ascribed to the Chaldeans, the Egyptians, the Arabians, and likewise to the Chinese;[A1]amongst whom, it is pretended, astronomical observations are to be found of almost as early a date as the flood. But little credit is given to these reports of the Jesuits, who it is thought were imposed on by the natives; or else perhaps from motives of vanity, they have departed a little from truth, in their accounts of a country and people among whom they were the chief European travellers.
Not to mention the prodigious number of years in which it is said the Chaldeans observed the heavens, I pass on to what carries the appearance of more probability;[A2]the report that whenAlexander took Babylon, astronomical observations for one thousand nine hundred years before that time were found there, and sent from thence to Aristotle. But we cannot suppose those observations to have been of much value; for we do not find that any use was ever after made of them.[A3]
The Egyptians too, we are told, had observations of the stars for one thousand five hundred years before the Christian era.What they were, is not known; but probably the astronomy of those ages consisted in little more than remarks on the rising and setting of the fixed stars, as they were found to correspond with the seasons of the year;[A4]and, perhaps, forming them into constellations. That this was done early, appears from the book of Job, which has by some been attributed to Moses, who is said to have been learned in the sciences of Egypt.[A5]“Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion? Canst thou bring forth Mazzaroth in his season, or canst thou guide Arcturus with his sons?” Perhaps too, some account might be kept of eclipses of the sun and moon, as they happened, without pretending to predict them for the future. These eclipses are thought by some to have been foretold by the Jewish prophets in a supernatural way.
As to the Arabians, though some have supposed them the first inventors of astronomy, encouraged to contemplate the heavens by the happy temperature of their climate, and the serenity of their skies, which their manner of life must likewise have contributed to render more particularly the object of their attention; yet it is said, nothing of certainty can now be found to induce us to think they had any knowledge of this science amongst them before they learned it from the writings of Ptolemy, who flourished one hundred and forty years after the birth of Christ.
But notwithstanding the pretensions of other nations, since it was the Greeks who improved geometry, probably from its first rudiments, into a noble and most useful science; and since we cannot conceive that astronomy should make any considerable progress without geometry, it is to them we appear indebted forthe foundations of a science, that (to speak without a metaphor) has in latter ages reached the astonishingly distant heavens.
Amongst the Greeks, Hipparchus[A6]deserves particular notice; by an improvement of whose labours Ptolemy formed that system of astronomy which appears to have been the only one studied for ages after, and particularly (as was said before) by the Arabians; who made some improvements of their own, and, if not the inventors, were at least the preservers of astronomy. For with them it took refuge, during those ages of ignorance which involved Europe, after an inundation of northern people had swallowed up the Roman empire; where the universally prevailing corruption of manners, and false taste, were become as unfavourable to the cause of science, as the ravages of the Barbarians themselves.
From this time, we meet with little account of astronomical learning in Europe[A7]until Regiomontanus,[A8]and some others, revived it in the fifteenth century; and soon afterwards appeared the celebrated Copernicus,[A9]whose vast genius, assisted by such lights as the remains of antiquity afforded him, explained the true system of the universe, as at present understood. To the objection of the Aristotelians, that the sun could not be the centre ofthe world, because all bodies tended to the earth, Copernicus replied, that probably there was nothing peculiar to the earth in this respect; that the parts of the sun, moon and stars, likewise tended to each other, and that their spherical figure was preserved amidst their various motions, by this power; an answer that will at this day be allowed to contain sound philosophy. And when it was further objected to him, that, according to his system, VenusandandMercury ought to appear horned like the moon, in particular situations; he answered as if inspired by the spirit of prophecy, and long before the invention of telescopes, by which alone his prediction could be verified, “That so they would one day be found to appear.”
Next follows the noble Tycho,[A10]who with great labour and perseverance, brought the art of observing the heavens to a degree of accuracy unknown to the ancients; though in theory he mangled the beautiful system of Copernicus. The whimsical Kepler, too, (whose fondness for analogies frequently led him astray, yet sometimes happily conducted him to important truths) did notable services to astronomy: and from his time down to the present, so many great men have appeared amongst the several nations of Europe, rivalling each other in the improvement of astronomy, that I should trespass on your patience were I to enumerate them. I shall therefore proceed to what I proposed in the second place, and take notice of some of the most important discoveries in this science.
Astronomy, like the Christian religion, if you will allow me the comparison, has a much greater influence on our knowledge in general, and perhaps on our manners too, than is commonlyimagined. Though but few men are its particular votaries, yet the light it affords is universally diffused amongst us; and it is difficult for us to divest ourselves of its influence so far, as to frame any competent idea of what would be our situation without it.[A11]Utterly ignorant of the heavens, our curiosity would be confined solely to the earth, which we should naturally suppose a vast extended plain; but whether of infinite extent or bounded, and if bounded, in what manner, would be questions admitting of a thousand conjectures, and none of them at all satisfactory.
The first discovery then, which paved the way for others more curious, seems to have been the circular figure of the earth, inferred from observing the meridian altitudes of the sun and stars to be different in distant places. This conclusion would probably not be immediately drawn, but the appearance accounted for, by the rectilinear motion of the traveller; and then a change in the apparent situations of the heavenly bodies would only argue their nearness to the earth: and thus would the observation contribute to establish error, instead of promoting truth, which has been the misfortune of many an experiment. It would require some skill in geometry, as well as practice in observing angles, to demonstrate the spherical figure of the earth from such observations.[A12]
But this difficulty being surmounted, and the true figure of the earth discovered, a free space would now be granted for the sun,moon, and stars to perform their diurnal motions on all sides of it; unless perhaps at its extremities to the north and south; where something would be thought necessary to serve as an axis for the heavens to revolve on. This Mr. Crantz in his very entertaining history of Greenland informs us, is agreeable to the philosophy of that country, with this difference perhaps, that the high latitude of the Greenlander makes him conclude one pole only, necessary: He therefore supposes a vast mountain situate in the utmost extremity of Greenland, whose pointed apex supports the canopy of heaven, and whereon it revolves with but little friction.
A free space around the earth being granted, our infant astronomer would be at liberty to consider the diurnal motions of the stars as performed in intire circles, having one common axis of rotation. And by considering their daily anticipation in rising and setting, together with the sun’s annual rising and falling in its noon day height, swiftest about the middle space, and stationary for some time when highest and lowest, he would be led to explain the whole by attributing a slow motion to the sun, contrary to the diurnal motion, along a great circle dividing the heavens into two equal parts, but obliquely situated with respect to the diurnal motion. By a like attention to the moon’s progress the Zodiac would be formed, and divided into its several constellations or other convenient divisions.
The next step that astronomy advanced, I conceive, must have been in the discovery attributed to Pythagoras;[A13]who it is said first found out that Hesperus and Phosphorus, or the Evening and Morning Star, were the same. The superior brightness of this planet, and the swiftness of its motion, probably first attracted the notice of the inquisitive: and one wandering star being discovered, more would naturally be looked for. The splendor of Jupiter, the very changeable appearance of Mars, and the glittering of Mercury by day light, would distinguish them. Andlastly, Saturn would be discovered by a close attention to the heavens. But how often would the curious eye be directed in vain, to the regions of the north and south, before there was reason to conclude that the orbits of all the planets lay nearly in the same plane; and that they had but narrow limits assigned them in the visible heavens.
From a careful attendance to those newly discovered celestial travellers, and their various motions, direct and retrograde, the great discovery arose, that the sun is the centre of their motions; and that by attributing a similar motion to the earth, and supposing the sun to be at rest, all the phænomena will be solved. Hence a hint was taken that opened a new and surprizing scene. The earth might be similar to them in other respects. The planets too might be habitable worlds. One cannot help greatly admiring the sagacity of minds, that first formed conclusions so very far from being obvious; as well as the indefatigable industry of astronomers, who originally framed rules for predicting eclipses of sun and moon, which is said to have been done as early as the time of Thales;[A14]and must have proved of singular service to emancipate mankind from a thousand superstitious fears and notions, whichjugglingjugglingimpostors (the growth of all ages and countries) would not fail to turn to their own advantage.
For two or three centuries before and after the beginning of the Christian era, astronomy appears to have been held in considerable repute; yet very few discoveries of any consequence were made, during that period and many ages following.
The ancients were not wanting in their endeavours to find out the true dimensions of the planetary system. They invented several very ingenious methods for the purpose; but none of them were at all equal, in point of accuracy, to the difficulty of the problem. They were therefore obliged to rest satisfied with supposing the heavenly bodies much nearer to the earth than in fact they are, and consequently much less in proportion to it. Add to this, that having found the earth honoured with an attendant, while they could discover none belonging to any of the other planets,theysupposed it of far greater importance in the Solar System than it appears tousto be: And the more praise is due to those few, who nevertheless conceived rightly of its relation to the whole.
Tycho took incredible pains to discover the parallax of Mars in opposition; the very best thing he could have attempted in order to determine the distances and magnitudes of the sun and planets. But telescopes and micrometers were not yet invented! so that not being able to conclude any thing satisfactory from his own observations, he left the sun’s parallax as he found it settled by Ptolemy, about twenty times too great. And even after he had reduced to rule the refraction of the atmosphere, and applied it to astronomical observations, rather than shock his imagination by increasing the sun’s distance, already too great forhishypothesis, he chose to attribute a greater refraction to the sun’s light, than that of the stars, altogether contrary to reason; that so an excess of parallax might be balanced by an excess of refraction. Thus when we willingly give room to one error, we run the risk of having a whole troop of its relations quartered upon us. But Kepler afterwards, on looking over Tycho’s observations, found that he might safely reduce the sun’s parallax to one minute; which was no inconsiderable approach to the truth. Alhazen,[A15]an Arabian, had some time before, discovered the refraction of light in passing through air; of which the ancients seem to have been entirely ignorant. They were indeed very sensible of the errors it occasioned in their celestialmeasures; but they, with great modesty, attributed them to the imperfections of their instruments or observations.
I must not omit, in honour of Tycho, to observe that he first proved, by accurate observations, that the comets are not meteors floating in our atmosphere, as Aristotle,[A16]that tyrant in Philosophy, had determined them to be, but prodigious bodies at a vast distance from us in the planetary regions; a discovery the lateness of which we must regret, for if it had been made by the ancients, that part of Astronomy (and perhaps every other, in consequence of the superior attention paid to it), would have been in far greater perfection than it is at this day.
I had almost forgot to take notice of one important discovery made in the early times of Astronomy, the precession of the equinoxes. An ancient astronomer, called Timocharis, observed an appulse of the Moon to the Virgin’s Spike, about 280 years before the birth of Christ. He thence took occasion to determine the place of this star, as accurately as possible; probably with a view of perfecting the lunar theory. About four hundred years afterwards, Ptolemy, comparing the place of the same star, as he then found it, with its situation determined by Timocharis,[A17]concluded the precession to be at the rate of one degree in an hundred years; but later astronomers have found it swifter.
Whatever other purposes this great law may answer, it will produce an amazing change in the appearance of the heavens; and so contribute to that endless variety which obtains throughout the works of Nature. The seven stars that now adorn our winter skies, will take their turn to shine in summer. Sirius, that now shines with unrivalled lustre, amongst the gems of heaven, will sink below our horizon, and rise no more for very many ages! Orion too, will disappear, and no longer afford our posterity a glimpse of glories beyond the skies! glittering Capella, that now passes to the north of our zenith, will nearly describethe equator:[A18]And Lyra, one of the brightest in the heavens, will become our Polar Star: Whilst the present Pole Star, on account of its humble appearance, shall pass unheeded; and all its long continued faithful services shall be forgotten! All these changes, and many others, will certainly follow from the precession of the equinoxes; the cause of which motion was so happily discovered and demonstrated by the immortal Newton: A portion of whose honors was nevertheless intercepted by the prior sagacity of Kepler, to whom I return.
Kepler’s love of harmony encouraged him to continue his pursuits, in spite of the most mortifying disappointments, until he discovered that admirable relation which subsists between the periodic times of the primary planets, and their distances from the sun; the squares of the former being as the cubes of the latter. This discovery was of great importance to the perfection of Astronomy; because the periods of the planets are more easily found by observation, and from them their several relative distances may be determined with great accuracy by this rule. He likewise found from observation, that the planets do not move in circles; but in elipses, having the sun in one focus. But the causes lay hid from him, and it was left as the glory of Sir Isaac, to demonstrate that both these things must necessarily follow from one simple principle, which almost every thing in this science tends to prove does really obtain in Nature: I mean, that the planets are retained in their orbits by forces directed to the sun; which forces decrease as the squares of their distances encrease.
Kepler also discovered that the planets do not move equally in their orbits, but sometimes swifter, sometimes slower; and that not irregularly, but according to this certain rule; That in equal times, the areas described by lines drawn from the planet to the sun’s centre, are equal. This, Sir Isaac likewise demonstrated must follow, if the planet be retained in its orbit by forces directed to the sun, and varying with the distance in any manner whatsoever. These three discoveries of Kepler, afterwards demonstrated by Newton, are the foundation of all accuracy in astronomical calculations.[A19]
We now come to that great discovery, which lay concealed from the most subtle and penetrating geniuses amongst mankind, until these latter ages; which so prodigiously enlarged the fields of astronomy, and with such rapidity handed down one curiosity after another, from the heavens to astonished mortals, that no one capable of raising his eyes and thoughts from the ground he trod on, could forbear turning his attention, in some degree, to the subject that engages us this evening.
Galileo, as he himself acknowledges, was not the first inventor of the telescope, but he was the first that knew how to make a proper use of it.[A20]If we consider that convex and concave lenses had been in use for some centuries, we shall think it probable that several persons might have chanced to combine them together, so as to magnifydistantobjects; but that the small advantage apparently resulting from such a discovery,either on account of the badness of the glasses or the unskilfulness of the person in whose hands they were, occasioned it to be neglected.
But Galileo, by great care in perfecting his telescope, and by applying a judicious eye, happily succeeded; and with a telescope magnifying but thirty times, discovered the moon to be a solid globe, diversified with prodigious mountains and vallies, like our earth; but without seas or atmosphere. The sun’s bright disk, he found frequently shaded with spots, and by their apparent motions proved it to be the surface of a globe, revolving on its axis in about five and twenty days. This it seems was a mortifying discovery to the followers of Aristotle; who held the sun to be perfect without spot or blemish.[A21]Some of them, it is said, insisted that it was but an illusion of the telescope and absolutely refused to look through one, lest the testimony of their senses should prove too powerful for their prejudices.
Galileo likewise discovered the four attendants of Jupiter, commonly called his satellites:[A22]Which at first did not much please that great ornament of his age, the sagacious Kepler. For by this addition to the number of the planets, he found their Creator had not paid that veneration to certain mystical numbers and proportions, which he had imagined. Let us not blush at this remarkable instance of philosophical weakness, but admire the candour of the man who confessed it.
Galileo not only discovered these moons of Jupiter, but suggested their use in determining the longitude of places on the earth; which has since been so happily put in practice, that Fontenelle does not hesitate to affirm, that they are of more useto Geography and Navigation,[A23]than our own moon. He discovered the phases of Mars and Venus; that the former appears sometimes round and sometimes gibbous, and that the latter puts on the shapes of our moon: And from this discovery, he proved to a demonstration, the truth of the Copernican System.[A24]Nor did that wonderful ring, which surrounds Saturn’s body, without touching it, and which we know nothing in nature similar to, escape his notice; though his telescope did not magnify sufficiently to give him a true idea of its figure.
Amongst the fixed stars too, Galileo pursued his enquiries. The Milky-Way, which had so greatly puzzled the ancient Philosophers, and which Aristotle imagined to be vapours risen to an extraordinary height, he found to consist of an innumerable multitude of small stars; whose light appears indistinct and confounded together to the naked eye. And in every part of the heavens, his telescope shewed him abundance of stars, not visible without it. In short, with such unabated ardour did thisgreat man range through the fields of Astronomy, that he seemed to leave nothing for others to glean after him.
Nevertheless, by prodigiously encreasing the magnifying powers of their telescopes, his followers made several great discoveries; some of which I shall briefly mention. Mercury was found to become bisected, and horned near its inferior conjunction, as well as Venus. Spots were discovered in Mars, and from their apparent motion, the time of his revolution on an axis nearly perpendicular to its orbit, was determined. A sort of belts or girdles, of a variable or fluctuating nature, were found to surround Jupiter, and likewise certain spots on his surface, whence he was concluded to make one revolution in about ten hours on his axis; which is likewise nearly perpendicular to his orbit. Five[A25]moons or satellites were found to attend Saturn, which Galileo’s telescope; on account of their prodigious distance, could not reach:[A26]And the form of his ring was found to be a thin circular plane, so situated as not to be far from parallel to the plane of our equator; and always remaining parallel to itself. This ring, as well as Saturn, evidently derives its light from the sun, as appears by the shadows they mutually cast on each other.
Besides several other remarkable appearances, which Hugenius[A27]discovered amongst the fixed stars, there is one in Orion’s Sword, which, I will venture to say, whoever shall attentively view, with a good telescope and experienced eye, will not find his curiosity disappointed. “Seven small stars, (says he,) of which three are very close together, seemed to shine through a cloud, so that a space round them appeared much brighter thanany other part of heaven, which being very serene and black looked here as if there was an opening, through which one had a prospect into a much brighter region.” Here some have supposed old night to be entirely dispossessed, and that perpetual daylight shines amongst numberless worlds without interruption.
This is a short account of the discoveries made with the telescope. Well might Hugenius congratulate the age he lived in, on such a great acquisition of knowledge: And recollecting those great men, Copernicus, Regiomontanus, and Tycho, so lately excluded from it by death, what an immense treasure, says he, would they have given for it. Those ancient philosophers too, Pythagoras, Democritus, Anaxagoras, Philolaus, Plato, Hipparchus; would they not have travelled over all the countries of the world, for the sake of knowing such secrets of nature, and of enjoying such sights as these?
Thus have we seen the materials collected, which were to compose the magnificent edifice of astronomical Philosophy; collected, indeed, with infinite labour and industry, by a few volunteers in the service of human knowledge, and with an ardour not to be abated by the weaknesses of human nature, or the threatened loss of sight, one of the greatest of bodily misfortunes! It was now time for the great master-builder to appear, who was to rear up this whole splendid group of materials into due order and proportion. And it was, I make no doubt, by a particular appointment of Providence, that at this time the immortal Newton appeared. Much had been done preparatory to this great work by others, without which if he had succeeded, we should have been ready to pronounce him something more than human. The doctrine of atoms had been taught by some of the ancients. Kepler had suspected that the planets gravitated towards each other, particularly the earth and moon; and that their motion prevented their falling together: and Galileo first of all applied geometrical reasoning to the motion of projectiles. But the solid spheres of the ancients, or the vortices of Des Cartes,[A28]were still found necessary to explain the planetary motions; or if Kepler had discarded them, it was only to substitute something else in their stead, by no means sufficient to account for those grand movements of nature. It was Newton alone that extended the simple principle of gravity, under certain just regulations, and the laws of motion, whether rectilinear or circular, which constantly take place on the surface of this globe, throughout every part of the solar system; and from thence, by the assistance of a sublime geometry, deduced the planetary motions, with the strictest conformity to nature and observation.
Other systems of Philosophy have been spun out of the fertile brain of some great genius or other; and for want of a foundation in nature, have had their rise and fall, succeeding each other by turns. But this will be durable as science, and can never sink into neglect, until “universal darkness buries all.”
Other systems of Philosophy have ever found it necessary to conceal their weakness, and inconsistency, under the veil of unintelligible terms[A29]and phrases, to which no two mortals perhaps ever affixed the same meaning: But the Philosophy of Newton disdains to make use of such subterfuges; it is not reduced to the necessity of using them, because it pretends not to be of nature’s privy council, or to have free access to her most inscrutable mysteries; but to attend carefully to her works, to discover the immediate causes of visible effects, to trace those causes to others more general and simple, advancing by slow and sure steps towards the great First Cause of all things.
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 withknowledge, 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 oflight, and of theeye, 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.[A30]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.[A31]
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 probablytheir 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 thousandsof 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?[A32]
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 phasesfromfromfull 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[A33]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 wingsof 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.
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:[A34]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.
How far indeed the inhabitants of the other planets may resemble man, we cannot pretend to say. If like him they were created liable to fall, yet some, if not all of them, may still retain their original rectitude. We will hope they do: the thought is comfortable.—Cease, Galileo, to improve thy optic tube: and thou, great Newton, forbear thy ardent search into the distant mysteries of nature: lest ye make unwelcome discoveries. Deprive us not of the pleasure of believing that yonder radiant orbs, traversing in silent majesty the etherial regions, are the peacefulseats of innocence and bliss: where neither natural nor moral evil has ever yet intruded; where to enjoy with gratitude and adoration the creator’s bounty, is the business of existence. If their inhabitants resemble man in their faculties and affections, let us suppose that they are wise enough to govern themselves according to the dictates of that reason their creator has given them, in such manner as to consult their own and each other’s true happiness, on all occasions. But if, on the contrary, they have found it necessary to erect artificial fabrics of government, let us not suppose that they have done it with so little skill, and at such an enormous expence, as must render them a misfortune instead of a blessing. We will hope that their statesmen are patriots, and that their kings, if that order of beings has found admittance there, have the feelings of humanity.—Happy people! and perhaps more happy still, that all communication with us is denied. We have neither corrupted you with our vices, nor injured you by violence. None of your sons and daughters, degraded from their native dignity, have been doomed to endless slavery by us in America, merely becausetheirbodies may be disposed to reflect or absorb the rays of light, in a way different fromours. Even you, inhabitants of the moon, situated in our very neighbourhood, are effectually secured, alike from the rapacious hand of the haughty Spaniard, and of the unfeeling British nabob. Even British thunder impelled by British thirst of gain, cannot reach you: And the utmost efforts of the mighty Frederick, that tyrant of the north and scourge of mankind, if aimed to disturbyourpeace, becomes inconceivably ridiculous and impotent.
Pardon these reflections; they rise not from the gloomy spirit of misanthropy. That being, before whose piercing eye all the intricate foldings and dark recesses of the human heart become expanded and illuminated, is my witness with what sincerity, with what ardor, I wish for the happiness of the whole race of mankind: how much I admire that disposition of lands and seas, which affords a communication between distant regions, and a mutual exchange of benefits:[A35]how sincerely I approveof those social refinements which really add to our happiness, and induce us with gratitude to acknowledge our great Creator’sgoodness:—how I delight in a participation of the discoveries made from time to time in nature’s works, by our Philosophic brethren in Europe.
But when I consider, thatluxuryand her constant followertyranny, who have long since laid in the dust, never to rise again, the glories of Asia, are now advancing like a torrent irresistible, whose weight no human force can stem, and have nearly completed their conquest of Europe; luxury and tyranny, who by a vile affectation of virtues they know not, pretend at first to be the patrons of science and philosophy, but at length fail not effectually to destroy them; agitated I say by these reflections, I am ready to wish—vain wish! that nature would raise her everlasting bars between the new and old world; and make a voyage to Europe as impracticable as one to the moon. I confess indeed, that by our connections with Europe we have made most surprising, I had almost said unnatural, advances towards the meridian of glory; but by those connections too, in all probability, our fall will be premature. May the God of knowledgeinspire us with wisdom to prevent it: let our harbours, our doors, our hearts, be shut against luxury. But I return to my subject, and will no longer indulge these melancholy thoughts.
Some have observed, that the wonderful discoveries of the microscope ought to go hand in hand with those of the telescope; lest whilst we contemplate the many instances of the wisdom and power of divine Providence, displayed in the great works of creation, we should be tempted to conclude that man, and other less important beings of this lower world, did not claim its attention. But I will venture to affirm, without at all derogating from the merits of those who have so greatly obliged the world with the success of their microscopical enquiries, that no such danger is to be apprehended. Nothing can better demonstrate the immediate presence of the Deity in every part of space, whether vacant or occupied by matter, than astronomy does. It was from an astronomer St. Paul quoted that exalted expression, so often since repeated; “In God we live, and move, and have our being.” His divine energy supports that universalsubstratumon which all corporal substances subsist, that the laws of motion are derived from, and that wingslightwith angelic swiftness.
If the time would permit, how agreeable the task to dwell on the praises of Astronomy: to consider its happy effects as a science, on the human mind. Let the sceptical writers forbear to lavish encomiums on their cobweb Philosophy, liable to be broken by the smallest incident in nature. They tell us it is of great service to mankind, in banishing bigotry and superstition from amongst us. Is not this effectually done by Astronomy? The direct tendency of this science is to dilate the heart with universal benevolence, and to enlarge its views. But then it does this without propagating a single point of doctrine contrary to common sense, or the most cultivated reason. It flatters no fashionable princely vice, or national depravity. It encourages not the libertine by relaxing any of the precepts of morality; nor does it attempt to undermine the foundations of religion. It denies none of those attributes, which the wisest and best of mankind, have in all ages ascribed to the Deity: Nor does it degrade the human mind from that dignity, which is ever necessary to make it contemplate itself with complacency. None of these things does Astronomy pretend to; andif these things merit the aim of Philosophy, and the encouragement of a people, then let scepticism flourish, and Astronomy lie neglected; then let the names of Berkeley, and Hume, become immortal, and that of Newton be lost in oblivion.
I shall conclude this part of my discourse with the words of Dr. Barrow—It is to Astronomy we owe “that we comprehend the huge fabric of the universe, admire and contemplate the wonderful beauty of the divine workmanship, and so learn the invincible force and sagacity of our own minds, as to acknowledge the blessings of heaven with a pious affection.”
I now come, in the last place, to point out some of the defects of Astronomy at this day. Which I am induced to undertake by the hopes I entertain that some of those defects may be removed under the auspices of this society, and of you my fellow citizens, who have so zealously promoted its institution. “The advantages arising from Astronomy, the pleasure attending the study of it, the care with which it was cultivated by many great men among the ancients, and the extraordinary attention paid to it in Europe by the present age,” all contribute to recommend it to your protection, under which we have the best reason to expect that it will flourish.
The mildness of our climate and the serenity of our atmosphere, perhaps not inferior to that of Italy, and likewise our distant situation from the principal observatories in the world (whence many curious phænomena must be visible here that are not likely to be observed any where else) are so many circumstances greatly in our favour.
And I trust there will not be wanting men of genius, to arise in this new world, whose talents may be particularly adapted to astronomical enquiries. Indeed I am persuaded that nature is by no means so nigardly in producing them, as we are apt to imagine. Some are never tempted forth from obscurity, some are untimely snatched away by death, a striking instance whereof we have in Horrox; and many are accidentally led to other pursuits.
The Astronomy of comets is still in its infancy; not that the attention of the learned and ingenious has at all been wanting for more than a century past; but because it will necessarily require many ages to bring it to perfection. I wish we were in a condition to promote it in some degree, by carefully observingsuch comets as may appear. As yet we scarce dare affirm that any one has or will return a second time. It has never, that I know of, been certainly proved by observation, that a comet has descended within a parabolic orbit, and until that is done we have only a coincidence of periods and orbits (none of which have been very precise) to depend on for their return. Far less are astronomers able to determine the changes that may, and probably do, happen in their orbits[A36]and velocities in every period, so as to predict their nearer or more remote approach to the earth or any planet. Whether their business be to repair or destroy, whether they are worlds yet in formation or once habitable worlds in ruins; whether they are at present habitable and regular attendants of our Sun only, or whether they are the vast links that connect the distant parts of creation by surrounding more suns than one, we know not.
If we descend to the Planetary System, there are still many things wanting to compleat Astronomy.
The orbits of the primary planets have at one time been supposed moveable with various irregularities, at other times fixed and permanent. It seems now generally granted, that according to the theory of gravity they must change their situations; yet not long since, some great astronomers warmly contended that this change was altogether insensible.
According to the best tables we now have, the planes of the orbits of Jupiter, the Earth and Mercury are immoveable, though the orbits themselves have a progressive motion in their planes. On the contrary, the poles of the orbits of Saturn, Mars and Venus are supposed to revolve about the poles of the earth’s orbit, with such velocities as at present nearly reconcile calculation to appearances. But there is good reason to apprehend that such a supposition is not true in fact, and a mistake in this matter will have some important consequences. More probable is it, that the poles of the orbits of all the planets, the earth not excepted, revolve about some common centre. The several quantities of these motions, I am confident, are to be had from observation, and not from theory alone. If such a motion of the earth’s orbit be admitted, it will account for the diminution[A37]of the obliquityof the ecliptic; which seems now incontestible; and that in whatever manner we divide the forces producing such motion, amongst the two superior planets and Venus, or even amongst all of them. And I should suspect the further diminution of obliquity, from this cause, will amount to about one degree and an half.
But as Astronomy now stands, it seems doubtful whether this change is owing to a deviation in the diurnal or annual motion of the earth; which introduces a very disagreeable uncertainty in conclusions drawn from some nice and useful observations.
The Lunar Astronomy has been brought so much nearer to perfection, by thecelebratedcelebratedMayer,[A38]than could have been expected, that I shall mention no deficiency in it, but this. We do not certainly know whether that apparent acceleration of the moon’s motion, which Mayer with other great astronomers has admitted, ought to be attributed to a real increase of velocity in the moon, or to a diminution of the earth’s diurnal motion. If to the former, the destruction of this beautiful and stupendous fabric, may from thence be predicted with more certainty than from any other appearance in Nature: But if to the latter, it may be prettily accounted for, by Dr. Halley’s ingenious hypotheses concerning the change of variation in the magnetical needle. The Doctor supposes the external crust or shell of the earth to contain a nucleus detatched from it, and that the impulse which first caused the diurnal motion, was given to the external parts, and from thence in time communicated to the internal nucleus, by means of an intervening fluid; but not so as perfectly to equal the velocity of the superficial parts of the globe. Whence it will follow, that the external shell of the earth is still communicating motion to the internal parts, and losing motion itself proportionably. The diurnal motion must therefore become slower and slower, yet can never be retarded, by this cause, beyond certain limits; nor can we conceive that any inconvenience will follow.
There is another physical question relating to the moon, which to me appears extremely curious; it is this—Whence is it thatthe moon always turns the same side to us? or, which is the same thing, How comes the moon’s rotation on her axis, and her monthly revolution about the earth, to be performed in the same time? None I believe will suppose it to be accidental, nor will the astronomer be easily satisfied with a final cause. Was it not originally brought about by a natural cause which still subsists? Can the attraction of any foreign body change a rotatory motion into a libratory one, and a libratory motion into rest, in spaces so very free from all resistance as those wherein the planets move? There are other defects in Astronomy that are purely optical. Removing of those, depends on the further improvement of telescopes, or rather on the more judicious use of them, at times and places the most favourable.
In speaking of telescopic discoveries I purposely reserved those made on Venus for this place, because they are still uncertain. Burratini in Poland first discovered spots in Venus, then Cassini in Italy; and afterwards Bianchini got a sight of them. But from all their observations it is uncertain, whether Venus revolves on its axis once in 23 hours, or once in 24 days. Perhaps it does neither. Nor is their determination of the axis’ situation much more satisfactory. These spots on Venus are not to be seen but through an excellent telescope and a pure atmosphere.
In the year 1672 and 1676 Cassini saw a small star near Venus, which he thought might be a satellite attending on her. It appeared to have the same phase with Venus. In 1740 Mr. Short with a telescope of 16 inches saw a small star at the distance of ten minutes from Venus, which from its apparent shape he likewise thought might be a satellite. And in 1761 Mr. Montaigne, in France, saw what he took to be the satellite of Venus, on the 3d, 4th, 7th and 11th of May.[A39]But whether Venus has a satellite or not, must still be left amongst the doubtful things of Astronomy.
The spots on the sun, and those on the surfaces of several planets, have been many years observed without our approaching any nearer towards discovering their nature and cause. Dr. Wilson of Glasgow, has lately succeeded in advancing one stepat least, with respect to those of the sun. He has proved from observation that those spots are vast cavities, whose bottoms lie far below the general surface of the sun, and whose sloping sides form the border which we generally see surrounding them. If I should venture to add one conjecture of my own, to those of this ingenious gentleman, I would suppose that those prodigious cavities in the surface of the sun, some of them capable of containing half our earth, are not repeatedly formed by unaccountable explosions of a semifluid substance, but permanent and solid, like the cavities within the moon. And that it is the dark matter sometimes lodging in them, that distinguishes them, and is only accidental.
The diurnal rotations of Saturn and Mercury are yet unknown; but when further improvements shall be made in the art of using telescopes, this circumstance will hardly escape the vigilance of astronomers.
These are a few of the many things that are still left to the industry of the ingenious in this science.
But if all higher and more sublime discoveries are not reserved for us in a future and more perfect state; if Astronomy shall again break those limits that now seem to confine it, and expatiate freely in the superior celestial fields; what amazing discoveries may yet be made amongst the fixed stars! That grand phænomenon the Milky-Way seems to be the clue that will one day guide us. Millions of small stars compose it, and many more bright ones lie in and near it, than in other parts of heaven. Is not this a strong indication that this astonishing system of worlds beyond worlds innumerable, is not alike extended every way, but confined between two parallel planes, ofimmeasureable, though notinfiniteextent? Or rather, is not the Milky-Way a vein of a closer texture, running through this part of the material creation? Great things are sometimes best explained by small and small by great. Material substances, such as we daily handle, have been thought composed of impenetrable particles in actual contact: then again it has seemed necessary to suppose them at a distancefromfromeach other, and kept in their relative situations byattractionandrepulsion. Many appearances require that those distances should be very great in proportion to the size of the particles. Hence some, with no small reason, have concluded that matter consists of indivisible points enduedwith certain powers. Let us compare these smaller portions of it with that great aggregate of matter which is the object of Astronomy;Lightwill then appear to have as free passage through a piece of glass, as the comets have in the planetary regions; and several other new considerations will arise.
If instead ofdescendingweascendthe scale. If we consider that infinite variety which obtains in those parts of nature with which we are most intimate: how one order of most curiously organized bodies, infinitely diversified in other respects, all agree in being fixed to the earth, and receiving nourishment from thence: how another order have spontaneous motion, and seek their food on different parts of the earth, whilst by gravity they are confined to its surface, but in other respects diversified like the former. How athirdfloat in, and below the surface of, a dense fluid, of equal weight with their bodies, which would soon prove fatal to both the others: And afourthconsisting of a vast variety too, have this property in common, that by a peculiar mechanism of their bodies, they can soar to great heights above the earth, and quickly transport themselves to distant regions in a fluid so rare as to be scarcely sensible to us. But not to pursue this boundless subject any further, I say, when we consider this great variety so obvious onourglobe, and ever connected by some degree of uniformity, we shall find sufficient reason to conclude, that the visible creation, consisting of revolving worlds and central suns, even including all those that are beyond the reach of human eye and telescope, is but an inconsiderable part of the whole. Many other and very various orders of things unknown to, and inconceivable by us, may, and probably do exist, in the unlimited regions of space. And all yonder stars innumerable, with their dependencies, may perhaps compose but the leaf of a flower in the Creator’s garden, or a single pillar in the immense building of the Divine Architect.
Here is ample provision made for the all-grasping mind of man!
If it shall please that Almighty Power who hath placed us in a world, wherein we are only permitted “to look about us and to die;” should it please him to indulge us with existence throughout that half of eternity which still remains unspent; and to conduct us through the several stages of his works; here is ample provision made for employing every faculty of the humanmind, even allowing its powers to be constantly enlarged through an endless repetition of ages. Let us not complain of the vanity of this world, that there is nothing in it capable of satisfying us: happy in those wants, happy in those restless desires, forever in succession to be gratified; happy in a continual approach to the Deity.
I must confess that I am not one of those sanguine spirits who seem to think, that when the withered hand of death hath drawn up the curtain of eternity, almost all distance between the creature and creator, between finite and infinite, will be annihilated. Every enlargement of our faculties, every new happiness conferred upon us, every step we advance towards the perfection of the divinity, will very probably render us more and more sensible of his inexhaustible stores of communicable bliss, and of his inaccessible perfections.
Were we even assured that we shall perish like the flowers of the garden, how careful would a wise man be to preserve a good conscience, during the short period of his existence; because by his very constitution, which he cannot alter, this is his pride and glory, and absolutely necessary to his present happiness; because this would insure to him at the approach of death, the soothing reflection, that he was going to restore, pure and uncorrupted, that drop of divinity within him, to the original ocean from whence it was separated. How much more anxiously careful ought we to be, if we believe, as powerful arguments compel us to believe, that a conduct in this life depending on our own choice, will stamp our characters for ages yet to come. Who can endure the thought of darkening his faculties by an unworthy application of them here on earth, and degrading himself to some inferior rank of being, wherein he may find both his power and inclination to obtain wisdom and exercise virtue, exceedingly diminished? On the other hand, if that humble admiration and gratitude, which sometimes rises in our minds when we contemplate the power, wisdom and goodness of the Deity, constitutes by far the most sublimely happy moments of our lives, and probably will forever continue to do so, there cannot be a stronger incitement to the exercise of virtue and a rational employment of those talents we are entrusted with, than to consider that by these means we shall in a few years be promoted to a more exalted rank amongst the creatures of God, haveour understandings greatly enlarged, be enabled to follow truth in all her labyrinths with a higher relish and more facility, and thus lay the foundation of an eternal improvement in knowledge and happiness.