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heat, are curious, and highly interesting. He seems to have demonstrated that the different prismatic colours have different grades of temperature; that radiant heat, as well as light, is not only refrangible, but also subject to the laws of dispersion, arising from its different refrangibility; that those rays of light which have the greatest illuminating power are the yellow, and those which have the greatest heating power the red; and, of course, that, contrary to the general belief, the maximum of illumination, and the maximum of heat, do not coincide."

ASTRONOMY.

Though this subject is mentioned last, it holds a very conspicuous place among those branches of mechanical philosophy which have received great accessions of discovery and improvement during the century in question. At the beginning of this period the Principia of the immortal NEWTON had given a new face to astronomical science. Much had been done by his predecessors, and especially by the sagacious KEPLER, to prepare the way for his grand. discoveries; but it was reserved for this luminary of the first magnitude to shed a degree of light on the laws of our planetary system, which has served to guide every exertion, and point out the way to all progress which has since been made. It was he who first applied the simple principle of gravitation to account for the movements of the celestial bodies; who laid down the laws of this great and all pervading attraction; and thence, by the assistance of a sublime geometry, deduced the revolutions of the planetary orbs, both primary and

w See Transactions of the Royal Society for 1800.

secondary, including the minute irregularities of each, with some errors indeed, but with a degree of conformity to nature and subsequent observation, which must ever astonish and delight the inquiring mind. The British philosopher leaving astronomy in this improved state, no wonder that those who came after him should at once, with growing ardour, and with greater ease, pursue a course which he had so happily marked out.

At the beginning of the century under review, we find FLAMSTEAD, the first Astronomer Royal of England, devoting himself to this science with great zeal and success. He particularly directed his attention to the fixed stars; and after a series of patient and most laborious observations, published, in 1719, a catalogue of stars, more extensive and accurate than had ever been formed by one man. To him, both in office, and in astronomical fame, succeeded Dr. HALLEY, who made a number of important discoveries, and useful publications. Among many others which might be mentioned, he discovered the Acceleration of the Moon, and gave a very ingenious method of finding her parallax. He composed tables of the Sun, the Moon, and all the planets. He also recommended the mode of ascertaining the Longitude by Lunar Observations; a mode which has been since much improved, and generally adopted; and which is, at present, the most certain guide of the mariner. After him, at the head of the Royal Observatory was placed Dr. BRADLEY, who greatly distinguished himself as a practical astronomer. He was the first who made observations with sufficient accuracy to detect the smaller inequalities, in the motions of the planets and fixed stars. By means of this accuracy, he discovered, in 1727, the aberration of the stars, a phenomenon produced by the compound motion of the earth, and

the rays of light; and furnishing new proof, both of the materiality and amazing velocity of light, and also of the reality of that motion which had been ascribed to the earth. The same gentleman, in 1737, discovered the mutation of the earth's axis-that libratory motion, which is occasioned by the inclination of the moon's orbit to the ecliptic, and the retrograde revolution of her nodes; thus, in the course of ten years, making two of the most important additions to astronomical knowledge that the century produced.

While these noble and successful exertions were making in Great-Britain, to improve the science of astronomy, the philosophers of France were employing themselves in the same field of inquiry, and with very honourable success. The real figure of the globe we inhabit had not been, before this time, satisfactorily ascertained. M. CASSINI, the Astronomer Royal at Paris, believed its figure to be that of a prolate spheroid, or, in other words, that the polar diameter was greater than the equatorial; while NEWTON had been led, by his principles, to a conclusion directly opposite, and had taught that it must be an oblate spheroid, or flatted at the poles. To determine the question, between these contending powers, the French Royal Academy of Sciences, under the authority and patronage of LEWIS XV. resolved to have two degrees of the meridian measured, the one as near the Equator, and the other as near the Pole, as póssible. For this purpose, one company of philosophers, consisting of Messrs. GODIN, CONDAMINE, and BOUGUER, to whom the King of Spain added Don ULLOA and Don JUAN, was dispatched, in 1735, to South-America; and another Company, consisting of Messrs. MAUPERTUIS, CLAIRAULT, CAMUS, LE MONIER, and OUTHIER, attended by Professor CELSIUS, of Upsal, were sent to Lapland,

These companies, after devoting several years to the task committed to them, and encountering nu merous difficulties in the prosecution of it, at length completed their design. The result proved to be an ample confirmation of NEWTON's opinion; for a degree near the Pole being found to measure more than one near the Equator, they necessarily inferred that the polar degree must be part of a larger circle.*

At the beginning of the eighteenth century, our knowledge of the Moon was extremely defective. Since that period, so many discoveries have been made respecting this attendant on our earth, and the laws of her motion have been so ably and diligently investigated, that this part of astronomy may now be ranked among those which are most fully known and understood. For these investi gations we are indebted to CLAIRAULT, D'ALEMBERT, EULER, MAYER, SIMPSON, WALMSLY, BURG, BOUVARD, DE LA GRANGE, DE LA PLACE, and others. By the labours of these great astronomers, the inequalities in the moon's motion have been detected, ascertained, and reduced to a system; accurate Lunar Tables have been formed; and the theory of this satellite has been carried to such a degree of perfection, that her place in the heavens may be computed with a degree of preci sion, which would have been pronounced, a few years ago, altogether impossible. With respect to the condition and aspect of the moon's surface, many important discoveries have been made, and much valuable information given to the world, by M. SCHROETER," a celebrated astronomer of Goet

It is impossible to recollect the attempt by M. BERNARDIN DE ST. PIERRE, in his Studies of Nature, to revive the opinion of CASSINI on this subject without surprize. That so learned and ingenious a man should oppose such distinct mathematical demonstration, is one of those caprices of respectable minds not easy to be accounted for.

y See Selenotopographische Fragmente, &c. by JOHAN. HYERONYMUS SCHROETER, 4to. 1791.

tingen, and by Dr. HERSCHEL, of Great-Britain; who, by the aid of very powerful and accurate instruments, and with the skill and perseverance for which they are so eminently distinguished, have made surprizing progress in investigating this department of the lunar phenomena.

When NEWTON died, several of the inequalities of the planetary motions, arising from the disturbing forces of various bodies, were with difficulty reconciled with the astronomical principles which he had laid down. These inequalities have been successively investigated since that time, their causes ascertained, their laws fixed, their perfect consistency with the Newtonian theory demonstrated, and thus a very formidable objection to that theory satisfactorily removed.—It is known to mathematicians, that this celebrated philosopher, calculating the effect of the sun's force, in producing the precession of the equinoxes, fell into an error, and made it less, by one half, than the truth. The true quantity of this motion was first determined by M. D'ALEMBERT, in 1749; who also, in the course of his inquiries, more fully explained the nutation of the earth's axis, which had been discovered a few years before by Dr. BRADLEY. With no less diligence the inequalities in the revolutions of all the planets, and especially of Jupiter and Saturn, have been examined, ascertained, and reduced to regular principles. In these difficult investigations, many astronomers have employed themselves, in the course of the last century, and by their labours rendered important services to this science; but, perhaps, none of the number deserve more honourable distinction than EULER, DE LA PLACE, and DE LA GRANGE, whose accurate observations, and rigid and delicate analyses, with a view to explore the anomalies in

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