easily explained. Cassini's curve has not the same advantage and besides, when the two foci are very distant from each other, it takes a course, which it is physically impossible any planet should follow.

Auzout was also an excellent observer; he had a perfect knowledge of astronomical instruments, and improved and extended the use of the micrometer, which was first invented by Huygens. It is said, that, when he presented to Lewis XIV his observations on the comet of 1644, he first suggested the idea of building an observatory, and furnishing it with instruments. The observatory at Paris, begun in 1667, was finished in 1672, six years after the foundation of the Academy of Sciences. England closely followed the example, and the observatory at Greenwich was built in 1676.

There are certain speculative sciences, as geometry, algebra, rational mechanics, &c., in which great progress can be made only by men of a sedentary life, meditating in the silent retreat of the closet. There are others, in which we must pass from theoretical study to practical application, make experiments, and traverse various countries: such are physics, natural history, and particularly astronomy, which frequently requires comparative observations made in different places very remote from each other.

Abbé Picard, distinguished by his dexterity and skill in selecting and managing the proper instruments for observations, performed several useful labours; and among them the project, that had been frequently attempted, of measuring the Earth, with a precision on which geography and navigation might establish certain bases: for the measures of the Greeks

and arabs, and even those of some modern philosophers, wanted this stamp, or at least had nothing to warrant their accuracy. He measured the arc of the heavens comprised between Amiens in Picardy, and Malvoişine on the confines of the Gâtinois and Hurepoix : then by a comparison of this measure with that of the correspondent terrestrial arc, determined by means of a series of triangles connected with each other, and of which the first was constructed on a known base, he concluded, that the length of a degree of the Earth was 57060 toises nearly. Hence it followed, that the entire length of a great circle of the globe was 20541600 toises.

In 1672 Richer was sent to Cayenne, which is within five degrees of the equator, to make various astronomical observations there. He was particularly directed, to observe the planet Mars, which Picard, then in Denmark, and Cassini and Roemer in Proyence, were observing at the same time; that from all these observations, made in such distant places, they might be able to deduce the parallax of this planet, to which the attention of most astronomers was turned, as they hoped it would throw great light on the theory of parallaxes.

As soon as Richer was preparing to begin his observations, he made one that had not been foreseen, and which was much more important than all that had been proposed. As a measure of time he carried with him a pendulum, which swung seconds accurately at Paris; but when he came to make use of it at Cayenne, he found, that the pendulum performed it's oscillations too slowly; and that to make it swing seconds with precision,

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precision, it was necessary to shorten it about a line and a quarter. This singular observation being transmitted to Paris, Huygens immediately discovered the physical reason of it; which was, that, in consequence of the rotatory motion of the Earth round it's axis, the centrifugal force was greater toward the equator than in the latitude of Paris, and consequently must occasion a greater diminution of the natural and primitive gravity there: whence it fol lowed, by a farther consequence, founded on the theory of the motion of pendulums, that the pendulum swinging seconds at Cayenne must be shorter, than the pendulum swinging seconds at Paris. Huygens gave likewise a calculation of the progressive flattening of the Earth in proceeding from the equator toward the poles.

Some years after Newton likewise found a flattening in the same direction, but somewhat greater than that of Huygens, because these two illustrious gcometricians set out with suppositions respecting the nature of primitive gravity a little different from each other. Huygens considered it as constant, and directed to the centre: Newton as the result of all the reciprocal attractions of the molecules of the terrestrial globe, which leaves the centre a little on one side.. In this grand problem we find experiment preceded and enlightened theory, and France had the honour of furnishing the data, which were to be employed in it's solution. We shall hereafter see the vast and expensive undertaking which she executed, to determine the truc dimensions of the Earth.

We

We may likewise reckon in the number of our astronomical discoveries that of the propagation of light, which was made about the same time. Roemer, the author of this discovery, who was born in 1644 and died in 1710, was indeed a dane by birth: but at the time he made it he was fixed in France by the munificence of Lewis XIV, and was one of the first members of our Academy of Sciences. After the satellites of Jupiter had become known, astronomers sought diligently to determine their motions; and Dominic Cassini had gone so far as to construct tables, which accurately represented their revolutions, and their eclipses occasioned by the shadow of Jupiter. But Roemer, who assiduously observed the first satellite, perceived, that in eclipses it emerged from the shadow at certain times a few minutes later, and at others a few minutes sooner, than it ought to have done according to the tables. He also, on comparing these times together, found, that the satellite emerged from the shadow later, when the Earth was carried by it's annual motion to a distance from Jupiter, and sooner, when it approached the planet. Hence he formed this ingenious conjecture, which was soon converted into demonstration, that the motion of light is not instantaneous, as des Cartes had thought, and as was then still believed, but that it took up a certain space of time to arrive from the luminous body at the eye of the observer. According to his first calculations, it was about seven minutes in traversing the radius of the Earth's orbit: but he afterward found, that the velocity of the luminous atoms was a little greater

greater than this. There is not a more remarkable phenomena in the natural philosophy of the heavens, or one more essential as an element in astronomical theories; hence it must secure immortality to the name of Roemer.

England produced at all times astronomers of the first order. Here we shall remark among others

Hooke, Flamsteed, and Halley.

Hooke, who was born in 1635, and died in 1702, was not merely a great observer in every branch of astronomy: we are indebted to him for the first idea of the system of universal gravitation, that can be called at all explicit. He made the three following suppositions. 1st: All the celestial bodies have not only an attraction or gravitation toward their own centre, but they mutually attract each other in their sphere of activity. 2dly: All bodies, which have a simple and direct motion, would continue to move in a right line, if some force were not incessantly turn ing them out of it, and compelling them to describe a circle, an ellipsis, or some other more complicated curve. 3dly: Attraction is so much the more powerful, as the attracting body is more near. All these bases enter into Newton's system; but what characterizes the discovery of the latter is the law of attraction, which he found out, and which was unknown to Hooke,

Flamsteed was born in 1646, and died in 1720. As soon as the observatory at Greenwich was built, he was appointed to the management of it by Charles II, and began to make there that numerous series of observations of all kinds, recorded in his Historia

Celestis,