it. He extended this proposition to all bodies whatever, and established the principle that all particles of matter attract each other directly as their mass, and inversely as the square of their distances.' Having arrived at this principle, Sir Isaac Newton saw that the great phenomena of the system of the world might be deduced from it. By considering gravity, at the surfaces of celestial bodies, as the result of the attractions of all their particles, he ascertained these remarkable truths, that the attracting force of a body, or of a spherical stratum, upon a point placed without it, is the same as if its mass was compressed into its centre. He proved that, by the motion of the rotation of the Earth, it ought to be flattened at the poles; and he determined, theoretically, the law of the variation of the degrees and of gravity, suppose it be homogeneous. He saw that the action of the Sun and Moon on the terrestrial spheroid ought to produce a motion in its axis of rotation-to make the equinoxes retrograde-to elevate the waters of the ocean-and to produce, in this fluid mass, the oscillations which are observed under the name of tides. Finally, he was convinced that the lunar irregularities were produced by the combined action of the Sun and Earth on the Moon. In short, Sir Isaac Newton perfectly established the existence of the principle of gravity, which he discovered; but the developement of its consequences, and its advantages, has, in a great measure, been the work of his successors. Nevertheless, the importance and extent of his discoveries, the great number of original and profound conceptions, which have been the germ of the most brilliant theories of geometricians of the last century, and arranged with much elegance, insure to his Principia a pre-eminence over all other productions of human intellect. The theories described in this work, and the truths which it has displayed, will re ASTRONOMICAL OCCURRENCES. 197 main an everlasting monument of the profundity of that genius which has revealed to mankind the greatest law in the universe. [To be continued.] Astronomical Occurrences. THE Sun enters the sign Leo at 37 m. past 1 in the morning of the 23d of July. TABLE Of the Sun's Rising and Setting every fifth Day of Table of what is to be added to the apparent Time on the Dial, to obtain true Time by the Clock for every 5th Day of July. (See Occurrences for January) : July 1, to the time on the dial add m. S. 3 23 And we have true time by the clock. Mercury is at his inferior conjunction at 20 m. after 8 in the morning of the 2d of July, and he will appear stationary on the 13th. Venus is at her superior conjunction at past 8 in the evening of the 31st. Jupiter is at his quadratures on the 24th, at 20 m. past 10 in the morning. The Moon enters her first quarter at 28 m. past 9 in the morning of the 3d day of July: she is full at 21 m. past 12 at noon of the 9th day; enters her last quarter at 46 m. past 12 at noon on the 17th; and she is at change, or new Moon, at 9 m. past 11 at night of the 24th: she enters her first quarter at 25 m. past 2 in the afternoon of the 31st. The Moon is in conjunction with ß m at 13 m. past 1 in the morning of the 6th. At 17 m. past 1 at noon on the 6th, the Moon and the planet Herschel will be in conjunction. On the 25th, the planet Mars and a will be in conjunction; the star will be 43 south of the planet. There will be but one eclipse of Jupiter's first satellite, which will happen on the 31st, at 14 m. past 9 in the evening. Of the second satellite there will be two eclipses, viz. on the 13th, at 29 m. past 9 in the evening; and on the 20th, at 37 m. past 9 at night. The eclipses of Jupiter's satellites, as has been observed in a former volume, afford the readiest, and, for general practice, the best method of settling the longitudes of places at land; and it is by their means, principally, that geography has been so much reformed since the invention of telescopes, and the construction of tables for calculating the time of their happening; the position of the most distant places being determined with equal accuracy as that of the nearest. It was,' says Dr. Maskelyne, the late Astronomer Royal, hoped that some means might be found of using proper telescopes on ship-board to observe these eclipses; and could this be effected, it would be of great service in ascertaining the longitude of a ship from time to time. In my voyage to Barbadoes, under the directions of the Commissioners of Longitude, in 1763, I made a full trial of the late Mr. Irwin's Marine Chair, proposed for this purpose, but could not derive any advantage from the use of it; and, considering the great power requisite in a telescope for making these observations well, and the violence as well as the irregularities of the motion of a ship, I am afraid the complete management of a telescope on ship-board will always remain among the desiderata.' The telescopes, proper for observing the eclipses of Jupiter's satellites, are common refracting telescopes of from 15 to 20 feet in length, or reflecting telescopes of 18 inches or 2 feet focal length, and telescopes of Mr. Dollond's construction with two object glasses from 5 to 10 feet; those of 46 inches focal length, and 3 inches aperture, constructed with three object glasses, which are as manageable as reflecting telescopes, and perform as much as those of 10 feet with two object glasses only. The eclipses of Jupiter's satellites are observed by astronomers at land, as well to provide materials for improving the theories and tables of their motions, as for the sake of comparison with the corresponding observations which may be made by persons in different parts of the globe, whereby the longitude of such places will be accurately ascertained. The eclipses, carefully calculated and set down in the Nautical Almanack, will serve to advertise observers of the times when they should attend to these observations. The person who is under any meridian different from Greenwich must turn his difference of longitude into time, and add it to, or subtract it from, the time of the eclipse set down in the Ephemeris, according as he is to the east or west of Greenwich, to find the mean time at which the eclipse will happen at his meridian nearly. He must, farther, take care to regulate his watch or clock by mean time, or, at least, to know the difference, as well in order to apprise him of the time to look out for the eclipse, as for ascertaining the apparent time exactly at which he shall observe it. The observer, being in a place whose longitude is well known, should be settled at his telescope three minutes before the expected time of an immersion or emersion of the satellite; but, if the longitude of the place be uncertain to 2 or 3 degrees, answering to 10 or 12 minutes of time, he ought to fix himself to his telescope 10 or 12 minutes sooner than is mentioned above. The Naturalist's Diary. First April, she with mellow showers More wealth brings in than all those three. THE succession of the four sweet months,' which Herrick has described with so much truth and nature, leads us, by insensible degrees, to enjoy the beauties of SUMMER, which may be said to commence with July, although our Almanacks tell us that it began on the 21st of the last month. In May and June the Spring glowed with all the mixtures of colorific radiance,' but now opening beauty and increasing variety are succeeded by the more uniform scenes of maturity and perfection. Nature, in our temperate regions, appears now to have nearly finished her annual work. Something of her variety she begins to lose in this season. Nothing, indeed, can be more beautiful than the verdure of the orchards and woods, but the shades of hue which they exhibit are no longer so agreeable. The meadows begin to whiten, and the flowers that adorn them are mowed down. Now the mower, Laying his long scythe aside, Sleeps on some bank with daisies pied, Till roused to toil again. The corn gradually assumes a yellow hue, and the colours that decorate the rural scene are no longer so How lately did the glowing beauty and numerous. |