SUBSTANCES may either be examined in a state of rest, or as acting upon each other, and producing changes on each other. The knowledge derived from the first of these views is called Natural History; that which we obtain by the second, is distinguished by the name of Science. But bodies cannot act upon each other without producing motion, and the motions produced by such actions are of two kinds; either so great as to be visible to our senses, and capable of being measured by the space passed over; or so small as not to be distinguishable by our senses, except by the effects produced. The phenomena connected with the first of these kinds of motions constitute what is called Natural Philosophy, or Mechanical Philosophy, in this country, and on the Nature of meContinent, Physics. The phenomena connected with the imperceptible mo-losophy. tions belong to the science called Chemistry. chanical phi Mechanical Philosophy comprehends under it a great many very important Division, phenomena, and has been sub-divided into different subordinate heads. A variety of arrangements has been proposed, attended each with its peculiar advantages and defects. Our object is only to consider it as far as it is treated of in the Philosophical Transactions. Now all the topics connected with it, as far as they occupy a part of that work, may be arranged under the following heads: These will constitute the subject of the nine following chapters. CHAP. I. OF ASTRONOMY. Origin of astronomy, THE Science of Astronomy furnishes the noblest proof of what the human intellect is capable. The only true mode of advancing, patient and assiduous observation, was early perceived by its cultivators; and for 2000 years past astronomers have devoted themselves to the observation of the heavenly phenomena, with a degree of persevering industry, which has only been equalled by the progress which they have made. No other science can boast of the same degree of perfection. It may be considered in some measure as complete Astronomy constitutes one of the most prominent departments in the Philosophical Transactions; and owes much of its progress to the splendid exertions of some of the most conspicuous Members of the Royal Society. From the foundation of the Royal Observatory of Greenwich, in 1675, the place of Astronomer Royal has been filled by a succession of men of the first eminence, who have furnished a series of observations of infinite importance to navigation and astronomy. Whether that race be at length extinct by the death of Dr. Maskelyne, or whether succeeding Astronomers Royal will display any portion of the vigour and abilities which illustrated their predecessors, remains to be seen. The History of Astronomy has been treated so amply and so well, in a great variety of books, which are in every body's hands, that we may be excused for passing it over in a very concise manner. The Chaldeans and Egyptians are conceived to have turned their attention to astronomy at a very early period; and attempts have been made to show that the Chinese, and the inhabitants of India, are in possession of tables indicating observations coeval with, or even anterior to, the commonly received era of the creation. But these observations, notwithstanding the zeal, the skill, and the acknowledged abilities of the ingenious supporters of them, have not met with that favourable reception from astronomers, which was expected by their propagators; and have been almost unanimously rejected as erroneous and impossible. The oldest astronomical observations on record were made at Babylon, 719 and 720 years before the commencement of the Christian era. Ptolemy, who has transmitted them to us, employed them for determining the period of the moon's mean motion; and therefore, in all probability, had none more ancient on which he could depend. The Greeks borrowed their first notions of astronomy, as of all the other sciences, from the Egyptians. Thales, of Miletus, who lived in the seventh Thales Timocharis. But the real foundation of astronomy was not laid by the Greeks till the establishment of the Alexandrian school by Ptolemy Soter, and his successors. Ptolemy Philadelphus built a magnificent observatory, and supplied it with the requisite books and instruments. The first astronomers, who were placed in this building, were Aristyllus and Timocharis, who lived about 300 years Aristyllus and before Christ, and observed, with accuracy, the position of the principal stars of the zodiac. The little that we know respecting their labours, we obtain from the writings of Ptolemy, who quotes them sometimes. About the same time flourished Aristarchus, of Samos, who is celebrated by the ancients for his genius and discoveries. He pointed out an ingenious way of determining the Aristarchus. comparative distances of the sun and moon from the earth; and demonstrated that the sun was at least 20 times further off than the moon. Though this determination was greatly below the truth, yet it was a considerable step at the time, and extended the bounds of the universe to the eyes of astronomers. He also supported the opinion, that the earth and the planets move round the sun; an opinion previously advanced by Pythagoras, and some of his immediate disciples; but which the imperfect state of astronomy and mathematics did not enable them to support by such arguments as were calculated to give it general credit. Eratosthenes was another astronomer of the Alexandrian school, to whom Eratosthenes. the science was much indebted. He prevailed upon Ptolemy Euergetes to erect, in the portico of the observatory, different large instruments for observing the stars. It was by means of these instruments that most of the subsequent † Phil. Trans. 1811. p. 220. * Herodotus. Lib. I. § 74. flis measure ment of a degree. Hipparchus. observations of the Grecian astronomers were made. Eratosthenes likewise observed the obliquity of the ecliptic, and found it 23° 51′ 13′′; a determination which has been of some service to astronomers, in determining the change of the obliquity which has taken place since that time. But the most famous of his experiments was his measurement of a certain portion of the earth's surface. There was a deep pit at Syena, an island in the Nile, and almost at the southern extremity of Ptolemy's dominions. Eratosthenes had observed, that on the day of the summer solstice, the whole bottom of this pit was illuminated at mid-day; and, as objects for some distance round Syena cast no shadow on that day, he concluded that Syena was situated directly under the tropic of Cancer. He next concluded, that Alexandria was in the same meridian with Syena, and that the distance between them was 5000 stadia. He then determined, by observation, the difference between the latitude of the two places. From all this, he settled the length of a degree at 250,000 stadia: but as we do not know the length of the stadium which he used, this measurement is of no use at present. It cannot be very accurate, because several of the data, upon which it was founded, are erroneous. Alexandria and Syena are not under the same meridian, their longitude differs by several degrees. The distance between the two places was not measured, but only estimated, and of course could not be accurate. All the fragments of the works of Eratosthenes still remaining have been inserted in the magnificent edition of Eratus, published at Oxford in 1672. But the true founder of astronomy, as a science, was Hipparchus, of Bithynia, who observed in Alexandria during 35 years, between 160 years before Christ, and 125 years B. C. We know very few particulars of his life; but his astronomical labours are well known, and constitute a memorable era in the science. His first care was to determine the length of the year, which he did by a great many successive determinations of the exact times of the equinoxes and solstices. But, in order to be able to determine the point with greater exactness, he had recourse to the determination of the summer solstice, about 150 years before, by Aristarchus, of Samos. The result of his calcu lation was, that the year wants about five minutes of 365 days 6 hours. His next object was to determine the rate of the sun's motion; and he showed, by numerous and accurate observations, that this luminary moves fastest in winter and slowest in summer; so that the summer half year is about nine days longer than the winter half year. In order to account for this, Hipparchus supposed the sun to move uniformly in an eccentric circle, the distance of the centre of which, from that of the earth, was th of radius, and he placed the apogee in the sixth degree of Gemini. In these determinations Ptolemy coincides with Hipparchus; though it is now known that they made the eccentricity about 4th too great. Hipparchus also devoted much of his attention to the moon. He determined the length of a lunation, by the same means that he had ascertained the length of the year. He measured the inclination of the moon's orbit to the ecliptic, which he found amount to five degrees. He determined, likewise, the motion of the apsides and nodes with much greater exactness than had formerly been done. He thought of an ingenious mode of ascertaining the size and distance of the sun and moon; and his construction, though not sufficiently delicate for the purpose to which he applied it, is still employed in the calculation of eclipses. He found the distance of the sun 1200 semi-diameters of the earth, and his horizontal parallax 3'. The mean distance of the moon from the earth he found equal to 59 of these semi-diameters. From this he concluded that the diameter of the earth was 3 times as great as that of the moon; and the diameter of the sun 5 times as great as that of the earth. Though these numbers are far below the truth, they served, however, to extend considerably the bounds of the universe, and were a much nearer approximation than had been formerly obtained. logue of the Hipparchus did not attempt to form any theory of the planetary motions; but he made many accurate observations respecting them, in order to put it in the power of his successors to contrive a theory to explain their motions. But perhaps the greatest of all the labours of Hipparchus was his catalogue of the Makes a catastars, and the discovery of the precession of the equinoxes, to which he was stars. led by that catalogue. It was the appearance of a new star in the heavens, during his time, that induced him to undertake that laborious task, in order to enable future astronomers to know whether any new stars had made their appearance after his time. He not only drew up a catalogue of the principal fixed stars, with their exact positions, but he appears to have constructed a solid sphere, and to have placed them upon it in their proper situations, and likewise to have projected this sphere upon paper, for the facility of transportation. On comparing his own observations with those of Arystillus and Timocharis, made 150 years before, he ascertained that all the stars had changed their place since that time two degrees in the order of the signs: a discovery which has been since amply confirmed, and is well known by the name of The Precession of the Equinoxes. Hardly any additions were made to astronomy (for it is unnecessary to mention Posidonius, of whom we know little,) till the period of Ptolemy; whom antiquity, perhaps a little unjustly with regard to Hipparchus, have dignified with the name of the first of astronomers. Ptolemy was born in Ptolemais, in Ptolemy, Egypt, and flourished during the reigns of Adrian and Antoninus, about the year 125 of our era. Ptolemy contrived the first system of astronomy, and consigned it in his great work, which still remains, and which is usually known by the name of Almageste; because it was first made known to Europeans by |
