ERRATA.

Page

46, line 5, for "13th," read "30th."

56, line 6 from bottom, for " discrepancies," read "discrepances." 57, for lines 3d and 4th, read “gives for the compression

deduced from arcs of the meridian." 59, Note.-The effect of local attraction on the pendulum is so great, that it has rendered the experiments made with that instrument for the purpose of ascertaining the compression of the earth very uncertain. Mr. Baily, President of the Astronomical Society, has devoted much attention to the investigation of this subject. He finds that the experiments of Captain Foster, whose early loss is so justly lamented, give a compression of those of Cap. tain Sabine give the mean of the French and Russian experiments give 67; from the mean of the whole Mr. Baily deduces the compression to be; but even this is

not conclusive.

267.23;

9748

64, line 7, for "92246700," read "95296400."-Line 8, for "ninetytwo," read" ninety-five."

Note.-If the computation be made with the more accurate parallax 8.5776, the sun's distance is 95070500 miles.

10

924'

67, line 8, the quantity 159, representing the compression of Jupiter, was not deduced from Encke's comet, but from the perturbations of Juno.

Note.-Professor Airy has recently determined the most accurate estimation of the value of the mass of Jupiter to be 18 deduced from the elongation of the fourth satellite: he has also found that the mass of the whole Jovial system is 648.70. showing how small a proportion the mass of the satellites bears to that of the planet.

68, line 8, for "886860," read "886952."

1

88, lines 7 and 8 from bottom, for " radius," read "diameter."

99, line 7, for "poles," read pole."

128, lines 1 and 2 from bottom, for " volume," and "volumes," read

"atom" and "atoms.'

129, lines 1 and 3, for "volumes" and "volume," read "atoms"

SECTION I.

ALL the knowledge we possess of external objects is founded upon experience, which furnishes facts; and the comparison of these facts establishes relations, from which induction, the intuitive belief that like causes will produce like effects, leads to general laws. Thus, experience teaches that bodies fall at the surface of the earth with an accelerated velocity, and with a force proportional to their masses. By comparison, Newton proved that the force which occasions the fall of bodies at the earth's surface, is identical with that which retains the moon in her orbit; and induction led him to conclude that, as the moon is kept in her orbit by the attraction of the earth, so the planets might be retained in their orbits by the attraction of the sun. By such steps he was led to the discovery of one of those powers with which the Creator has ordained that matter should reciprocally act upon matter.

Physical astronomy is the science which compares and identifies the laws of motion observed on earth with the motions that take place in the

B

heavens; and which traces, by an uninterrupted chain of deduction from the great principle that governs the universe, the revolutions and rotations of the planets, and the oscillations of the fluids at their surfaces; and which estimates the changes the system has hitherto undergone, or may hereafter experience changes which require millions of years for their accomplishment.

The accumulated efforts of astronomers, from the earliest dawn of civilization, have been necessary to establish the mechanical theory of astronomy. The courses of the planets have been observed for ages with a degree of perseverance that is astonishing, if we consider the imperfection and even the want of instruments. The real motions of the earth have been separated from the apparent motions of the planets; the laws of the planetary revolutions have been discovered; and the discovery of these laws has led to the knowledge of the gravitation of matter. On the other hand, descending from the principle of gravitation, every motion in the solar system has been so completely explained, that the account of no astronomical phenomenon can now be transmitted to posterity of which the laws have not been determined.

Science, regarded as the pursuit of truth, which can only be attained by patient and unprejudiced investigation, wherein nothing is too great to be