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ADVERTISEMENT.

IN presenting to the public this third and concluding volume of Natural Philosophy, it seems requisite to offer a few remarks concerning the plan adopted for the arrangement of the originally disjointed portions. To those who would have wished that it had been more systematic, it may be observed that the several Treatises were composed so that the subjects might, as much as possible, be individually perfect, and independent of one another; and this necessarily tended, not only to occasion repetitions when collected into volumes, but, in some degree, to break the thread which would have united them in one general science.

The object of Natural Philosophy is the investigation of those principles which are to be considered as inherent in matter, and by the agency of which the changes in the relative positions, modifications, and internal combinations of the several masses (or separate bodies) are produced. These are,—

I. The Effects of Force or Impulse, and its modes of Propagation.

II. The Effects of the Pressure and Motion of non-elastic Fluids.

III. The Effects of Air, and similar elastic Fluids.

IV. The Effects of Caloric, or the principle of Heat.

V. The Phenomena of Light and Colours.

VI. The Phenomena and Effects of the Electric and Magnetic Fluids.

VII. The Effects of the principle of Gravitation.

The FIRST of the preceding divisions is investigated in the three Treatises on Mechanics, in Vol. I.; and in the Introduction to Mechanics, Vol. II.

The SECOND is treated of under the titles Hydrostatics and Hydraulics, Vol. I., and the Introduction to Hydrostatics, Vol. II.

The THIRD is treated of under the titles Pneumatics, Vol. I., and Introduction to Pneumatics, Vol. II.

The FOURTH is treated of under the titles Heat, Vol. I., and Thermometer and Pyrometer, Vol. II.

The FIFTH is copiously treated under the several heads of Optics, Double Refraction, and Polarization of Light, Vol. I.; and of Introduction to Optics, Sir Isaac Newton's Optics, and Optical Instruments, Vol. II.

The SIXTH, with an account of the latest discoveries, will be found under Electricity, Galvanism, Magnetism and Electro-magnetism, Vol. II. And The SEVENTH is the Astronomy of the present volume, an Introduction to which is prefixed to Vol. II.

To the Treatises on Astronomy, and the History of that science, it was judged proper to append Mathematical Geography. The Treatise on Physical Geography is less connected with the principal portion of the volume, but naturally follows that division of Geography which is termed Mathematical, Navigation, as far as it is a science, is wholly dependent on Astronomy, and as such is not considered to be out of place.

The discoveries of modern Chemistry have raised it from an art to the rank of a science; and there would have been no impropriety in including it in the general system of which we now speak. In such an arrangement it would have stood thus :

VIII. The Effects of Corpuscular Attraction on the Combination and

Decomposition of Bodies;

and the Treatises on the subject would have formed a FOURTH Volume of Natural Philosophy. It has, however, been judged advisable to follow the usual practice; and, therefore, the several Treatises (of which four have already appeared) will, when completed, be collected and published in a sepa rate volume, under the title of CHEMISTRY.

October, 1834.

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8, for of motion,' read of his motion.'
16 from the bottom, for ES, Es, read ES1, Es,.
13 from the bottom, for Es, read Es ̧·
14, for s,s,s3, read s ̧§283•

33, for t read t.

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34, for s,t,, read s1t.

41 and 44, for t' e'r', read t,' e' ra'.
4 in the note, for S, read S.
12 in the note, for ES1, read ES,.
45, for t, r', read t'er.
last, for S, read S1.

add to the first note, 'At the points Z and Y therefore these
circles are for a short space parallel to each other, or the
sun has no perceptible motion Northward or Southward:
at the point r his course makes an angle = ZY with the
equator, which is necessarily its greatest inclination to it:
or his motion Northward is then most rapid: the conclusions
referred to in the note in page 23, col. i.'

13, in the note, for Ps, Pt, read Ps, Pt.

31 and 32, for a motion from left to right in those already referred to, read from left to right where the North pole is above the horizon.'

35, for on its' read'on in its.'

10, for '1108' read '1103'; and for '133100' read' 1331000.' 35, for 'from' read 'for.'

47,

61,

2,

64,

1,

81,

top line to be transferred to the foot of column 2.

111,

figure 31 reversed.

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12, prefix VI. to mark the section.

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for CHAPTER XIII,' read 'CHAPTER XII.'

HISTORY OF ASTRONOMY.

96,

1,

10, for 'being carried' read 'carrying.'

ASTRONOMY.

Introductory Observations. IN treating of any science which is grounded upon physical facts and appearances, two courses are generally open. We may begin with a statement of the results observed, and by gradual investigation extricate from them the principles on which they depend: or else, if these principles have been ascertained, we may begin by stating them, and may deduce from them the consequences which would follow on the supposition of their truth; and finally, by comparing these consequences with the appearances presented by Nature, and finding them to correspond, we may satisfy ourselves of the truth of those principles which we originally assumed. The former is necessarily the course of discovery; the latter is often the most concise and convenient method of instruction after the discovery has been made. In some cases there is little practical distinction between the two methods; for instance, the fundamental principle of Hydrostatics is the equal pressure of fluids in all directions; and the fact that they do press so is one of the first and most obvious results of observation and experiment; and from the time that it is ascertained, the experimental and hypothetical mode of discussing the subject may very nearly coincide. In other sciences, on the contrary, the first effect of observation is to lead us to conclusions very distant from the fundamental principles which we finally adopt, or even at variance with them. In these cases the simpler and shorter mode of instruction will generally be the second which we have mentioned. Among these sciences Astronomy is eminently distinguished; for almost all the immediate results of observation are contrary to its true principles, and it is not without much labour and reasoning that the truth can be extricated from the mass of error in which it is involved. Astronomy has been a favourite study from the earliest periods:

it is only within the last two hundred years that its true principles have been at all generally received; it is only from the time of Newton that they have been adequately explained.

In the present treatise, nevertheless, the results of observation will not be explained from principles assumed in the first instance, but the principles of astronomy will be deduced, as far as they can be so without complicated mathematical investigation, from observation. There are several reasons which seem to render this the most desirable course of proceeding, although adopted at the sacrifice of much conciseness, and of any very logical precision of arrangement.

The present treatise is principally addressed to a class of readers not habitually accustomed to severe reasoning, and is intended for those who know nothing of astronomy when they enter upon its perusal; and to them the course which we have preferred will probably be at once more interesting and more intelligible than the other. The general appearances of the heavens, the succession of day and night, the apparent courses of the heavenly bodies, are objects of interest and curiosity to all, however ignorant of the laws which regulate them, or the consequences which may be deduced from them. And they are not only interesting, but to a certain extent familiar; sufficiently so to perplex the reader of statements apparently at variance with them, and to deprive him of the greatest satisfaction that the student of a new science can feel, the power of at once comparing his deductions with facts, and convincing himself experimentally of the soundness of his reasonings by the accuracy of the results to which they lead him. A mind habituated to close reasoning upon merely hypothetical truth may be satisfied with out such confirmation; yet it is agreeable to all, and to those which have not been thus exercised it is almost necessary; but, on the hypothetical system,

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