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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 unler 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 E'ectricity, 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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ASTRONOMY.
Page 5, column 2, line 17 from the bottom, dele each of.'

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26, 1, 41 and 44, for the rol, read t,' elry!
28,

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12 in the note, for ES', read ES,.
29,

45, fort, elr,', read l,' dri'.
last, for S, read S,.

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 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.' 31, 2, 13, in the note, for Ps, Pt, read Ps, Pe. 1, 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.' 61,

2, 10, for • 1108' reall'1103 '; and for '133100' read 1331000.” 64, 1, 35, for "from read' for.' 81, 1, top line to be transferred to the foot of column 2. 111,

figure 31 reversed. 231, 1, 12, prefix VI. to mark the section.

253, 1, for CHAPTER XIII,' read CHAPTER XII.' HISTORY OF ASTRONOMY.

96, 1, 10, for ' being carried' read 'carrying.'

39,

47,

ASTRONOMY.

Introductory Observations.

it is only within the last two hundred In treating of any science which is years that its true principles have been grounded upon physical facts and ap- at all generally received; it is only from pearances, two courses are generally the time of Newton that they have been open. We may begin with a statement adequately explained. of the results observed, and by gradual In the present treatise, nevertheless, investigation extricate from them the the results of observation will not be principles on which they depend: or else, explained from principles assumed in if these principles have been ascer- the first instance, but the principles of tained, we may begin by stating them, astronomy will be deduced, as far as and may deduce from them the con- they can be so without complicated sequences which would follow on the mathematical investigation, from obsersupposition of their truth; and finally, vation. There are several reasons which by comparing these consequences with seem to render this the most desirable the appearances presented by Nature, course of proceeding, although adopted and finding them to correspond, we at the sacrifice of much conciseness, may satisfy ourselves of the truth and of any very logical precision of of those principles which we origi- arrangement. nally assumed. The former is ne- The present treatise is principally cessarily the course of discovery ; addressed to a class of readers not hathe latter is often the most concise bitually accustomed to severe reasoning, and convenient method of instruction and is intended for those who know after the discovery has been made. nothing of astronomy when they enter In some cases there is little prac- upon its perusal; and to them the tical distinction between the two me- course which we have preferred will prothods; for instance, the fundamental bably be at once more interesting and principle of Hydrostatics is the equal more intelligible than the other. The pressure of fluids in all directions; and general appearances of the heavens, the the fact that they do press so is one of succession of day and night, the appathe first and most obvious results of oh- rent courses of the heavenly bodies, are servation and experiment; and from the objects of interest and curiosity to all, time that it is ascertained, the experi- however ignorant of the laws which remental and hypothetical mode of dis- gulate them, or the consequences which cussing the subject may very nearly may be deduced from them. And they coincide. In other sciences, on the con- are not only interesting, but to a certain trary, the first effect of observation is extent familiar; sufficiently so to perplex to lead us to conclusions very distant the reader of statements apparently at from the fundamental principles which variance with them, and to deprive him of we finally adopt, or even at variance the greatest satisfaction that the student with them. In these cases the simpler of a new science can feel, the power of and shorter mode of instruction will at once comparing his deductions with generally be the second which we have facts, and convincing himself experimentioned. Among these sciences As- mentally of the soundness of his reasontronomy is eminently distinguished; for ings by the accuracy of the results to almost all the immediate results of obser- which they lead him. A mind habivation are contrary to its true principles, tuated to close reasoning upon merely and it is not without much labour and hypothetical truth may be satisfied with reasoning that the truth can be extri- out such confirmation; yet it is agreecated from the mass of error in which it able to all, and to those which have not is involved. Astronomy has been a fa- been thus exercised it is almost necesvourite study from the earliest periods: sary; but, on the hypothetical system,

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