CHAPTER VIII

.--THE TWO ANTAGONIST DOCTRINES OF GEOLOGY.

_Sect._ 1. Of the Doctrine of Geological Catastrophes. 586 _Sect._ 2. " " " Uniformity. 588 {19}

_ADDITIONS TO THE THIRD EDITION._

## BOOK VIII.--ACOUSTICS.

SOUND.

The Velocity of Sound in Water. 599

## BOOK IX.--OPTICS.

Photography. 601 Fluorescence. 601

UNDULATORY THEORY.

Direction of the Transverse Vibrations in Polarization. 603 Final Disproof of the Emission Theory. 604

## BOOK X.--THERMOTICS.--ATMOLOGY.

THE RELATION OF VAPOR AND AIR.

Force of Steam. 606 Temperature of the Atmosphere. 607

THEORIES OF HEAT.

The Dynamical Theory of Heat. 608

## BOOK XI.--ELECTRICITY.

General Remarks. 610 Dr. Faraday's Views of Statical Electrical Induction. 611

## BOOK XII.--MAGNETISM.

Recent Progress of Terrestrial Magnetism. 613 Correction of Ships' Compasses. 616 {20}

## BOOK XIII.--VOLTAIC ELECTRICITY.

MAGNETO-ELECTRIC INDUCTION.

Diamagnetlc Polarity. 620 Magneto-optic Effects and Magnecrystallic Polarity. 621 Magneto-electric Machines. 623 Applications of Electrodynamic Discoveries. 623

## BOOK XIV.--CHEMISTRY.

THE ELECTRO-CHEMICAL THEORY.

The Number of Elementary Substances. 625

## BOOK XV.--MINERALOGY.

Crystallography. 627 Optical Properties of Minerals. 629 Classification of Minerals. 630

## BOOK XVI.--CLASSIFICATORY SCIENCES.

Recent Views of Botany. 631 " " Zoology. 634

## BOOK XVII.--PHYSIOLOGICAL AND COMPARATIVE ANATOMY.

VEGETABLE MORPHOLOGY. 636 ANIMAL MORPHOLOGY. 638 Final Causes. 642

## BOOK XVIII.

GEOLOGY. 646

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## BOOK VIII.

_THE SECONDARY MECHANICAL SCIENCES._

HISTORY OF ACOUSTICS.

. . . . . . Go, demand Of mighty Nature, if 'twas ever meant That we should pry far off and be unraised, That we should pore, and dwindle as we pore, Viewing all objects unremittingly In disconnexion dead and spiritless; And still dividing, and dividing still, Break down all grandeur, still unsatisfied With the perverse attempt, while littleness May yet become more little; waging thus An impious warfare 'gainst the very life Of our own souls. WORDSWORTH, _Excursion_.

. . . . . . Ἐσσυμένη δὲ Ἠερίην ἀψῖδα διεῤῥοίζησε πεδίλῳ Εἰς δόμον ἉΡΜΟΝIΗΣ παμμητόρος, ὁππόθι νύμφη Ἴκελον οἶκον ἐναίε τύπῳ τετράζυγι κόσμου Αὐτοπαγῆ NONNUS. _Dionysiac_. xli. 275.

Along the skiey arch the goddess trode, And sought Harmonia's august abode; The universal plan, the mystic Four, Defines the figure of the palace-floor. Solid and square the ancient fabric stands, Raised by the labors of unnumbered hands.

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## BOOK VIII.

INTRODUCTION.

_The Secondary Mechanical Sciences._

IN the sciences of Mechanics and Physical Astronomy, Motion and Force are the direct and primary objects of our attention. But there is another class of sciences in which we endeavor to reduce phenomena, not evidently mechanical, to a known dependence upon mechanical properties and laws. In the cases to which I refer, the facts do not present themselves to the senses as modifications of position and motion, but as _secondary qualities_, which are found to be in some way derived from those primary attributes. Also, in these cases the phenomena are reduced to their mechanical laws and causes in a secondary manner; namely, by treating them as the operation of a _medium_ interposed between the object and the organ of sense. These, then, we may call _Secondary Mechanical Sciences_. The sciences of this kind which require our notice are those which treat of the sensible qualities, Sound, Light, and Heat; that is. Acoustics, Optics, and Thermotics.

It will be recollected that our object is not by any means to give a full statement of all the additions which have been successively made to our knowledge on the subjects under review, or a complete list of the persons by whom such additions have been made; but to present a view of the progress of each of those branches of knowledge _as a theoretical science_;--to point out the Epochs of the discovery of those general principles which reduce many facts to one theory; and to note all that is most characteristic and instructive in the circumstances and persons which bear upon such Epochs. A history of any science, written with such objects, will not need to be long; but it will fail in its purpose altogether, if it do not distinctly exhibit some well-marked and prominent features. {24}

We begin our account of the Secondary Mechanical Sciences with Acoustics, because the progress towards right theoretical views, was, in fact, made much earlier in the science of Sound, than in those of Light and of Heat; and also, because a clear comprehension of the theory to which we are led in this case, is the best preparation for the difficulties (by no means inconsiderable) of the reasonings of theorists on the other subjects.

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