PREFACE.

'IX

principles which are known to exercise an influence on celestial phe-nomena, as well as the study of those facts respecting the structureof the celestial bodies which admit of being explained by reference toestablished principles of physics. In accordance with this more enlargedsignification, the subjects noticed in the greater portion of the remainderof the work ought to be considered as forming an essential part of Phy-sical Astronomy.

The invention of the telescope about the beginning of the seventeenthcentury furnished the astronomer with an instrument of observation, themighty efficacy of which can only be compared with the aid which theinfinitesimal calculus affords to the geometer in his researches on theeffects produced by The continuous agency of those forces which Natureemploys in her operations. Armed with such an instrument, the sagaciousGalileo was soon enabled to announce a multitude of discoveries in theheavens, of startling novelty and of the highest importance. Myriads ofstars whose existence had eluded the scrutinies of the naked eye, werenow seen to illumine the unfathomable regions of space. The investiga-tion of the cosmical arrangement of the celestial bodies, and the studyof their individual structure, were problems unexpectedly found to bewithin the reach of the human faculties. This department of astronomicalscience, no less remarkable for the sentiments which it is calculated toinspire respecting the grandeur of the material universe, than for themultitude of instructive and delightful views of the physics of thecelestial regions which it unfolds, has been prosecuted with ardour by asuccession of eminent astronomers from Galileo ’s time down to the pre-sent day.

The fourteenth chapter exhibits a view' of the progress of researcheson the physical constitution of the bodies of the solar system, and alsoincludes an account of the various discoveries by which it has been en-riched in modern times. The observations of the solar spots have sug-gested some highly interesting speculations respecting the great centralbody which forms the source of the light and heat of the system. TheMoon , from her comparative proximity, has naturally given rise to muchphysical enquiry. The observations of the planets have disclosed a mul-titude of facts of a highly interesting character. Their rotatory move-ments round fixed axes with corresponding elliptical figures, and thediversified appearance of their surfaces, constitute striking points ofanalogy between them and the Earth . The remarkable phenomena visiblein the polar regions of Mars , the belts of Jupiter and Saturn , and thewondrous rings of the latter planet, have all furnished abundant materialsof observation and research. Nor are the satellites wanting in physicalfeatures of an important character. The relation of equality betweentheir periods of rotation and revolution, which a variation of their bright-ness, in several instances, has served to establish, constitutes a strikingpoint of analogy between them and the terrestrial satellite. The pheno-