PREFACE.

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mena accompanying their transits and occultations are also suggestive ofsome interesting speculations.

This chapter introduces a name which occupies a prominent place in theremaining portion of the work—it is the immortal name of William Her-schel. To the bulk of the intelligent class of readers this illustrious indi-vidual appears in the character of an astronomer distinguished by his skil-ful construction of huge telescopes, which he employed with marvelloussuccess in exploring the heavens. To the student who has advanced withinthe precincts of astronomical science, he forms a more exalted object of ad-miration, as an observer of almost unrivalled acuteness and sagacity, whoseexquisite faculty of discernment frequently enabled him to arrive at resultsfar beyond the scope of the mere instrumental resources available to him;and as a philosopher of the highest order, who, by his originality of thoughtand capacity for comprehensive speculation, succeeded in establishing theprinciples of Sidereal Astronomy upon a broad and indestructible basis.

The fifteenth chapter contains an account of the progress of enquiryon the physical constitution of Comets . These mysterious bodies, beyonddoubt, perform some important function in the economy of nature, whichcan only be ascertained by attentive observations of the phenomena whichaccompany their various apparitions.

The sixteenth chapter is devoted to those physical principles whoseinfluence in disturbing the apparent positions of the celestial bodies, orin modifying the features of celestial phenomena, must necessarily betaken into account before astronomical observations can be rendered avail-able as the groundwork of ulterior enquiry. It comprehends an accountof the progress of researches on Precession , Refraction , Aberration, Nuta-tion, Diffraction , and Irradiation . In the history of Refraction the mightynames of Newton and Laplace reappear with transcendant lustre. Thecorrespondence between Newton and Flamsteed, published by the lateMr. Daily, has supplied some interesting materials connected with theresearches of Newton on this subject. The uncertainty which so longexisted respecting the construction of Newton ’s table of refractions, whichHalley originally communicated to the Royal Society —whether it wasbased upon some physical theory of the subject, or whether it was cal-culated merely by an empiric process—has been effectually removed by thecorrespondence above referred to. It appears that Newton studied pro-foundly the theory of Astronomical Refraction , and succeeded in deter-mining the results corresponding to various hypotheses respecting thephysical constitution of the atmosphere. His suggestion to Flamsteed,recommending the practice of noting the indications of the barometer andthermometer, as a desirable accompaniment to astronomical observations,constitutes a striking illustration of the sagacity by which that greatphilosopher was distinguished above ordinary enquirers.

The subjects of Aberration and Nutation are introduced, with anaccount of the original discovery of these phenomena by the immortal