PREFACE TO THE FIFTH EDITION. I AM informed by the Publishers that this Ffith Edition is required to meet the demand of a somewhat wider class of students than those for whom the Lectures were originally intended. This circumstance, as well as the advances made in Astronomical science during the period which has elapsed since the Lectures were delivered, have rendered it desirable that some very slight modifications should be made in the text, and that some additions should be made in the form of Appendix. I am happy to state that Mr. Stirling has been at liberty to prepare the modifications and additions to which I allude, and to undertake the general editing of the book. And I now issue the work to the public, with the increased confidence derived from the assistance of a friend on whose ability I place complete reliance. ROYAL OBSERVATORY, GREENWICH, 1866, June 9th. G. B. AIRY. INTRODUCTION. IN conversing with persons who are not officially attached to Observatories, or in other ways professionally cognizant of the technicalities of practical Astronomy, but who, nevertheless, display great interest in the science of Astronomy generally, I have frequently been struck with two remarks. The first is, that these persons appear to regard the determination of measures, like those of the distance of the Sun and Moon, as mysteries beyond ordinary comprehension, based perhaps, upon principles which it is impossible to present to common minds with the smallest probability that they will be understood; if they accept these measures at all, they adopt them only upon loose personal credit; in any case, the impression which the statement makes on the mind is very different from that created by a record of the distance in miles between two towns, or of the number of acres in a field. The second remark is, that when persons well acquainted with the general facts of Astronomy are introduced into an Observatory, they are, for the most part, utterly unable to understand anything which they see; they are impressed perhaps with the apparent complexity of subsidiary parts of the Astronomical instruments, and they imagine that the fundamental |