PREFACE

TO

THE FIFTH EDITION.

THE rapid progress of science renders it necessary frequently to revise and bring up elementary works to the existing state of knowledge, under penalty of their becoming obsolete. In former editions of this work, this has been done, so far as it could be done without incurring the necessity of an almost total typographical reconstruction. But Astronomy, within the last few years, has been enriched by so many and such considerable additions, that it has been considered preferable (another edition being called for), not indeed to recast the general plan of the work, but to incorporate these in it in due order and sequence, thereby materially enlarging the volume, and giving it in many respects the air of a new work. The articles thus introduced are distinguished from those of the former editions between which they have been inserted by the addition to the last current number of an italic letter- thus, between Arts.

$94. and 395. will be found inserted 394. a, 394. b, and 394. c. The enclosure of any passage in brackets

] indicates its having been introduced in the Fourth or some subsequent Edition. The index references in this as in former editions, being to the articles and not to the pages, are thus preserved. Together with these recent accessions to our knowledge, I have taken the opportunity of introducing several things which might justly have been noted as deficient in the former editions, -as, for instance, the account of the methods by which the mass of the Earth has been determined, and that of the successful treatment, and it is presumed final subjugation, of those rebellious ancient Solar Eclipses which have so much harassed astronomers. A brief account of M. Foucault's remarkable pendulum experiments, and of that beautiful instrument, the gyroscope, is introduced: as are also notices of Professor Thomson's speculations on the origin of the Sun's heat, and his estimate of its average expenditure, as well as of some curious views of M. Jean Reynaud, on the secular variation of our climates, supplementary to those put forward in former editions of this work. I could have wished that its nature and limits would have permitted some account of Mr. Cooper's magnificent contributions to sidereal astronomy, in his catalogue of upwards of 60,000 previously unregistered ecliptic stars; of Mr. Bishop's ecliptic charts and those of M. Chacornac; of Mr. Carrington's elaborate circumpolar catalogue; and of Mr. Jones's immense work on the zodiacal light, forming the third volume of the account of the

United States' Japan Expedition, which reached me too late to allow of drawing up a fitting analysis of his results. These gentlemen will severally please to accept, however, this respectful tribute of my admiration for their important labours. Some new speculations are also hazarded; as, for instance, on the subject of the Moon's habitability, the cause of the acceleration of Encke's comet, &c., and a few numerical errors are corrected which have hitherto escaped notice and public comment as blemishes,—as for example, in some of the numbers in Art. 422. in the explanation of the phænomena of a lunar eclipse, in the evaluation of the total mass of the atmosphere, Art. 242., and in the distance of the Moon, Art. 403. (for which, however, I am not answerable).

In the numerical statement of special astronomical elements, it is unavoidable that slightly different values of the same quantity should from time to time come to be substituted for those before received, as its determination acquires additional exactness. To have altered the figures in such cases wherever they occur, throughout the letter-press, would have entailed a great probability of error and confusion; and, as the Synoptic Tables of astronomical elements at the end of the work have been carefully revised in conformity with the best current authorities, the reader is requested, whenever he may observe any discrepancy of this nature, to prefer the tabulated values.

Several of the wood-cuts, which were originally drawn correctly, have been inverted right hand fo: left by the engraver. So far as explanation goes

xvi

PREFACE TO THE FIFTH EDITION.

this is not of the slightest moment. To a reader in the Southern Hemisphere, they are right as they stand; and one in the Northern has only to imagine himself so situated.

Collingwood, Feb. 17, 1858.

JOHN F. W. HERSCHEL.

NOTE (added in 1865.) The views of M. Reynaud, above referred to, though no doubt original on his part, are however completely anticipated by certain speculations of Sir Charles Lyell, in his "Principles of Geology," (p. 110, Ed. of 1830) expanded and further reasoned out to conclusions identical with those of M. Reynaud, in a paper by the author of the present work, "On the Astronomical Causes which may influence Geological Phenomena," read before the Geological Society on December 15th 1830, and published in the Transactions of that Society. This, however, had completely escaped his recollection when perusing the works of M. Reynaud referred to in the note to Art. 369. c, which occurring while the additions to the Fifth Edition were in preparation, with all the force of novelty, led to their insertion in Art. 369. b, and to the above notice of them in the preface.