10 MARCH 1877, Page 19

COMETS.*

Tuvalu are few subjects at once more interesting or more perplexing than cometary astronomy. The remarkable ap- pearance presented by many comets, their strange apparent movements on the heavens, and their scarcely less strange real movements in space, have long attracted the attention not only of students of science, but of all thinking minds. The explanations which science has been able to afford of some of the peculiarities presented by comets have certainly not tended to diminish the interest with which these bodies have been studied. Other peculiarities remain unexplained,—siay, as research extends they seem to grow less and less explicable. We have learned to trace the motions of comets in their elliptic or, in some cases, hyperbolic orbits (truly parabolic orbits can no more have any real existence than truly circular orbits), but how they came to be travelling in those paths we cannot understand. We trace a connection between comets and meteors, finding that meteors follow in the track of certain at least among the comets, but we have not yet learned how these trains of attendant bodies came to travel as they do, sometimes thousands of millions of miles behind their parent comet. We have deter- mined in some sort, by means of spectroscopic analysis, what the nuclei and heads and tails of comets are made of (not their ele- mentary structure, but whether they consist of gaseous or other matter), and yet we seem farther than ever from understanding how the head is formed around the nucleus, or how the tail is to all seeming driven away by some mighty repulsive force from the head.

M. Guillemin has been fortunate in his subject, for the mind of man is naturally drawn to the mysterious and the perplexing. In some respects, also, XL Guillemin's qualities as a writer suit him specially for the subject he has here chosen. He is admirable in description, weak in explanation, and in the case of comets, there is much more to be described than can possibly be explained.

The opening chapter, dealing with beliefs and superstitions relative to comets, is full of interesting matter. Two sections are devoted to the ideas of the ancients, two to those of the middle ages and of ignorant persons in more recent times. It will not be thought a serious fault in the last-named sections that a large amount of space is devoted to French ideas and stories about comets. It ifs, in fact, a relief for an English reader to turn to a book where the well-worn English stories about comets are scarcely mentioned, while others less known to us are related in fulL

In the second chapter, Guifismiu considers the history of cometic astronomy up to the time of Newton. The merely historical part of this chapter is excellent, but the last section, dealing with Newton's discovery of the true nature of cometary orbits, is less satisfactory. A brief study of the portions of the Principia relating to comets would have well repaid our author,

• The World of Cornets. By AmOdde Gninemin. Trembled by Tames Welsher. London : Sampson Low, Marston, and Co. 1871. and would have enabled him to make this section at once more complete and more interesting. The third chapter, relating to the motions and orbits of comets, rather describes what those motions and orbits are, than explains how astronomers have been able to prove that the cornea move as they do. In this, too, as in the following chapter, on periodical comets, we are struck by the omission of all attempt to give absolute measures of cometary motions at different distances from the sun, in orbits of different extent. There is no feature of cometary motion which is better worth indicating than the law according to which a. comet's -velocity at any given distance from the sun depends only on the mean distance of its orbit from the sun, that mean distance being in fact half the longer axis of the comet's path if the path is an ellipse. When the velocity equals or exceeds a certain definite value the law still bolds, but the path is no longer a closed one, and unless the comet while in our sun's neighbourhood is perturbed in such a way as to lose the excess of velocity, it will pass away, after circuiting round our sun, into the depths of interstellar space, returning to the region whence it came if its velocity at any given distance exactly equals -the critical value, but to a different region if the velocity exceeds -that value. The point here considered is not only interesting in itself, but is essential to the due consideration of various theories which have been advanced as to the origin of comets and of those meteoric systems which have been associated with comets. A careful consideration of the point would have saved M. Guillemin from much loose reasoning, and not a few erroneous statements, in a later chapter on the connection between comets and meteors. He had shown in his Le Ciel that he had bestowed little attention on the mathematics of the subject, or even on the simpler facts of the case. For example, he had actually described and illustrated by an elaborate figure a theory according to which the August meteors which salute our earth from the direction of the constellation Perseus, and the November meteors which radiate from the constellation Leo (neither, there- fore, travelling near the plane of the ecliptic), are both produced by the same nearly circular meteoric system, which, thus crossing our earth's orbit in two places, must lie in the plane of the ecliptic, near which (even) those meteors do not travel. This is much as though a meteorologist should suggest that rain in England and snow in North America had both fallen from the same cloud over the Brazils. No blunder so stupendous as this char- acterises his explanation of cometic and meteoric astronomy in -the present volume. Yet it is but too clear that in the interval between writing his Le Ciel and his Les Cometes Guillemin has not been able to master the elements of celestial dynamics. For example, he repeats Schiaparelli's statement that an endless -stream of meteors may be converted into a closed elliptical ring by planetary perturbations—a statement altogether incorrect, if the entire original ring is referred to, and which requires care- ful explanation and limitation, if only a minute part of the -original ring is meant—and he simply leaves the matter there. "We shall not," says he, "enter more particularly into the de- tails of this remarkable theory, but shall confine ourselves to pointing out the analogy existing between the nebulous currents which give birth to meteoric swarms and the nebulosities of the comets." The real fact is, as Mr. Glaisher points out in a useful chapter added to this part of the work, that all that Schiaparelli ean be said to have established is the existence of a close con- nection between comets and meteors. Schiaparelli's ideas re- specting the origin of cometic and meteoric systems, or rather respecting their introduction into the solar system, are not only undemonstrated, but are unquestionably inconsistent with the laws of dynamics.

A large portion of the work before us is devoted to various theories which have been formed respecting comets' tails. In this part of cometary astronomy science has hitherto been utterly defeated. A vast array of facts have been gathered together, but it has been found impossible to marshal them effectively. The theory that the matter forming the tails of comets has been re- pelled from the head by some solar force is fall of difficulties, and labours under this serious disadvantage, that it requires us to admit the existence of a physical force of whose action we have as yet had no direct experience. Yet no other theory seems competent to deal with the observed facts. In particular, as Professor Norton, of Yale College, New England, was the first, we believe, to point out, the thwart striations of the tail of Donati's comet of the year 1858 appear utterly inexplic- able on any other hypothesis. In passing, we must note that, by an unfortunate omission, this feature—the most re- markable and significant cometic phenomenon yet observed—is

not noticed in the present work, either by the author or the translator. In considering Tyndall's theory of the formation of comets' tails by the sun's chemical rays, Guillemin mistakenly asserts that the theory requires the head of the comet to be ac- companied by an invisible sphere of vapour equal in diameter to the length of the comet's tail. Tyndall, however, supposes the visible actinic cloud to be formed in exceedingly tenuous vapour already in position where the tail is seen, not carried along with the comet. A great deal too much space in the present work is given to the ideas of MM. Roche and Faye, which are of little scientific value.

When it was announced that Guillemin's Les Contetes was to be translated by Mr. Glaisher, it was hoped that to the interest of the original work as a history of comets would be added the value which an explanation by Mr. Glaisher of the many interest- ing mathematical and physical relations involved in cometary astronomy might be expected to possess. It is with some re- gret that we find Mr. Glaisher has not attempted this task, ex- cept in relation to a few minor details. Ample room could have been found for ouch additional matter, by the removal of several useless sections of the original work. For in- stance, besides large portions relating to M. Roche's hypo- theses, the three sections of the last chapter might have been either greatly abridged or (better) omitted entirely. In the present position of cometary astronomy no interest attaches to such questions as, "Are comets habitable ?" (chap. xv., sect. 1) ; "What would become of the earth if a comet were to make it its satellite ?" (sect. 2) ; and, "Is the moon an ancient comet 2" (sect. 3). The additions which Mr. Glaisher has made are in the main useful, but not always so. Thus, Professor Tait's theory of the constitution of comets might have been omitted, seeing that as yet Professor Tait has not condescended to adopt the course which is usual when a new theory is advanced,—viz., to show that the theory in some degree accords with observed facts. When the history of a single comet has been shown to be even barely reconcilable with Professor Tait's theory, it will be time for astronomers to discuss the theory. At present, it is a mere mathematical plaything.

As a translation, the present work, is on the whole, satisfactory. Occasionally French idioms and expressions have been too literally translated. For instance, an English writer would scarcely speak of circumstances "rendering impossible the recognition" of a comet. (p. 54.) But in translating a large book like the present, it is almost impossible to avoid mistakes of this sort.