25 JANUARY 1851, Page 15

DR. NICHOL'S PLANETARY SYSTEM. * DR. Mellor, may take the same

place in astronomical as South- wood Smith and the late Andrew Combe in medical science. If he has not himself advanced astronomy, he has extended its know- ledge and its study, not merely by rendering the great but very often scattered discoveries of modern astronomers accessible and intelligible, but by the manner in which he popularized the science. So-called " popular " compendiums were rife enough be- fore him, but they were of a dry school-book sort ; the elements were presented, and perhaps plainly, but after the plainness of a " rule " in grammar or arithmetic. Dr. Nichol was the first who brought to astronomical compilation the knowledge of an astro- nomer and the spirit of philosophy, united to breadth of view and warmth of manner. Ile not only elevated compilation himself, but set an example to others. Since the appearance of The Architec- ture of the Heaven.; greater largeness, -vigour, and life, seem to have been infused into most books that treat of any section of cosmogony.

The present volume is designed as an introduction to astro-

• The Planetary System ; its Order and Physical Structure. By J. T. Nichol, LL.D., Professor of Astronomy in the University of Glasgow, Author of "The Architecture of the Heavens," &c. Published by Hailliere. nomy, for • "that numerous class who cannot boast of acqui- sitions in mathematics," and is preliminary to a more profound and elaborate work which Dr. Nichol is preparing. In strictness, the theme is confined to a description of the solar system, and an investigation of the principles which sustain and rule it. But Dr. Nichol is not the man to confine himself too narrowly to his subject. The probable formation of the system enables him to enter, not so much upon the nebular hypothesis, as the demonstrative specu- lations of La Place with regard to the formation of the planetary system ; assuming the existence of a central body like the sun, in- stead of attempting to show with the elder Herschel how it was formed. The appearance of comets in our system takes the reader "beyond the visible diurnal sphere," into space. The powerful telescopes of modern times enable Dr. Nichol to describe the val- lies and mountains of the moon, to speculate upon the nature and constitution of its matter or groundwork, since we must not say earth, and even to pierce through the atmosphere of the sun to the luminary itself. The general subject of the book has been frequently handled in a jogtrot conventional manner by various compilers, as well as by Dr. Nichol in his Phanomena of the Solar 'System. Novelty is given to the matter of the present work by the introduction of the cognate subjects we have already mentioned, and by em- bracing the latest discoveries in astronomy. He imparts inter- est by his genial feeling, his eloquent if somewhat diffuse style, and by the warmth of his devotional reflections. The plan and treatment of the subject, however, is the great source of va- riety and freshness. Dr. Nichol embraces the history of astro- nomical discovery, and notices its great discoverers in connexion with the account of the planetary system. This method not only presents the student with an outline of the history of the science, bat, as the discoveries were made in the order of the palpable, he is enabled to present the simplest to the reader first, and to impress principles firmly in the mind, by noting the early errors and the causes of them. In the same way, he is able to combine a human interest with the exposition of an abstract science. The fact that the earth moves in its orbit round the sun, that the sun itself turns upon its axis, and similar commonplace truths of astronomy, possess a living interest -when they are associated with the long prevalence of erroneous notions in the world, the opposition Co- pernicus encountered, and the persecutions to which Gallileo was subjected. In later times and less priestridden countries, per- secution was not to be feared ; but a biographical interest still con- tinues, and one beyond a merely scientific attraction rises up with the law of progression. It is interesting to see how each discovery is imperfect, or leaves a difficulty which future inquiries have to solve, down even to the last planet Neptune : its discovery was facilitated and in part suggested by Bode's law, but the planet proved on discovery to be an exception to the rule.

We have mentioned how the history of error is made use of to explain or enforce truth. The natural difficulty experienced by the ancients in discovering the actual movement of our system may be taken as an example.

"The obstacles in the way of our discovering the actual character of the planetary system are twofold : they arise from the position and circum- stances within which we view it ; and both act in the same way, viz, they withdraw the truth from simple or direct inspection, and reduce us to the necessity of inferring or deducing it from what is only apparent. In the first place, we look at the system as an external object sideways. If from some point far aloft—the eye in a position similar to that from which one examines the diagram of Plate I.—we descried the planets rolling below as on a great plain, no doubt regarding their motions could possibly remain, as each would be seen steadfastly describing its allotted circle : but the oppor- tunity of thus discerning the phenomena we would disentangle is evidently denied us. In consequence of our residing on the earth, we are not above that system, but in the midst of it ; and, because of our system's special structure, we are limited to a view like the following—Suppose a ball set in motion so as to revolve in a circle on the surface of a table ; it actual mo- tion, with all its circumstances, would be seen directly by an eye raised above the table and looking down on it; but if the eye were only at the height of the table, and therefore constrained to look on the rolling globe sideways, the circular motion would not be a direct perception but an inference. The visible fact would simply be, that the ball passes alternately by some path to the right and left of the eye; but what the path is, whether merely a straight line through which the ball passes backwards and forwards, or a curve line, circular or otherwise, this could not be ascertained, unless after observation much more minute and prolonged than that which revealed mere alternations in the object's apparent place. But there is a second cha- racteristic of our position as observers, which renders the task of evolving the true planetary paths more difficult still. We do not look at these moving bodies from a position of rest; and, as the motion of the earth is not felt or originally understood, we are inclined to suppose that all apparent changes of place in external bodies spring from their actual motions ; so that we are obliged almost to mix up with what is real a large amount of what is un- real, viz, all those alterations in the relative positions of our globe and the other planets which are caused by our own positive but unfelt revolutions. Notice a body of cavalry on a field-day imitating a skirmish : it would be difficult for even an attentive spectator to follow and record the complex ca- reers of these horsemen; but how different the condition of any actual rider in the imaginary affray !—how unlikely that, by the utmost stretch of atten- tion, he could discern so as to narrate, when the scene terminated, the exact conduct of the nearest of his comrade', the directions and distances of his various courses. And with regard to the planets, such a process of discrimi- nation is rendered yet more hopeless, by the fact that there are no fixed neighbouring bodies by comparison with whose known stability we might infer our own changes or measure those of our companions. The stars are fixed, indeed, in relation at least to our evolutions ; but they are so remote that the motion of the most distant planet—the one whose orbit has the widest range—could not be detected through effect of any apparent change thereby impressed on their positions; nay, the entire sphere of the planetary orbits, seen from the nearest of these luminaries., could not be discriminated without great difficulty from that users point which marks the position of the BUD. I am not sure, indeed, that in the whole history of science there is aught more instructive than the chapter recording the mode by which these serious hinderances were withdrawn from our way to a right under- standing of the structure of the heavens. In the -first place, we have the surprismg,efforts of the Greeks, artificial undoubtedly, and, like other phy- sical speculations of that remarkable people, chiefly distinguished for their logical ingenuity; but pregnant with interest as a milestone in the progress of humanity, and the ground also of all succeeding achievements."