which the mind of the philosopher employs itself;the history of those branches of knowledge for the last three hundred years abundantly teaches us. Accordingly, the existence of clear ideas applied to distinct facts will be discernible in the History of Science, whenever any marked advance takes place. And, in tracing the progress of the various provinces of knowledge which come under our survey, it will be important for us to see, that, at all such epochs, such a combination has occurred; that whenever any material step in general knowledge has been made, whenever any philosophical discovery arrests our attention;-some man or men come before us, who have possessed, in an eminent degree, a clearness of the ideas which belong to the subject in question, and who have applied such ideas in a vigorous and distinct manner to ascertained facts and exact observations. We shall never proceed through any considerable range of our narrative, without having occasion to remind the reader of this reflection. Successive steps in Science.-But there is another remark which we must also make. Such sciences as we have here to do with, are, commonly, not formed by one single act;-they are not completed by the discovery of one great principle. On the contrary, they consist in a long-continued advance; a series of changes; a repeated progress from one principle to another, different and often apparently contradictory. Now, it is important to remember that this contradiction is apparent only. The principles which constituted the triumph of the preceding stages of the science, may appear to be subverted and ejected by the later discoveries, but in fact they are, (so far as they were true,) taken up into the subsequent doctrines and included in them. They continue to be an essential part of the science. The earlier truths are not expelled but absorbed, not contradicted but extended; and the history of each science, which may thus appear like a succession of revolutions, is, in reality, a series of developements. In the intellectual, as in the material world, Nec manet ut fuerat nec formas servat easdem, Sed tamen ipsa eadem est. All changes, nought is lost; the forms are changed, Yet that which has been is. Nothing which was done is useless or unessential, though it ceases to be conspicuous and primary. Thus the final form of each science contains the substance of each of its preceding modifications; and all that was at any antecedent period discovered and established, ministers to the ultimate developement of its proper branch of knowledge. Such previous doctrines may require to be made precise and definite, to have their superfluous and arbitrary portions expunged, to be expressed in new language, to be taken up into the body of science by various processes; but they do not on such accounts cease to be true doctrines, or to form a portion of the essential constituents of our knowledge. Terms record Discoveries.-The modes in which the earlier truths of science are preserved in its later forms, are indeed various. From being asserted at first as strange discoveries, such truths come at last to be implied as almost self-evident axioms. They are recorded by some familiar maxim, or perhaps by some new word or phrase, which forms part of the current language of the philosophical world; and thus asserts a principle, while it appears merely to indicate a transient notion;-preserves as well as expresses a truth; —and, like a medal of gold, is a treasure as well as a token. We shall frequently have to notice the manner in which great discoveries thus stamp their impress upon the terms of a science; and, like great political revolutions, are recorded by the change of the current coin which has accompanied them. Generalization.The great changes which thus take place in the history of science, the revolutions of the intellectual world, have, as a usual and leading character, this, that they are steps of generalization;-transitions from particular truths to others of a wider extent, in which the former are included. This progress of knowledge, from individual facts to universal laws, from particular propositions to general ones, and from these to others still more general, with reference to which the former generalizations are particular, is so far` familiar to men's - minds, that without here entering into further explanation, its nature will be understood sufficiently to prepare the reader to recognise the exemplifications of such a process, which he will find at every step of our advance. Inductive Epochs; Preludes; Sequels.—In our history, it is the progress of knowledge only which we have to attend to. This is the main action of our drama; and all the events which do not bear upon this, though they may relate to the cultivation and the cultivators of philosophy, are not a necessary part of our theme. Our narrative will therefore consist mainly of successive steps of generalization, such as have just been mentioned. But among these, we shall find some of eminent and decisive importance, which have more peculiarly influenced the fortunes of physical philosophy, and to which we may consider the rest as subordinate and auxiliary. These primary movements, when the Inductive process, by which science is formed, has been exercised in a more energetic and powerful manner, may be distinguished as the Inductive Epochs of scientific history; and they deserve our more express and pointed notice. They are, for the most part, marked by the great discoveries and the great philosophical names which all civilized nations have agreed in admiring. But, when we examine more clearly the history of such discoveries, we find that these epochs have not occurred suddenly and without preparation. They have been preceded by a period, which we may call their Prelude, during which the ideas and facts on which they turned were called into action;-were gradually evolved into clearness and connexion, permanency and certainty; till at last the discovery which marks the epoch, seized and fixed for ever the truth which had till then been obscurely and doubtfully discerned. And again, when this step has been made by the principal discoverers, there may generally be observed another period, which we may call the Sequel of the epoch, during which the discovery has acquired a more perfect certainty and a more complete developement among the leaders of the advance; has been diffused to the wider throng of the secondary cultivators of such knowledge, and traced into its distant consequences. This is a work, always of time and labour, often of difficulty and conflict. To distribute the history of science into such epochs, with their preludes and sequels, if successfully attempted, must needs make the series and connexion of its occurrences more distinct and intelligible. Such periods form resting-places, where we pause till the dust of the confused march is laid, and the prospect of the path is clear. Inductive Charts.-Since the advance of science consists in collecting by induction general laws from particular facts, and in combining several laws into one higher generalization, in which they still retain their former truth, we might form a Chart, or Table, of the progress of each science, by setting down the |