ANCIENT PHYSICS.
Though the phenomena of the material world could not but early excite the curiosity of a being who, hie man, receives his strongest impressions from without, yet an accurate knowledge of those phenomena, and their laws, was not to be speedily acquired. The mere extent and variety of the objects were, indeed, such obstacles to that acquisition, as could not be surmounted but in the course of many ages. Man could hot at fink perceive from whst point he mist begin his inquiries, in what direction he must-carry them on, or by what rules he mast be guided. He was like a traveller going forth to explore a vabt and unknown wilderness, in which a multitude of great and interesting objects. presented themselves on .every side, while there was no path .for 'him to follow, bo rule to direct 'his survey, and where the art of observing, and the instruments of observation, must -equally be the work of his own invention. In these circumstances, the selection of the 'objects to be studied was the effect of instinct rather than of reason, or of the passiOns and enio dons, more than Of the understanding. When things new and unlike &rise whieh.occdr red in the court of every day's .experience presented. they •excitied Wonder or sarprise, and .created an anxiety to discover some )principle which might connect them with the appearances commonly .observed. About thee last, mernielt no desire to 'be ther informed ; but when the common order of things was violated, and something new or singular was produced, they began to examine into the fact, and attempted to inquire into the cause. Nobody sought to know why a stone tell to the ground, why smoke ascended, or why the stars revolved round the earth. But if a fiery meteor shot across the heavens, if the flames of a volcano burst forth,—or if ad earthquake shook the foundations of the world, terror and curiosity were both awakened ; and when the former emotion had sub sided, the latter was sure to become active. Thus, to trace a resemblance between the events with which the observer was most familiar, and those to which he was less accustom ed, and which had excited his wonder, was the first object of inquiry, and produced the first' advances towards generalization and philosophy.
This principle, which it were easy to trace, from tribes the most rude and barbarous, to nations the most highly refined, was what yielded the first attempts toward classification and arrangement, and enabled man, out of individuals, subject to perpetual change, to form certain fixed and permanent objects of knowledge,—the species, genera, orders, and classes, into which he has distributed these individuals. By this effort of mental abstraction, he has created to himself a new and intellectual world, free from those changes and vicissi tudes to which all material things are destined. This, too, is a work not peculiar to the philosopher, but, in a certain degree, is performed by every man who compares one thing with another, and who employs the terms of ordinary language.
Another great branch of knowledge is occupied, not about the mere arrangement and classification of objects, but about events or changes, the laws which those changes observe, and the causes by which they are produced. In a science, which treated of events and of change, the nature and properties of motion came of course to be studied, and the ancient philosophers naturally enough began their inquiries with the definition of motion, or the determination of that in which it consists. Aristotle's definition is highly characteristical of the vagueness and obscurity of his physical speculations. He calls motion " the act of a being in power, as far as in power,"—words to which it is impossible that any distinct idea can ever have been annexed.
The truth is, however, that the best definition of motion can be of very little service in physics. Epicurus defined it to be the " change of place," which is, no doubt, the sim plest and best definition that can be given ; but it must, at the same time, be confessed, that neither he nor the moderns who have retained his definition, have derived the least advantage from it in their subsequent researches. The properties, or, as they are called, the laws of motion, cannot be derived from mere definition ; they must be sought for in ex.