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The most powerful electric forces had been resorted to; but no evolution of galvanic influence, no shock, however strong, would affect the magnetic needle. Experimenters were accustomed to witness the most intense electric action when the current was broken, or the accumulated power discharged; here, therefore, they expected to find the greatest effect of a magnetic kind.
But these modes of action were of a kind offering no analogy to those really concerned in the cases in question. This, however, was not perceived, till Ersted discovered the true point of connexion of electricity and magnetism. He succeeded, by a very slight change in the arrangement from that with which his predecessors had been so long and so fruitlessly working. By using an unbroken galvanic circuit, he instantly found an influence on the magnetic needle: not by violent concentration of forces but by a peculiar diffusion of them. And the whole system of action by transverse currents was almost immediately developed and followed out into all its correlative trains of consequences.
(4.) Newton published his Principia before any instance of the periodical return of a comet had been established, or even imagined. Yet, on comparing the masses of these bodies and their distances with those of the planets, he caught an analogy, and did not hesitate to speak positively of their describing orbits about the sun, and to recommend to future astronomers to verify their returns by comparison of
observations *. It is superfluous to notice how completely this idea has been borne out by subsequent discoveries.
(5.) In the extension of the law of gravitation from the fall of a stone on the earth to the motions of the most distant planet or the most erratic comet, we have a remarkable instance where a conclusion is made from effects which we observe near us, to those of the same kind which are produced in the remotest regions of space. Let us compare this with a parallel case in time. We observe the daily formation of rounded pebbles by the action of the waves on fragments of rock on the sea-shore; and we find the incessant continuance of that action for a long time give rise to accumulated beds of shingle.
Now, over large tracts of land, at considerable elevations above the sea, we find immense beds of pebbles presenting precisely the same appearance of rolled and rounded fragments, as those we now observe in the progress of formation in the sea. It is, then, by the same process of reasoning which connects the gravitation of a stone with that of the moon, or the remotest planet or comet, that we connect the formation of beds of pebbles at the present day with that of similar beds in ages of remote antiquity, when the present dry land formed the bottom of the ocean; or, rather, was gradually emerging from it, through such a long succession
* See Principia, lib. iii. prop. 39, corr. 3, and prop. 41 at the
of ages as would alone suffice for the production of the immense beds of rolled gravel which we find deposited over a large part of the surface of the globe.
Force of Physical Analogies.
PHILOSOPHICAL induction, then, proceeds mainly by seizing upon analogies between known orders of facts, known relations of cause and effect, and cases where the existence of such relations is unknown, but where the circumstances render it probable that they also subsist. Such circumstances, perhaps quite casual and unimportant in the eyes of the ordinary observer, suggest in an instant, to the practised mind of the philosophical inquirer, a train of relations in which the analogy is maintained. He proceeds to verify his idea: a single experimental instance often suffices to confirm it; and at most, a very few repetitions and variations in the circumstances and conditions, satisfy him that his analogy is correct, and the uniformity of the law by which physical action. is determined becomes established.
In fact, so essential to induction is the dependence on analogy, that in the very use of the terms, "observation," "experience," and the like, by many writers, to describe the grounds of our belief in physical events, it is evident that they mean to include essentially the reference to analogy, and not barely to facts actually witnessed. Unless this be the case,
indeed, their meaning would, in some cases, be involved in absurdity and contradiction.
Thus, then, the most important part of the process of induction consists in seizing upon the probable connecting relation by which we can extend what we observe in a few cases to all. In proportion to the justness of this assumption, and the correctness of our judgment in tracing and adopting it, will the induction be successful. The methods by which a facility in discovering such relations, and a readiness in forming such judgment, may be attained and improved, are precisely the objects principally to be kept in view by the philosophical student who would prepare himself for the work of interpreting the phenomena of the natural world. The analogies to be pursued must be those suggested from alreadyascertained laws and relations. This, in proportion to the extent of the inquirer's previous knowledge of such relations subsisting in other parts of nature, will be his means of guidance to a correct train of inference in that before him.
And he who has, even to a limited extent, been led to observe the connexion between one class of physical truths and another, will almost unconsciously acquire a tendency to perceive such relations among the facts continually presented to him. The truth of the remark to which we have been thus led is amply confirmed by the history of philosophical discovery.
In point of fact, discoveries, commonly termed
inductive, have very seldom been really attained by the mere process of amassing collections of individual facts. It has been almost invariably the case that hypothesis has preceded observation; and that the discoverer has in truth only verified, by an appeal to experiment, the general theory which he had already imagined. The happy selection of such hypotheses is that which characterizes, and in fact constitutes, philosophical genius. And a just appreciation of the use of such imaginary provisional assumptions, eminently distinguishes the rational inquirer from the speculative visionary. The true philosopher neither discards hypothesis on the one hand, nor yields himself up to it on the other; but rates it at its proper value, and turns it to its legitimate use. He is always ready to reject an assumed theory the moment he finds it unsupported by fact: but if it be once duly substantiated, to adopt it, and be prepared to follow it out into all its legitimate consequences, however at variance with received notions-however contrary to established prejudices-however opposed to the prepossessions, the bigotry, the cherished delusions of mankind.
And the more extensive his acquaintance with nature, the more firmly is he impressed with the belief that some such relation must subsist in all cases, however limited a portion of it he may be able actually to trace. And it is by the exercise of an unusual skill in this way, that the greatest philosophers have been able to achieve their triumphs in