arrival at the ground, was regularly discharged with sparks, and the analogy converted into an identity.

(2.) Every one had been accustomed for ages, before the time of Newton, to observe, that bodies fall to the ground as soon as support is withdrawn. They were equally familiar with the fact, that the moon circulates periodically about the earth. But no one ever perceived any relation or imaginable connexion between these two classes of facts. Nay, the peripatetics, maintaining that the heavenly motions' were of an essentially different kind from the terrestrial, led men to the belief that these two cases could not possibly have any common relation.

cause.

The penetrating mind of Newton, however, instantly perceived a connexion between them. He considered that a body launched into space would continue to move off in a straight line, unless made to deviate from that path by the action of some other Now the moon does not go off in a rectilinear path, but has her course continually bent from such direction into a curvilinear orbit round the earth; and the degree in which it is thus bent, or the amount of deviation from the straight course, is in fact, so much of a real fall towards the earth: the moon is actually falling like a stone: and the amount of its fall can be measured; since astronomical observation has given the size and form of its orbit, and the rapidity of its motion, that is, the amount of the deviation. Also the amount of the fall of a stone near the earth's surface

is known. It becomes a matter of calculation to compare them. Newton made the comparison, and found the two effects precisely in the inverse proportion of the squares of the distances from the earth's centre. This was the precise proportion which would agree with the supposition of that law of central force, which, on abstract mechanical principles, ought to give rise to elliptic orbits, and to certain relations expressed by numerical laws between the magnitudes of those orbits and the motions in them. These were the very same as those numerical relations which had been found by Kepler long before to subsist in the planetary revolutions.

Thus the single circumstance of the analogy between the moon's motion and that of a stone falling to the ground, sufficed as a clue to the whole system of planetary motions, and the establishment of the principle of universal gravitation.

(3.) Physical philosophers had been long seeking to establish (what there was every reason to suspect) the existence of at least a close connexion, if not absolute identity, between electricity, galvanism, and magnetism. There were many points of resemblance in what was known of the nature of those agents; experiments had been multiplied, and many curious facts and results had been accumulated. But all this collection of facts had not afforded a real induction. And the reason was, that the inquirers had been guided either by no principle of analogy, or by such as was incorrect.

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 end.

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,