which retains that planet in its orbit, and keeps it revolving as it does around our own earth? It was a splendid conjecture, and we may be sure that Newton instantly set all his sagacity at work to verify it. If the moon, he considered, be retained in her orbit by a gravitation towards the earth, it is in the highest degree probable that the earth itself, and the other planets which revolve around the sun, are, in like manner, retained in their orbits by a similar tendency towards their central and ruling luminary. Proceeding then, in the meantime, upon this supposition, he found by calculation, and by comparing the periods of the several planets and their distances from the sun, that, if they were really held in their courses by the power of gravity, that power must decrease in a certain proportion, according to the distance of the body upon which it operated. This result he had already anticipated from the consideration that, although we could not detect any such diminution within the comparatively small distance to which our experience was limited, the fact was yet consistent with the whole analogy of nature. Supposing, then, this power, when extended to the moon, to decrease at the same rate at which it appeared to do in regard to the planets which revolved around the sun, he next set himself to calculate whether its force, at such a distance from the earth, would in reality be sufficient to retain that satellite in its orbit, and to account for its known rate of motion. This step of the discovery was marked by a very singular circumstance, and one strikingly illustrative of the truly philosophic character of this great man's mind. In the computations which he undertook for the purpose of his investigation, he naturally adopted the common estimate of the magnitude of the earth, which was at that time in use among our geographers and seamen. Indeed, no other yet existed for him to adopt but it was even then known to scientific men to be loose and inaccurate. It allowed only sixty English miles to a degree of latitude, instead of sixty-nine and a half, which is the true measurement. The consequence was, that the calculation did not answer; it indicated, in fact, a force of gravity in the moon towards the earth, less by one-sixth than that which was necessary to give the rate of motion actually possessed by that satellite. Another might have thought this but a trifling discrepancy, and, in such circumstances, might have taxed his ingenuity to account for it in a variety of ways, so as still to save the beautiful and magnificent theory which it came so unseasonably to demolish. But Newton was too true a philosopher, too single-hearted a lover of truth, for this. In his mind the refutation was a complete one, and it was admitted as such at once. He had made his calculation with care, although one of its elements was false; it did not present the result it ought to have done, had his hypothesis been as true as it was brilliant ; and, in his own estimation, he was no longer the discoverer of the secret mechanism of the heavens. By an act of self-denial, more heroic than

any other recorded in the annals of intellectual pursuit, he dismissed the whole speculation from his mind, even for years. We need hardly state how gloriously this sacrifice was in due time rewarded. Had Newton, instead of acting as he did, obstinately persevered in the partially erroneous path into which he had thus been misled, it is impossible to say into how many additional misconceptions and mis-statements he might have been seduced, in order to cover the consequences of his first blunder or how much the simplicity of the grand truth which had revealed itself to him, as it were, for a moment in the distance, might have been eventually complicated and disfigured by the vain imaginations of the very mind which had discovered it. The progress of science would, no doubt, at last have swept away all these useless and encumbering fictions; but that honour would, probably, have been reserved for another than Newton. Committed to the maintenance of his adopted errors, and with his mental vision even unfitted in some measure for the perception of the truth, he might in that case have been the last to take in the full brightness of the day, the breaking of which he had been the first to descry. But by keeping his mind unbiassed, he was eventually enabled to verify all, and more than all, that he had originally suspected. No other speculator had yet followed him in the same path of conjecture; when, a few years after, upon obtaining more correct data, he repeated his calculation, and found it terminate in the very result he had formerly anticipated. And what a moment of triumph and inconceivable delight must that have been, when he saw at last that the mighty discovery was indeed all his own! It is said, that such was his agitation as he proceeded, and perceived every figure bringing him nearer to the object of his hopes, that he was at last actually unable to continue the operation, and was obliged to request a friend to conclude it for him.

Another very beautiful example of the way in which some of the most valuable truths of philosophy have been suggested, for the first time, by the simplest incidents of common life, is afforded by GALILEO'S discovery of what is called the isochronism of the pendulum, or the equality of its oscillations in point of time. It was while standing one day in the metropolitan church at Pisa, that his attention was first awakened to this most important fact, by observing the movements of a lamp suspended from the ceiling, which some accident had disturbed and caused to vibrate. Now this, or something of the same kind, was a phenomenon which, of course, had been observed many, thousands of times before. But yet nobody had ever viewed it with the philosophic attention with which it was on this occasion examined by Galileo. Or if, as possibly was the case, any one had been half unconsciously struck for a moment by that apparent equability of motion which arrested so forcibly the curiosity of Galileo, the idea had been allowed to escape

as soon as it had been caught, as relating to a matter not worth a second thought. The young philosopher of Italy (for he had not then reached his twentieth year) saw at once the important applications which might be made of the thought that had suggested itself to him. He took care, therefore, to ascertain immediately the truth of his conjecture by careful and repeated experiment; and the result was the complete discovery of the principle of the most perfect measure of time which we yet posHow striking a lesson is this for us when we discover, or think we discover, any fact in the economy of nature which we have reason

sess.

to believe has not previously been observed! Let it be at least verified and recorded. No truth is altogether barren; and even that which looks at first sight the simplest and most trivial may turn out fruitful in precious results.

It seems, after it is stated and described to us, to have been an exceedingly obvious thought which struck Galileo, when, after having

ascertained the regular oscillation of the pendulum, he proposed employing it as a measure of time. Some, indeed, may imagine that there was no such extraordinary merit as is generally supposed, even in the grand conjecture of Newton, and that it amounted, after all, merely to the application of a law to the movements of the heavenly bodies, which was already known to affect at least every body in the immediate neighbourhood of the earth. But these things are only simple after they are explained. Slight and transparent as we may think the veil to have been which covered the truths alluded to, and others of a similar nature, immediately before they were detected, it is yet an unquestionable fact that this veil had been sufficient to conceal them, for thousands of years, from the observations of all the world. The phenomenon of a heavy body swinging to and fro from a point of suspension, had been familiar to every generation from the very earliest times; and yet, although men had long been very desirous of possessing an accurate and convenient measure of time, and had resorted in different countries to a great variety of contrivances to attain that object, nobody before Galileo had thought of effecting it by means of the pendulum. And, in the same manner, with regard to the law of gravitation: the fact of bodies generally having a tendency to fall to the earth, must of course have forced itself upon the attention of the very earliest inhabitants of our globe, every day and hour of their existence. But yet Newton's application and extension of this law had occurred to nobody, not even to Galileo himself, who had not many years before been engaged in investigating the exact amount of its influence within the field in which alone it had hitherto been supposed to operate. And Newton not only applied the law of gravitation to the heavenly bodies; but, as the principle, when affecting bodies in the neighbourhood of the earth, was that of a force apparently constant, he had to discover and demonstrate the law of its variation.1

But, perhaps, the most striking illustration we can give of the strange manner in which important truths will sometimes hide themselves for a long while from observation, even after science has approached almost so near as to touch them, is to be found in the history of the different discoveries relating to the mechanical properties of the air. The know

"

1 "The assumption of an attractive force emanating from the sun,' says the writer of the article on Newton in the Penny Cyclopædia, "was at this time far from being a novelty; and it had even been asserted by Bouillaud [in his Astronomio Philolaica, published in 1645] that, if such a force really existed, its intensity would vary inversely as the square of the distance from the attracting body; but neither Bouillaud nor those who entertained similar opinions had given any proof, either empirically or otherwise, of what they had asserted; and

certainly none appear to have attempted to establish that the forces which retain the planets in their orbs were identical, as to their nature, with that which draws a stone, when let fall, to the surface of the earth. Newton showed that the law of the inverse square of the distance is that which really exists in nature; and, further, that this law was a necessary consequence of the analogy already discovered by Kepler between the periodic times and the mean distances of the planets."

ledge of its positive weight, or gravity, is as old as the days of Aristotle. Even its elasticity was well known to the ancients; one of whose philosophers, HERO of Alexandria, had, about a century before the birth of Christ, constructed upon that principle the fountain, or jet d'eau, which still goes by his name. The common suction-pump is a still older invention, the effect of which, depending, as it does, entirely on the pressure of the atmosphere, might have suggested the true philosophy of that subject, it may be thought, to some one of its innumerable observers. But, in reality, although all the while the air was known to be really a heavy body, nobody for two thousand years found out the true reason why, on its removal from the barrel of the pump by the elevation of the piston, the water rose into the vacant space. The unlearned multitude attributed the phenomenon to a suction, or power of draught, in the pump, and gave it the name of the suction-pump accordingly. They saw a phenomenon which they did not understand, and they called its cause, of which they were ignorant, suction. But the theory of the philosophers was more irrational than that of the multitude: only that, professing to rest upon one of the great laws of nature, it looked somewhat more solemn and imposing. The water rises in the pump, it was said, upon the removal of the air, because Nature abhors a vacuum; and thus the matter rested, as we have said, for nearly twenty centuries—the alleged abhorrence of nature for a vacuum never having been established, either by experiment or reasoning, or in any other way, but at the same time being always so gravely propounded as a universal truth, that it never was questioned by anybody. Let us not, however, deride with too much levity these errors and follies of the old interpreters of Nature. We ourselves are only yet casting off the yoke of that ignorance, in the guise of wisdom, under which the men of other times were wont so submissively to bow; and, if not in physics, at least in other departments of knowledge, we are still too much given to accept mere words and phrases, in the place of philosophy. At least let what we are now to relate restrain a little the expression of our contempt for the philosophy of the schoolmen, as to the present matter, and our exultation in a superiority over them which we do not owe to ourselves.

The illustrious Galileo himself, unquestionably one of the greatest men that ever lived, even after advancing to the very confines of all we now know, stopped there, and could find nothing better to offer than the old solution of the difficulty, in a case attended with circumstances which to us would seem to have made the necessity for abandoning it obvious. A pump of more than thirty-two feet in height having chanced to be erected at Florence, while Galileo resided in that city, the philosopher, finding that the water would not rise as usual to its top, set himself immediately to endeavour to account for the unexpected