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all the generality in which Newton found it, had been promulgated even by the philosophy of ancient Rome and Greece. But yet Newton's application and extension of it 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. 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.
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 knowledge 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 sucking, 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 racuum; 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 any body. 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 phenomenon; and, after examining the case, came to the conclusion, that nature certainly abhorred a vacuum, but for the first two-andthirty feet only! It was his pupil TORRICELLI who first demonstrated the true cause of the phenomenon, by a most happily imagined experiment. The water, rising, as it does, only to a certain height, must, in fact, he remarked, be, not drawn, but pushed up into the barrel of the pump; and it can only be so pushed by the pressure of the atmosphere on the exposed portion of it. The thirty-two feet of water in the body of the pump are merely a counterbalance to a column of air of equal basis, reaching to the top of the atmosphere. But if so, it then oc-. curred to him, that another liquid, heavier or lighter than water, will, in similar circumstances, ascend to a correspondingly less or greater height, a less or greater quantity of it being, of course, required to balance the atmospheric column. Mercury, for example, is about thirteen times and a half as heavy as water; it ought to mount, therefore, only to the height of about twenty-eight inches, instead of thirtytwo feet. So, taking a glass tube of about three feet in length, and hermetically sealed (that is, made air-tight) at one end, he first filled it completely with mercury, and then closing it with his finger, reversed it, and plunged it into a basin of the same liquid metal; when, withdrawing his finger, he had the gratification of seeing the liquid in the tube, now forming one body with that in the basin, descend, until, exactly as he had anticipated, there remained suspended a column of twenty-eight inches
TORRICELLI.. PASCAL PRINCE RUPERT.
only. Now, by this experiment, in every way a most ingenious and beautiful one, Torricelli had in reality invented the instrument we now call the Barometer; and yet, strange to say, it was left to another to discover that he had done so. It was the great Pascal, a man of sublime and universal genius, who, upon hearing of Torricelli's experiment, first made the remark, that the inference which he had deduced from it might, if true, be confirmed beyond the possibility of dispute, by carrying the mercurial tube to a considerable elevation above the earth, when, the atmospheric column being diminished, that of the mercury, which was supposed to be its balance, ought to be shortened likewise in a corresponding proportion. It followed that we had thus, therefore, a measure of the weight of the atmosphere in all circumstances, and consequently of the height of any place to which we could carry the instrument. The experiment was performed, and the result was as Pascal had anticipated. In this way, at length, was completed a discovery, the first steps towards which had been made two thousand years before; during the whole of which period the phenomena best fitted to suggest it were matter of daily observation to every one: but which, nevertheless, at last escaped even several of the greatest philosophers who had made the nearest approaches to its developement.
To return, however, for a moment to the topic of the happy application of common facts to philosophical purposes. This subject is the more worth our attention, as it opens a field of invention and discovery to which all men have, in one sense, equal access; although it is only that mind which has been rightly prepared, by previous knowledge and reflection, which is in a condition to profit by the opportunity. Another example which may be given, is that of the famous Prince Rupert's supposed discovery of
the mode of engraving called mezzotinto, which is said to have been suggested to him by observing a soldier one morning rubbing off from the barrel of his musket the rust which it had contracted from being exposed to the night dew. The Prince perceived, on examination, that the dew had left on the surface of the steel a collection of very minute holes, so as to form the resemblance of a dark engraving, parts of which had been here and there already rubbed away by the soldier. He immediately conceived the idea that it would be practicable to find a way of covering a plate of copper in the same manner with little holes, which, being inked and laid upon paper, would undoubtedly produce a black impression; while, by scraping away, in different degrees, such parts of the surface as might be required, the paper would be left white wherever there were no holes. Pursuing this thought, he at last, after a variety of experiments, invented a species of steel roller, covered with points, or salient teeth, which, being pressed against the copper-plate, indented it in the manner he wished ; and then the roughness thus occasioned had only to be scraped down, where necessary, in order to produce any gradation of shade that might be desired. *
The celebrated modern invention of the balloon is said to have had an origin still more simple. ACcording to some authorities, the idea was first
suggested to Stephen MONTGOLFIER, one of the two brothers to whom we owe the contrivance, by the waving of a linen shirt, which was hanging before
Vertue,. the engraver, and others, assign this invention to Prince Rupert, and describe the accidental discovery as above. But some writers state that mezzotinto scraping was the invention of Lieut.-Col. de Siegen; that he thus engraved the portrait of the Landgravine of Hesse, in 1643; and that Prince Rupert learnt the art of him, and carried it into England, where he much improved it. See Heinecken, Idée des Estampes, p. 208.